# Climatology/Printable version

The current, editable version of this book is available in Wikibooks, the open-content textbooks collection, at
https://en.wikibooks.org/wiki/Climatology

# About

This book is useful for geography students and teachers for pre-university level for climate related subjects. Typically, this would be for an introduction to geography course which is taken by most under graduate student in colleges.

## How does climate affect our life and earth?

Climate is a broad term, but it always describes a long-term change of a climate system. Often 'climate' is used to mean the long-term mean state of the atmosphere, including temperature, humidity, and wind. In other contexts, 'climate' can include the oceanic state, the cryosphere (snow and sea-ice), the biosphere, and sometimes even the lithosphere (Earth's crust). The pattern of human life in any particular region is to a very large extent determined by the climate:--

##### Shelter:

The design of houses in different geographical regions of the world is determined by climatic conditions. In areas of heavy rainfall, the houses have sloping roofs. The material used in construction is, however, determined by the availability of construction material. The roof could be tiled or comprise bamboos covered with hay where clay for making tiles may not be available.
In dry regions like Rajasthan, flat stone pieces are used for roofing. Partition walls are made of wood in very cold countries. In regions of high temperature, there are open courtyards attached to each dwelling. In hill districts, houses are made on stilts with the ground floor reserved for cattle.

##### Dress:

Loose dresses are popular in regions where the climate is hot. In colder regions, people wear dresses that are body hugging. A very wide variety of head gears is used again depending on the climate. The Sola hat was so popular with the British people living in the hot plains of India. Where rainfall is abundant people wear head gears that provide protection against rain while people are at work.
The lungi is popular in the southern regions of the country where it is relatively hot but not so in colder region as Himachal Pradesh. Woollens are worn in cold regions while cotton clothing is worn in hot and temperate regions. Synthetic clothing may be suitable in cold climates but not so at places where people perspire during the day.

##### Wealth

The wealth of a country can also be impacted by climate. If a country has a very inhospitable climate, such as ice and snow, or desert, then it will not be easy for agriculture or other profitable industry to be carried out, so the country will not be able to sustain a very large population, or to give a good lifestyle to those it does sustain.

##### Occupations:

During the winter months when outdoor living is not easy, the people of Kashmir remain indoors and follow up occupations like making carpets, handicrafts; silk wares that can be weaved sitting indoors. Agriculture is practiced where rainfall is adequate.
People living in dry regions follow pastoral occupations moving from place to place along with their herds of cattle looking for water and grass where ever it may be available. Where rainfall is abundant, forestry comes up as an important occupation for the people.

##### Art and Culture:

Open air stage performances are popular in areas that are not very cold. A wide variety exists at places where people have opportunity to remain out doors for most part of the year.

##### Food habits:

Since climate determines the nature of crops that can be grown in any part, the food habit also get influence by the climate. In Bengal, the staple food is rice since heavy rainfall in that region is most suited for paddy cultivation. The northern parts of the country, wheat are the staple diet. Millets are popular in dry regions like Rajasthan.
Crops like Bajra are grown there since this crop can survive even with scant water supply. The people living in Tundra region have no chance of being vegetarians. The climate does not allow growing any cereals or food crops.

##### Migration:

People living in extremely cold climates are known to migrate to warmer places in search of jobs. Where the climates are temperate, people tend to live a more settled life.

##### Effect on environment :

As the climate warms, it changes the nature of global rainfall, evaporation, snow, stream flow and other factors that affect water supply and quality. Specific impacts include: Warmer water temperatures may affect water quality and accelerate water pollution.

## Authors
# Introduction

Climatology is a branch of atmospheric science as well as geographical and environmental science.Climate is a dynamical system that is based upon the long term averages and behaviour of Earth's atmosphere both on the global scale and the regional scale. This topic aims to help your understanding of the basics of climate science as well as the many impactors climate has on the Earth as well as the impactors the Earth has on the climate.This modern field of study is regarded as a branch of the atmospheric sciences and a subfield of physical geography, which is one of the Earth science. Climatology now includes aspects of oceanography]] and biogeochemistry. Basic knowledge of climate can be used within shorter term weather forecasting using analog techniques such as the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation (MJO), the North Atlantic oscillation (NAO), the Northern Annular Mode (NAM) which is also known as the Arctic oscillation (AO), the Northern Pacific (NP) Index, the Pacific decadal oscillation (PDO), and the Interdecadal Pacific Oscillation (IPO). Climate models are used for a variety of purposes from study of the dynamics of the weather and climate system to projections of future climate.Climatology, the science that studies climate, is a young science, with modern climate science only emerging from meteorology, oceanography, and geology in the late 20th Century, it is highly dependent of mathematical models and estimates that rely in a constant gathering of data, improved sensors and historical records (natural or human generated). Of course, people have been interested in the natural world, including movements of air and water, for a very long time. An in general the sciences are still very imprecise at short or very long time frames, even if precision tends to increase over large geographical areas. Meteorologists and atmospheric scientists often say that climate is what you expect; weather is what you get.
Early climate researchers include Edmund Halley, who published a map of the trade winds in 1686 after a voyage to the southern hemisphere. Benjamin Franklin (1706–1790) first mapped the course of the Gulf Stream for use in sending mail from the United States to Europe. Francis Galton (1822–1911) invented the term anticyclone.[4] Helmut Landsberg (1906–1985) fostered the use of statistical analysis in climatology, which led to its evolution into a physical science.
Climatology is compounded of two Greek words, klima+logos;"klima' means slope of the earth'and 'logos' means a discourse or study.The first distinct climate treaties were the works of Hippocrates, who wrote Airs, Water and Places in 400 B.C.E.

##### 'scientific Discoveries Related to Climate Science''''

Climatology was primarily an observational speculation prior to the scientific age. Gradually, devices for measuring and studying weather were invented and the keeping of systematic weather records began. Climatology, thus, began as the observation and description of weather on sub-continental and continental levels.The early nineteenth century marks the beginning of the scientific discovery of climate when ice ages in paleo-climate were first suspected. The natural greenhouse effect was also identified as an element of climate change. During first and second world war, effects of ground and upper air circulations were recognized. This led to the need for the statistical treatment of weather elements and its predictions. Earth observation through satellites and availability of huge quantum of data led to model the weather conditions and monitor the climate elements both at regional and global levels.

## References

* ↑ climatology by d.s lal

# Atmospheric Composition and Structure
## Meaning of Composition of Atmosphere

The literal meaning of composition is ‘ingredients’ or ‘constituents’ of something. In another words, it is a manner by which something is made up of. When we apply the same meaning with atmosphere, it signify the items or the elements with which our atmosphere is composed. Our atmosphere is composed of numerous gasesand other substances, hence, it is a mechanical mixture of the gases, water vapour and dust particles. Let us discuss about the composition of atmosphere.

## Composition of Atmosphere

The envelope of atmosphere around the earth,a mechanical mixtures of numerous gases and other substancesare very important to all living organisms of the planet. The four major gases – nitrogen, oxygen, argon and carbon dioxide together constitute 99.99% of the total volume of dry air. The maximum concentration is of nitrogen with more than 78 percent while the oxygen is a little less than 21 percent.
Major constituents of dry air, by volume :
| Gas | Gas | Volume$^{(A)}$ | Volume$^{(A)}$ |
| Name | Formula | in ppmv$^{(B)}$ | in % |
| Nitrogen | N$_2$ | 780,840 | 78.084 |
| Oxygen | O$_2$ | 209,460 | 20.946 |
| Argon | Ar | 9,340 | 0.9340 |
| Carbon dioxide | Template:CO2 | 413.32 | 0.041332 |
| Neon | Ne | 18.18 | 0.001818 |
| Helium | He | 5.24 | 0.000524 |
| Methane | CH$_4$ | 1.87 | 0.000187 |
| Krypton | Kr | 1.14 | 0.000114 |
| Not included in above dry atmosphere: ||||
| Water vapor$^{(C)}$ | H$_2$O | 0–30,000$^{(D)}$ | 0–3%$^{(D)}$ |
| notes: $^{(A)}$ volume fraction is equal to mole fraction for ideal gas only, also see volume (thermodynamics) $^{(B)}$ ppmv: parts per million by volume $^{(C)}$ Water vapor is about 0.25% by mass over full atmosphere $^{(D)}$ Water vapor strongly varies locally/ ||||

#### Nitrogen

Nitrogen is the most abundant found in atmosphere constituting 78.084 percent to the total volume of dry gases. This is almost chemically inactive and have nothing to do with any sort of chemical actions in the atmosphere. It does not combine freely with other elements, hence, it is termed as neutral substance. This gas is found beyond a height of 100 km, but its concentration is below 50 km height from the sea level. This gas is significant for the growth and reproduction in plants and animals. Certain bacteria in the soil are capable of converting a very small amount of atmospheric nitrogen into nitrates and fix it to the soils and water bodies to be consumed by animals and plants. This process is called as Nitrogen fixation. The nitrogen fixed in the earth’s surface is again converted and sent back to the atmosphere by bacterial action through a chemical reaction called denitrification.

#### Oxygen

It is the second largest gas of the atmosphere constituting 20.9476 percent of the total dry atmospheric gases. It is very essential for the survival of many of the living organisms of this planet. It is chemically very active gas. It is combined with several other elements and forms varied compounds. Oxygen is vital for combustion of fuels. When anything burns, oxygen is consumed and helps in burning that substance. Though oxygen is found beyond 100 km but it is reasonably in good proportion within 16 km of height.With increasing height, the amount of oxygen decreases very rapidly. On mountain slope, the available oxygen for breath is very scanty and the mountaineers are supposed to carry oxygen for them.

#### Argon

In terms of percentage, argon is the third largest gas in the atmosphere constituting 0.934 percent of total dry atmosphere . It is an inert gas and chemically it is inactive. It is also found in the earth’s crust and sea water. It is used in electric bulb and fluorescent lights.

#### Carbon-dioxide

It is the fourth abundant gas of the atmosphere. It is densest gas and found in lower parts. It is found upto a height of about 30 km, it is concentrated in the lower strata. Its percentage is very low, i.e., 0.04 percent but it is most vital for the growth of vegetative life of biosphere. It is transparent to the incoming solar radiation but does not allow to escape the same. And hence, it is called as greenhouse gas. It plays a very crucial role in increasing the global temperature.

#### Methane

Methane is the also a greenhouse gas which absorbs the radiation and cause more temperature of the air. Paddy cultivation also generates methane in the air. It is also produced from the wetlands and waterlogged soils and released in the atmosphere. Fossil fuel is also a source to release of methane in the atmosphere. Its amount in the atmosphere is variable.

#### Ozone

Ozone’s concentration lays in a belt between the heights of 15 to 50 km of atmosphere. Instead ofnormal two atoms of oxygen, ozone has three atoms of oxygen formed together denoted by O3. It is formed when atmospheric oxygen molecules are broken by ultraviolet solar radiation. It may even be formed at the time of electrical discharge during thunderstorms. This gas is also termed as variable as its formation and disintegration is dependent upon numerous activities. Though ozone is very less in quantity (0.00006 percent), this thin layer is very significant for the survival of living worldas it absorbsthe dangerous ultraviolet rays and protects the earth. Neon, helium, krypton, hydrogen, xenon are other minor gases. Some gases are still extremely less in quantity, they are termed as trace gases. Important among them are ammonia, carbon monoxide, sulphur dioxide, nitrogen dioxide, nitrous oxide and sulphur hexafluoride etc.

#### Water Vapour

Water vapour is small in amount but it is one of the most important part of atmosphere with respect to the distribution of vegetation and life. Water vapour exists all the time in the atmosphere but with varying degree of amount depending upon the season (temperature condition) and the supply of water for evaporation and evapotranspiration. Air is hardly completely dry. In summer, the water holding capacity of the air is large as the temperature is high while in winter it is low.

#### Aerosols

Aerosols are extremely fine-sized solid particles or liquid droplets which continue to be in suspended form in gas for very-very long time. They could be seen when their concentration is more otherwise they are invisible. Aerosols themselves are non-gaseous microscopic substance released in the atmosphere from various sources –natural and human created. They could be pollen, minute earthly dust, sea salt, carbon soot from burning fuels, volcanic dust etc. Human activities also help the aerosols to enter the atmosphere. Their concentration is more over the industrial and urban areas. Burning of fossil fuels and generation of smoke also pump the aerosols in the air.

## Meaning of Structure of Atmosphere

Structure means the arrangement of different part into one. In another words, it is the skeleton or organization or anatomy of a whole by looking at the relationships with its parts. According to this background, the study of different parts of the atmosphere and the relationship with its parts is said to be the structure of the atmosphere. Vertically, the atmosphere is divided into different layers/ parts. Therefore, the study of different layers is known as structure of atmosphere.

## Structure of Atmosphere

Based on chemical composition, the atmosphere is classified into two. They are homosphere and heterosphere.

### Homosphere

Homosphere is that part of atmosphere where the chemical composition of the air is uniform or similar. It is the lowest layer in terms of chemical composition. It extends from the earth’s/ ocean surface to about 85 km. the basis of the changes in temperature, the atmosphere is divided into five layers out of them, three lower layers falls under homosphere (i.e. within 85 km of altitude). They are troposphere, stratosphere and mesosphere.

##### Troposphere

It is the lowest and densest layer of the atmosphere. It extends till a height of about 8 km over pole but over equator, it is 18 km. About 80 percent of the total mass of the atmosphere lays in this layer. With increase in height, the temperature keeps on declining till the limit of this layer. On an average, the decrease in temperature with height is 60 Celsius par km. The upper boundary is known as troposphere laying between 8 and 18 km. At this level, the average temperature reaches to minus 500 to minus 600 Celsius.

##### Stratosphere

Stratosphere is the upward second layer as well as middle layer of the homosphere. It starts from tropopause to approximate height of 50 km. The temperature at the tropopause remains almost constant till the height of 20 km. After that, it starts increasing and continue the trend till the height of 50 km. At this level, the estimated temperature is about minus 100 to minus 150Celsius. Though the temperature is on rise, but there is no atmospheric turbulence. This layer is completely free from clouds and other weather conditions. That is why, it has an advantage for flying long-distance supersonic jets.

##### Mesosphere

Mesosphere is the third but the upper-most layer of the homosphere. After this layer, heterosphere starts. The literal meaning of mesosphere is the middle sphere. It is separated by tropopause below from troposphere and mesopause on the top from thermosphere. It is extended from 50 km to 85 km from the earth’s surface. The air pressure is very low. It is 1 millibar at the lower limit whereas it is 0.01 millibar at the highest limit. This layer is characterised by decreasing temperature and the coldest/ lowest atmospheric temperature is recorded in this layer. The lowest temperature estimated near the mesosphere is around minus 1300Celsius. It is colder that the lowest temperature recorded over Antarctic.

### Heterosphere

The atmosphere laying beyond the homosphere is termed as heterosphere. The term itself is self-explanatory and it is used for that part of atmosphere where the air is not uniform. In this part of atmosphere, the air is rare and the molecules are wide apart. Relatively heavier gas molecules are concentrated in the lower part whereas the lighter are forced to be above. Beyond 85 km height, the composition of the atmosphere with increasing altitude vary significantly. Different layers of prominently different gases are nitrogen layer, oxygen layer, helium layer and hydrogen layer are differentiated. However, the heterosphere, is divided into two main spheres –thermosphere and exosphere.

#### Thermosphere

This sphere extends from mesopouse. it is 85 km to about 650 km from earth. The temperature is on rise in this layer due to absorption of solar radiation by small amount of oxygen molecules present. It is highly dependent upon the solar activities. The temperature reaches beyond 12000C at an altitude of about 350 km but by 650 km it may even rise to 20000C. This much high temperature is primarily defined by average speed with which molecules are moving. Because of this, the temperature may be high.

#### Exosphere

Exo means external. Therefore, exosphere the external or the outer most layer of the atmosphere. Its lower boundary starts from the thermopause 650 km to 10000 km. This much distance is little less than the diameter of the earth. It is really a very big size of the limit of the atmosphere.

## Lesson Summary

* Different temperature gradients create different layers within the atmosphere. The lowest layer is the troposphere, where most of the atmospheric gases and all of the planet's weather are located.
* The troposphere gets its heat from the ground, and so temperature decreases with altitude. Warm air rises and cool air sinks and so the troposphere is unstable.
* In the stratosphere, temperature increases with altitude. The stratosphere contains the ozone layer, which protects the planet from the Sun's harmful UV. The higher layers contain few gas molecules and are very cold.

# Insolation

Solar irradiance (SI) is the power per unit area received from the Sun in the form of electromagnetic radiation as reported in the wavelength range of the measuring instrument. This integrated solar irradiance is called solar irradiation, solar exposure, solar insolation, or insolation.The sun is the primary source of energy for the earth. The sun radiates its energy in all directions into space in short wavelengths, which is known as solar radiation.
* The earth’s surface receives only a part of this radiated energy (2 units out of 1,000,000,000 units of energy radiated by the sun).
* The energy received by the earth’s surface in the form of short waves is termed as Incoming Solar Radiation or Insolation.
* The amount of insolation received on the earth’s surface is far less than that is radiated from the sun because of the small size of the earth and its distance from the sun.
* Moreover, water vapour, dust particles, ozone and other gases present in the atmosphere absorb a small amount of solar radiation.
* The solar radiation received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun.
* During the earth’s revolution around the sun, the earth is farthest from the sun on 4th July. This position of the earth is called aphelion. On 3rd January, the earth is nearest to the sun. This position is called perihelion.
* Due to this variation in the distance between the earth and the sun, the annual insolation received by the earth on 3rd January is slightly more than the amount received on 4th July.
* However, the effect of this variation is masked by some other factors like the distribution of land and sea and the atmospheric circulation. Hence the variation does not have a greater effect on daily weather changes on the surface of the earth.

## Methods of Solar Energy Receipt

The solar radiation received by the earth’s atmosphere and surface by direct radiation or diffuse energy. There are five way of solar radiations. They are:--
* Transmission
* Scattering
* Refraction
* Absorption
* Reflection

### Transmission

When the solar radiation reaches to the earth surface through a medium is known as transmission of solar radiation. The dictionary meaning of transmission is the passage of shortwave and longwave energy (electro-magnetic energy) through either the atmosphere or water but the sun’s energy is reaching to the earth without any medium from the space. It is also known as radiation. The atmospheric energy comprises shortwave radiation inputs (ultraviolet light, visible light, and near-infrared wavelengths) and longwave radiation outputs (thermal infrared) that pass through the atmosphere by transmission. In our atmosphere, all the radiation is not reaching to the earth surface or in other word, atmosphere is not opaque for all the radiation. This differential transmission causes the greenhouse effect in the atmosphere.Due to greenhouse effect of the atmosphere, the earth’s atmospheric temperature is hospitable. Without this effect, the living conditions could not be created.

### Scattering

Atmospheric gases and dust particles physically interact with incoming solar radiations through processes of scattering. A redirection of energy through refraction and reflections called scattering. The solid particles of dust, smoke, aerosols, sea salts sprays, pollutants atmospheric humidity, smoke shoots etc. available in the atmosphere are responsible for scattering of sun electromagnetic energy. It changes the direction of the light's movement without altering its wavelengths. This phenomenon is known as scattering and represents 7percent of Earth's reflectivity, or albedo. It is ‘unpredictable’ because of multiple reflections of electromagnetic waves by particles and surfaces. But inreflection, the direction of reflection is predictable. Dust particles, pollutants, ice, cloud droplets, and water vapor produce further scattering.

### Refraction

Refraction means bending of light. As solar radiation enters the atmosphere, it passes from one medium to another of atmosphere, from virtually empty space into atmospheric gases. The atmosphere itself is made up of different layers. The different layers have different density and change in density of the atmosphere causes the bending of incoming solar radiations at different angles. This refraction could also be seen with water as well. Itis known as refractionof incoming radiation. For example, prism refracts light passing through it, bending different wavelengths to different angles, separating the light into different component of colours to display the spectrum. In nature, rainbow is created when visible light passes through many raindrops and is refracted and reflected toward the viewer at a precise angle.

### Reflection

The reflection is an ability of the material where a portion of arriving energy strikes. Depending upon the surface characteristics, where it strikes, the quantum of reflectivity is determined. It might be completely bounced directly back into space with or without being absorbed. This returned energy is called reflection especially specular reflectance where angle of incidence and angle of reflectance is same. A mirror is one of the example of reflecting more than 90 per cent of the visible light incidence upon it. The term albedo is used to describe the amount of energy reflected back in percentage. Albedo is the reflective quality of a surface. It is an important control over the amount of insolation that is available for absorption by a surface. So,the proportion of insolation that is reflected back from the atmosphere, from the tops of the clouds, and earth surface including land and water both, without heating the receiving surface, is an albedo.

### Absorption

In our earth’s atmosphere due to the different composition, act as a barrier to the insolation or it absorbed some solar radiation, may be assimilated by the object is generally termed as atmospheric absorption. The different material on earth has different absorptive capabilities with different wavelength of solar radiation. It is happening in all electromagnetic radiations at certain spectral bands by the composition present in the atmosphere. The most efficient absorbers of solar radiation in this regardare water (H2O), carbon dioxide (CO2), ozone (O3), Oxygen (O2),nitrous oxide (NO2). The cumulative effect of the absorption by the various constituents can cause the atmosphere to close down completely in certain regions of the spectrum that leads to an increase in temperature. Generally, a good radiator is also a good absorber and a poor radiator is a poor absorber. As colour is concern, dark-colour surface are much more efficient absorbers of radiation in the visible portion of the spectrum than light-colour surface.
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## Factors Influencing Insolation

The amount of insolation received on the earth’s surface is not uniform everywhere. It varies according to the place and time. When the tropical regions receive maximum annual insolation, it gradually decreases towards the poles. Insolation is more in summers and less in winters. The major factors which influence the amount of insolation received are:--
* Solar output /constant
* The angle of incidence of the sun’s rays
* Duration of the day
* Earth distance from sun
* Transparency of the atmosphere

### Solar output /constant

At the top of the earth’s atmosphere receives insolation is expressed as the solar constant.It received at the top of the atmospheric surface (thermopause) on a perpendicular plane to the solar beam. The average insolation received at the thermopause i.e. 1368Wm2(Watt per square metre)energy (solar constant) in the form of short wave. Thus, it is termed as solar constant for that mean distance from the sun. These solar constant is varying over 1 Wm2by periodic disturbances and explosions in the solar surface basically related to sun spot. Sun spots are dark and cooler areas visible on the sun’s surface. The recent researches have shown that more and more energy is released when the sunspots are in large number. The number of sunspots also increases or decreases on a regular basis, creating a cycle of 11 years.

### The Angle of Incidence

Since the earth is a geoid resembling a sphere, the sun’s rays strike the surface at different angles at different places. This depends on the latitude of the place.The higher the latitude, the less is the angle they make with the surface of the earth. The area covered by the vertical rays is always less than the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases.The sun’s rays with small angle traverse more of the atmosphere than rays striking at a large angle. The angle at which sun rays strike on earth surface is called angle of incidence. Itcontrols the amount of insolation received at the earth’s surface. The amount of insolation is determined by time of the day (morning, noon and evening), the latitude (equator to poles) and season (summer, autumn, winter and spring).When the sun rays strike vertically or sun is directly overhead, the rays angle of incidence is 90 degree. The beam of rays spread on a smaller area in comparison to oblique/ slanting beam of sun rays.when sun is in a vertical position, the beam of light will spread over one mile but in the oblique position(with 300of angle) of the sun the same beam of light will spread over two miles.It is clear that the larger amount of radiant energy is lost in case of slanting rays than in vertical rays. Therefore, on anaverage, equatorial areas receive approximately 2.4 times more insolation than polar areas.

### Duration of The Day

The length of the day determines the duration of sunlight which affects the amount of solar radiation received by the earth’s surface. The longer period of sunshine, greater the quantity of solar radiation will be received by a portion of earth.For example, at the equator the length of days and nights is 12 hours in all the months but the tropics of Arctic and Antarctic sunshine duration varies between 0 and 24. On the autumn and springe quinoxes (September 23and March 21 respectively),the sun is overhead at the equatorat noon.The night and day all over the earth are equal on these days and maximum amount of insolation is received at the equator, and the amount of insolation decreasing towards the poles.It is caused by vertical sunshine at equator but with increasing latitudes, the rays become more and more slanting. Therefore, poleward, the received energy keeps on declining.

### Earth Distance from Sun

The earth is revolving around the sun in an elliptical orbit, resulting continuous change in the distance between sun and the earth on annual basis. It leads to seasonal variation in solar energy received by the earth.The mean distance between the earth and sun is about 149,600,000 kilometers(92,900,000 miles). When earth position is farthest(152 million km) from sun is known as ‘aphelion’ on July 4. It is perihelion(147 million km)occurs on January 3 each year which is the closest distance. During aphelion the northern hemisphere is facing the sun and therefore receives energy about 7 percent less than the perihelion (southern hemisphere).

### Transparency of the Atmosphere

Atmosphere is not transparent for all the radiation coming from the sun because of different composition and layers. It is also one of the controlling factors of insolation to reach earth surface. The atmosphere is composed of gases, water vapour and particulate matters.The atmosphere is a mixture of gases, such as nitrogen(N), oxygen (O2), Argon, carbon dioxide, Neon (Ne), Helium (He), Methane (CH4), Krypton (Kr), Ozone (O3), Nitrous oxide (N2O), Hydrogen (H) and Xenon (Xe). The atmosphere also contains water vapour, water in the gaseous state.

## Mechanisms of Heating and Cooling of Atmosphere

Heating and cooling of theatmosphere is performed by following processes:--
* Partial absorption of solar radiation by atmosphere
* Conduction
* Terrestrial radiation
* Convection
* Advection
* Latent heat of condensation
* Expansion and compression of the air

# Heat Budget
## Concept of Heat Budget

the meaning of budget is an estimate of your income and the expenditure of the same over a certain period of time. Heat is a sort of energy which our earth receives from the sun. Therefore, the energy received (your income) from the sun and its utilization (your expenditure) by/in the atmosphere as well as the surface (land and water surface) of the earth is basically heat budget. In this context, the heat budget has two main components incoming shortwave solar radiation and outgoing long-wave terrestrial radiation. Let us study them in brief:-

### Incoming Shortwave Solar Radiation

the sun is the prime source of almost all energy on the earth. This energy coming from the sun is known as solar radiation.Since the sun is extremely hot, it emits shortwave radiation in the form of electromagnetic waves. different types of shortwaves like x-rays, ultraviolet, visible and near infrared . the shortwave energy from the sun is not directly absorbed by the atmosphere.

### Processes Involved with Incoming Radiation

The uppermost atmosphere of earth receives about one part of energy out of two billion parts radiated from the entire sun’s surface.Processes Involved with energy is the cause of various interactions in the atmospheric systems.the processes involved with incoming radiations: There are three processes operating with the incoming solar radiation. They are:-
* Reflection
* Diffusion
* Scattering
* Absorption

#### Reflection

The meaning of reflection is returning something back from where something was coming. you bring a mirror to the sunlight, a bright beam of light is thrown away from the surface of the mirror. The angle of the reflection of light is dependent on the angle of the incidence. you find that the angle of incidence and angle of reflection is equal.The certain amount of incoming energy is lost and it does not participate in the heating process of atmosphere or the earth’s surface. It is generally expressed in percentage of the incident radiation reflected. It is also called as albedo or coefficient of reflection.

#### Diffusion and Scattering

The term diffusion refers to the spreading of something more widely from its centre to all directions. In scattering, sun energy or light is forced to deviate from the direction of propagation. When the sun energy passes through the atmosphere, it has to travel through numerous solid minute particles of aerosols and gases.In this process of passing through, the energy and light is diffused and scattered.The blue colour of the sky is due to selective scattering of sun light. Before dawn or after the sunset, the sky is red and it is due to diffusion and scattering of red visible light.

#### Absorption

The term absorption refers to a state of being engrossed or being captivated.It is a process by which something is absorbed by another thing. With reference to the absorption of solar radiation, it is done so by the atmosphere or the earth surface. Incoming solar radiation is absorbed by the different elements of atmosphere present at a particular point of time. These elements are gas molecules, water vapour, smoke and dust particles. They trap a part of solar energy during transmission through the atmosphere.

### Outgoing Longwave Terrestrial Radiation

The received energy from the sun heats the earth surface. Heated earth surface is not that hot hence, it re-emits the energy. This returned energy is in the form of long-wave radiation. In this process, the longwave energy is absorbed by the atmosphere and the atmosphere is warmed up.

### Processes Involved with Outgoing Radiation

The received energy by the earth as well as by the atmosphere is re-emitted back.It happens through different processes.The outgoing energy involves following processes:--
* Latent heat transfer
* Sensible heat transfer
* Emission by vapour and clouds
* Longwave radiation

#### Latent Heat Transfer

Latent heat is the energy absorbed or released from a substance due to changing phases. For example, when solid to liquid or from liquid to gas or even from solid to gas and vice versa. If a substance is changing from solid to liquid, it absorbs the energy from the surroundings so that its molecules are spread out. If the liquid is again changing its state to gas, it further requires more energy for the same reason.

#### Sensible Heat Transfer

Sensible heat is the energy needed to alter the temperature of a substance without any change in the state. It is possible by absorption of sunlight by the land surface or even the air is warmed up by gaining heat. Release of latent heat or the cool air coming in contact of warm air also cause the temperature to rise.

#### Emission by Vapour and Clouds

Emission means discharge or release of something. Huge amount of terrestrial energy is released through the vapour and clouds. In fact, the energy due to which the atmosphere was heated up, in general, is released through vapour and clouds as well.

#### Longwave Radiation

Little amount of energy is directly released to the space by direct longwave radiation.It means that this amount of energy is not trapped by the atmosphere.

## Heat Budget

The ideal heat budget of the earth is supposed to be a perfect balance between the incoming solar radiation and outgoing terrestrial radiation.It has to be a zero outcome of the incoming solar radiation and outgoing terrestrial radiation . The total incoming shortwave radiation reaching at the top of the atmosphere is considered to be 100 percent. The distributed and re-distributed of this 100% or 100 units energy is termed as heat budget of the earth. Out of these 100 units, 17 units are reflected back to the space by cloud cover. Air molecules scatter 8 units of the energy back to the space. Energy reached on earth surface is also reflected by some surfaces like snowcover, deserts or other bright surfaces. Their contribution in reflection is 6 units. Hence, the total reflection, from atmosphere (17 units), from air molecules (8 units) and from surface (6 units) is 31 units. This much of energy is not at all used in the heating of the atmosphere or the earth surface. Remaining 69 units are involved in heating of the atmosphere and the earth surface. Out of these 69 units, 19 units are trapped by the water vapour, dust particles and ozone and heats up the atmosphere. Four units are absorbed by clouds and the rest 46 units reach directly to the earth surface. The 31 units directly reflected back to space, 23 units utilized in the atmosphere and 46 units reaching the earth make the total 100 units of incoming solar radiation.
Out of 46 units received by the earth, 9 units are directly released back to space by longwave radiation without heating the atmosphere. Six units of the longwave radiation are absorbed by clouds, water vapour, carbon dioxide and ozone. The total energy used up in heating the atmosphere is 60 units; 37 units released by the earth surface (7 units –sensible heat; 24–units latent heat and 6 units absorbed by clouds, water vapour, carbon dioxide and ozone) and 23 units already absorbed during transmission of the solar energy (19–units absorbed by water vapour, dust particles and ozone and 4 units –absorbed by clouds).

## Heat Balance

Heating is the process of transfer of energy from a body of higher temperature to another body of lower temperature. The distribution heat energy is not uniform over the earth surface. It has numerous factors to affect. the maximum energy is received near the equator throughout the year. The seasonal variations of energy received are considerable with increasing latitudes.

# Temperature of the Atmosphere
## Mechanisms of Heating and Cooling of Atmosphere

Heating and cooling of theatmosphere is performed by following processes:--
* Partial absorption of solar radiation by atmosphere
* Conduction
* Terrestrial radiation
* Convection
* Advection
* Latent heat of condensation
* Expansion and compression of the air

### Partial absorption of solar radiation by atmosphere

The solar radiations are coming to the earth surface directly from the sun. It is in the form of short-wave radiation. They are so energized that atmospheric gases are unable to trap them. But the presence of some dust particles and water vapour in the lower level of troposphere are capable of holding some energy directly coming from the sun.About 20% of the total incoming solar energy is trapped by the dust particles and vapour in the atmosphere.

### Conduction

The literal meaning of the term conduction is passing on something by a medium without any perceptible movement by itself. It is the transfer of something from one part to the other without any physical movement. Air is a very poor medium of heat conduction. It is a very slow process of transferring heat in a mass of air. By this method, air is heated, but its importance is not that great. Because, the air is in the gaseous state and its particles (molecules or atom) are not very solidly compacted. A very thin layer which is very close to the earth surface is heated by conduction method .

### Terrestrial Radiation

The terrestrial radiation is the most important method of atmospheric heating. Out of the total solar electromagnetic radiations reaching at the top layer of the atmosphere, approximately 49% reaches to the earth’s surface.energy is reaching the earth surfacein the form of shortwaveelectromagnetic radiations from the sun. The heated earth radiates back the same but in the form of long wave electromagnetic or infrared radiations.Terrestrial radiations are a continuous affair all 24 hours throughout the year. During day time when sun is in the sky, the solar short wave incoming energy is greater than the energy lost from the earth surface (land and water).warming.htmlIt is also very true that there is a gradual decrease in temperature with increasing altitude within the troposphere. Greater temperature is recorded at the ground surface as the earth is heated first and then the heating of atmosphere starts.

### Convection

The earth’s surface is heated with incoming solar energy. The air in contact with the surface is in gaseous form. Earth’s surface heating results in heating of the airin its contact. But the air becomes less dense by heating. It further results into rising of the warmed/ expanded air molecules upward. Upward moving air molecules in large quantity createa convection. The occurrence of Hadley cell, Ferrel’s cell and Polar cell are examples of atmospheric convection. Therefore, convection transfers the heat energy received from the sun to the surface and from the surface to the atmosphere.

### Advection

The meaning of advection is transfer of something from one placeto another especially in horizontal direction. Atmosphere is a huge body of air and it has differences in terms of its pressure depending on several affecting factors. Due to varying pressure at local, regional and global level, atmospheric gases are continuously on move.The monsoonal air current movement is the example of regional advection while planetary permanent wind system signifiesthe global advection. All of them are transferring the heat from one area to another.

### Latent Heat

Heat absorbed or released due to change of the state of any matter is known as latent heat. During this process, there is no change of temperature of that matter. In another words, it is the heat that is required to change the matter to a higher state of matter.one example; when water changes from one state to another, for example, water vapour to liquid water and liquid water to solid water (ice), it absorbs or releases heat. The energy involved in this process is known as latent heat, popularlymeant for ‘hidden’ heat.

### Latent Heat of Condensation

It is the amount of heat energy released to the atmosphere when condensation takes place. when one gram vapour is changed to water and calories is released, it is called latent heat of condensation, because it is reaching to the atmosphere due to the process of condensation.

### Expansion and Compression of the Air

The mass is greater downward .It is due to this reason, any parcel of air, if it rises, going upward is expanded.Because the rising air is entering in the zone of less dense air, it results in expansion. Rising air expands and the intermolecular space is expanded and it causes the cooling in the air as well. It is also known as adiabatic cooling. That means, the cooling is caused by simply expansion of the volume of air.

## Factors Affecting Heating and Cooling of Atmosphere

The erth is a sphere and the atmosphere is encircling around it .The distribution of energy on the earth surface and in atmosphere is varying to a great degree particularly with respect to latitude. The distribution of heat is affected by several factors important among them are:-
* Latitude
* Altitude and nature of earth’s surface
* Distribution of land and water
* Nature of ocean currents
* Transparency of the sky
* Slope aspects Latitude

### Latitude

The light energy of the sun reaches to the earth’s surface to a maximum limit to 180 degree of angle.the low latitude areas are warmer and high latitude areas are colder.high latitude areas are much colder because of less effective heating.

### Altitude and Nature of Earth’s Surface

The atmosphere is normally warmed by the longwave terrestrial radiations. Hence, low altitude are a has more temperature than the high altitude areas.The nature of the rock also affects the atmospheric heating and cooling. The areas possessing bare rocks have more intense heating by the sun’s energy. That type of area also radiates back more and more received energy and the result is quick heating of the air laying there.

### Distribution of land and water

The earth surface is covered by land and water bodies.The land is an opaque to the incoming solar radiation while water is translucent.Whatever the heat energy reaches to the landbody, it is utilized to heat a thin layer of the land surface while the same amount of heat energy reaching to the water surface is penetrating to much deeper depths.

### Ocean currents

Heating and Cooling Low latitude are as are warmer while the high latitude are as are colder. The temperature of the ocean water is also affected by the temperature distribution over the globe. Ocean currents are flowing under the influence of planetary wind system as well as the regional shape of these a coasts. Since the ocean currents are very important medium of heat transfer through advection from low latitude to the high latitude.

### Transparency

Apart from the gases, several other minute suspended particle sand water vapour constitute the atmosphere. Though the gases are almost uniformly distributed, but other substances are varying at local and regional level. Their availability and quantity is season dependent also.

### Slope Aspects

Slope of the any region or mountain has direct bearing on the heating or cooling of theair.The south facing slope of northern hemisphere and north facing slope of southern hemisphere receive more energy than their counterpart.

## Horizontal Temperature Distribution

The energy coming from the sun is not uniform all through the globe .equatorial region is hot and its temperature is high throughout the year. Generally, from equator to polewards, temperature keeps on declining. The lowest temperature is at and near the pole.To represent distribution of temperature isotherms are used. Isotherm is an imaginary line joining the places with same temperature. If we draw the isotherm of certain time over the world map, we would be in a position to have spatial pattern of the distribution of temperature.We study about two position:--
* when the sun is overhead at Tropic of Cancer
* when the sun is overhead at Tropic of Capricorn

### Sun Overhead on the Tropic of Cancer(July)

The sun is overhead at the Tropic of Cancer by the end of third week of June (June 21st) at 23.50N. Entire northern hemisphere witnesses bright sun, greater insolation leading to high temperature throughout. But the maximum average monthly temperature is not recorded in June, but it so in July.July is taken to study ideal summer month for northern hemisphere. The highest temperature is recorded over large chunk of area comprising Sahara desert of northern Africa and desertic parts ofwest central Asia. This belt runs from Sahara desert, via Arabia to Thar. The high temperature zone is extended to the Indo-Gangetic plain as well asTibetan plateau. This zone attracts the monsoon winds as it has intense low pressure due to high temperature. This low pressure zone is characterized by inter tropical convergence zone (ITCZ). During northern hemisphere summer days, the isotherms turns towards northward over land as it is hotter thanwater. The condition is reversed on the oceans as the water bodies are not that hot as the land is. Hence, the isotherms turns southward on the oceans of the northern hemisphere. Another low pressure system is developed over the north western Mexico and south western USA due to more intense record of temperature.

### Sun Overhead on the Tropic of Capricorn (January)

The sun is overhead at the Tropic of Capricorn by the end of third week of December(December 21st)at 23.50S. Entire southern hemisphere witnesses bright sun, greater insolation leading to high temperature throughout. But maximum average monthly temperature is not recorded in December, but it is so in January.Therefore, case of January is taken to study ideal summer month for southern hemisphere. Both major continents –South America and Africa are tapered towards south. The maximum mean temperature of January is about 320C over a small area of western Australian desert. Over South America and Africa, it is around 270C. The area bounded by the 270C isotherm is wide over continents as well as on the Indian Ocean. The temperature gradient is increasing. It is greater in the northern hemisphere particularly over the large landmass of Asia and North America. The lowest temperature around -400C is recorded on the polar region of Canada, Iceland and Asian Siberia. Over the northern oceans, isotherms are turned towards pole whereas on landmass their bends are towards south. It is because of the transport of heat from equatorial region to poleward through prevailing winds and ocean currents.

## Inversion of Temperature

In certain conditions, temperature is not always declining with increasing altitude but it rises. This situation is known as inversion of temperature. The term, inversion, means opposite to the normal. Since normal is fall intemperature with altitude, under inversion, it rises with increasing height.It happens when the air near surface is cooler than upper air.

#### Ideal Conditions for Temperature Inversion:--

Temperature inversion takes place only under certain conditions.
* There has to be long and cool nights so that earth radiates received solar energy.
* There has tobe clear sky so that terrestrial radiation escapes.
* There has to be calm and stable air so thatvertical motion in the air is absent.

### Types of Temperature Inversion

there are several types of inversion of temperature.
* Ground inversion,
* Valley inversion,
* Subsidence inversion and
* Frontal inversion

#### Ground Inversion

Ground inversion occurs when the surface is cooled rapidly by earth radiation under clear sky. In this way, temperature above the ground is still warmer than air near the ground. When temperature near surface reaches todew level, the possibility of fog formation increases. Ground inversion is very common in the higher latitude areas or during winter in the plain even inthe tropical regions.

#### Valley inversion

Valley inversion takes place on the rolling topography, particularly in hilly areas. In such situation, mountainslope becomes cool in the night and the air with its contact gets cooler. Cool air creeps downward along the slope and occupies the valley.The warm air of the valley is pushed up and thus the inversion of temperature is evident.

#### Subsidence inversion

Subsidence inversion takes place mostly insubtropical high pressure belts orleeward side of the mountain where air subsides. In either of the cases, subsiding air gets warmed in thisprocess while the lower level preexisting air is cooler. The warming is achieved about 100C per km of descending air.

#### Frontal Inversion

This type of inversion of temperature takes place under the frontal formation of two different air masses. Whencold and heavier air mass under cuts warm sector occupied by warm air mass, the warm air is lifted up. The ground is occupied with cold air, and thus, inversion is observed.

# Air pressure
## Introduction

Atmosphere is an envelope of gases circling the earth. It is mainly made up colourless and odorless gases of Nitrogen, Oxygen, Carbon-dioxide and many more. Pressure can be defined as the force exerted by the atmosphere at a given place and time. Generally it is expressed in millibar (mb). The atmospheric pressure is determined by the mass of a column of air lying above a certain level or surface. The increase or decrease in pressure of the air is determined by various factors. Atmospheric pressure is a force of dry air exerted on a particular place or surface. The column of dry air causes the pressurization due to its own mass. The air above is exerting pressure on the lower level, and is also known as barometric pressure. Atmospheric pressure declines very rapidly with increasing height above the sea/earth surface.
The first atmospheric pressure measuring instrument was invented by E. Torricelliin in 1643. In Torricelli’s barometer, mercury was used. One inch of mercury pressure is equivalent to about 33.8639 mb at sea level. The air pressure is shown on maps using isobars -- lines that join sites with the same air pressure.

## Pressure Gradient

The atmospheric pressure gradient refers to the change in the pressure per unit distance between two places along a line on an isobar map of anyarea. The maximum pressure gradient is observed along a line perpendicular to the isobar. It is more when the isobars are closely spaced, but when it is widely spaced, the pressure gradient is lesser.

## Variation in the Atmospheric Pressure

The atmospheric pressure varies from place to place and from time to time. On the basis of time, diurnal and seasonal variations can be observed.

### Diurnal Variation

The periodic pressure change is observed on daily basis. Two highs and two lows of air pressure is seen each day. The high point occurs at 10am and 10pm while the low pressure occurs at 4pm and 4am. This is also called as semidiurnal observation in pressure variation. It is because of difference of 12 hours between them. The mean daily changes in the air pressure can be found out by calculating the average hourly observed pressure for a long time. Insolation, heating, cooling and radiation are the factors for the diurnal changes in the air pressure.

### Seasonal or Annual Variation

The amount of insolation received in a particular region varies from one season to another season. Because of this variation, seasonal or annual variation in the atmospheric pressure is found. The amount of insolation received in a particular region varies from one season to another season. Because of this variation, seasonal or annual variation in the atmospheric pressure is found.

### Factors Affecting Air Pressure

There are three important factors affecting the air pressure. They are temperature of the air, altitude and moisture. Briefly known as TAM.
* Temperature(T)
* Altitude (A)
* Moisture (M)

## Distribution of Atmospheric Pressure

Atmospheric pressure varies from place to place and season to season. Its distribution over the globe is not uniform. Variation is seen in both perspectives :--
* vertical
* horizontal

### Vertical Distribution of Atmospheric Pressure

The air is compressible.Its density is greater in lower layers as compared to the upper layers of the atmosphere.The atmospheric pressure decreases with increasing height. The vertical distribution of atmosphere is influenced by temperature, water vapors and altitude. In the higher altitude, atmosphere becomes thin and intermolecular space is more.

### Horizontal Distribution of Atmospheric Pressure

The distribution of atmospheric pressure on the earth is explained across the latitude. Distribution of air pressure on the earth's surface is shown by means of isobars.Isobars are line drawn through points of equal pressure. This is considered as horizontal distribution of atmospheric pressure. distribution of pressure belts is very distinct and classifiable.Based on the characteristics of different belts, they are grouped into four: They are:-
* Equatorial low pressure belt
* Sub-tropical high pressure belt
* Sub-polar low pressure belt
* Polar high pressure belt
Last three belt have two cases of each –northern and southern hemisphere. In fact the first belt has also two cases but both north and south cases of equatorial low forms a single belt. That is why, it is call as one.

#### Equatorial Low Pressure Belt

The Equatorial belt extends from 10° north to 10° south latitude. It is a thermally induced belt because here the temperature remains very high throughout the year due to the vertical sun’s rays.The average pressure in this belt is less rhan 1013millibars,but in the eastern hemisphere it is generally less than 1009milibars.there is almost an absence of horizontal movement of wind, the calm condition is termed as doldrum. The winds converging from both hemisphere’s high pressure belts results into a zone of convergence. It is known as inter tropical convergence zone (ITCZ). The risen air from the equatorial low reaches to the upper troposphere and dragged towards poles. By reaching in the tropics, air descends from 200to 350 latitudes in both the hemispheres. It is caused by cooling of the air. Cool air is heavier.

#### Sub-Tropical High Pressure Belts

These belts are extended from 20°to 35°latitudes in both hemisphere. These belts are situated over tropic of Cancer and tropic of Capricorn. There is subsidence of air from the upper troposphere in this zone. The wind moves equatorward to fill the temporarily created vacuum/ gap produced by rising air at the low pressure zone. Hence, an atmospheric cell is created by rising air at equator –moving up –getting drifted towards pole –getting subsided due to cooling –becoming heavier –climbing down at sub-tropical high and finally moving towards equator to fill the gap created by rising air,This cell (circular motion) is known asHedley cell .A calm and feeble wind is created in this region which is known as horse latitude. In early day sailing vessel with cargo of horses was very difficult under such calm conditions. The horses were thrown into thesea to reduce the load of the ship.

#### Sub-Polar Low Pressure Belts

These belts are found between 50° to 70° latitudes in both the hemispheres. These belts are induced due to ascend of air as a result of convergence of wind coming from sub-tropical high pressure belts (westerlies) and polar high pressure belts (easterlies).The air moving fromsub-tropical high to sub-polar low – rises above – gets cooled – diverted towards equatorward and descends at sub-tropical high – makes a cell (circulation motion) known as Ferrel’s cell.During winter season, because of high contrast of temperature between land and sea, this belt is broken into two low pressure centers in northern hemisphere one in the vicinity of the Aleutian Island, and other between Iceland and Green Land. During the summer season, the variation is less.

#### Polar High Pressure Belts

High pressure prevails over both the polar regions due to excessive cold condition. The cold climatic condition itself is caused by slanting sun’s ray at the poles.These pressure zones, thermal factor is more important than dynamic factor. The air coming from polar region –rises up at the sub-polar low –finally pushed towards pole and descends at the polar high. This also makes a cell known as Polar cell.

## Factors Controlling Pressure Belt System

There are two main factors controlling the pressure belt system over the globe are following:--
* Thermal factor
* Dynamic factor

### Thermal factor

Equatorial region receive intense solar energy throughout the year. Polar regions are extremely cold throughout the year due to less effective and inclined solar rays. More solar energy in the equatorial region causes the air to expand and thus low air pressure is created.At poles, due to extreme cold conditions the air is very cold and hence, high air pressure is observed. Therefore, the creation of equatorial low and polar high are caused by thermal factor. That is why, they are called asthermally induced pressure belts.

### Dynamic Factor

The equatorial low as well as polar highs are explained by the thermal factors. But sub-tropical highs and sub-polar lows are not coming into the line of thermal explanation. The warm air uplifted from the equatorial low is pushed towards pole and due to cooling it subsides over tropics. It is considered as a dynamic factor of the development of subtropical highs. wind blows from the subtropical highs to equatorial low as well as towards pole. Since pole has thermally induced high pressure. Both winds meet in the subpolar low around 50 to 70 degree latitudes in both the hemispheres. Therefore, subpolar lows also have dynamic origin.

## Changing Seasons and Pressure Belts

The earth’s axis is inclined by 66030’from the horizontal plane. On this inclined axis, it is revolving around the sun. Due to this reason, the orbit around the sun is elliptical in shape The sun’s rays are vertical on equator on 21 March and 22 September. This situation is known as equinox. Equinox means the day and night areequal throughout the globe. The wind coming from both the hemispheres converges at equator. It is inter tropical convergence zone (ITCZ). After 21 March, the northern hemisphere starts tilting towards the sun.January being on of the summer months in the southern hemisphere,the continent of Australia, Africa,and South America are warmer. there are distinct low pressure cell over them.Vertical sun rays in the northern hemisphere is marked by increasing energy.
This energy heats the areas/ air. Therefore, ITCZ migrates towards north along the thermal equator. Since the ITCZ is temperature induced low pressure (already discussed before) the migration of ITCZ is marked by the shifting of all pressure belts northward.The maximum northward departure of sun is seen by 22 June, the effective maximum average energy received in the northern hemisphere is observed in July. The northward maximum departure of ITCZ is seen upto 25 degree from equator .Land is getting heated much quickly as well as greater temperature is recorded there, but the same is not with water body/ oceans. Greater effective energy is accumulated over the land, ITCZ is also havethe tendency to depart more over the land. After 22 June, the retreat is seen and sun again shines vertically over the equator by 22 September. Further onward the southern hemisphere is inclined towards the sun. The same case is repeated in the case of southern hemisphere what it has been discussed above. By 21 December, sun shines vertically over the tropics of Capricorn. After this it starts northward migration and reached over the equator again by 21 March.

### Seasonal Variations during July

End of third week of June, the sun shines directly over the tropic of Cancer. After that sun’s rays starts turning towards south. July is the hottest month recorded in the northern hemisphere. Northward departures of the inclination of sun’s rays from last week of March increases the temperature in northern hemisphere. the ITCZ keeps on moving northwards. Its maximum departure is seen upto 25 degree north latitude in the large chunk of land of Asia. the ITCZ shifting is northwards, its shift is minimum on the large water bodies of Pacific and Atlantic.The effectiveness of the sun’s departure is observed till July as it is the hottest month for northern hemisphere, while the southern hemisphere witness the coldest conditions in July.

### Seasonal Variations during January

End of third week of September the sun’s rays become vertical towards southern hemisphere. The effectiveness of solar radiation increases in southern hemisphere with the passage of time, but simultaneously it keeps on decreasing in the northern hemisphere. It all happens due to inclined rays of the sun in the northern hemisphere.Increasing temperature causes the high atmospheric pressure region turnsinto a low air pressure zone. The continuous widespread high pressure belt is confined only to the southern parts of Pacific, Atlantic and Indian oceans. The subpolar low pressure belt is also reduces to a narrower belt in comparison to July. So the case of northern hemisphere is reversed in January in comparison to July. Everything is due to seasonal changes due to earth’s revolution round the sun. A huge area is under the influence of high atmospheric pressure.

# Wind
## Introduction

Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the Sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in the Solar System occur on Neptune and Saturn. Winds have various aspects: velocity (wind speed); the density of the gas involved; energy content or wind energy. Wind is also an important means of transportation for seeds and small birds; with time things can travel thousands of miles in the wind. Winds can shape landforms, via a variety of aeolian processes such as the formation of fertile soils, such as loess, and by erosion. Dust from large deserts can be moved great distances from its source region by the prevailing winds; winds that are accelerated by rough topography and associated with dust outbreaks have been assigned regional names in various parts of the world because of their significant effects on those regions. Wind also affects the spread of wildfires.

## Factor Affecting Wind Motion

* Horizontal Pressure (Gradient)
* Rotation of the Earth (The coriolis Force)
* Frictional Forces
* Centrifugal action of wind

### Horizontal Pressure (Gradient)

Pressure gradient is the change in the air pressure per unit of distance travelled along a certain line. In another words, it is an average change in barometric pressure per unit of distance along a certain linear direction in a given region. The pressure gradient determines the direction as well as the intensity of the wind blow. The winds always blow from the high pressure areas to the low pressure areas.Pressure gradient can be calculated by the change in the isobaric values (barometric pressure generally in millibars per km (distance)).

### Rotation of the earth - The Coriolis Force

Coriolis, practically speaking is not a force but it an effect which is observed on a mass of body in a rotating system. It results from the rotational movement of the earth and the movement of air in relation to the earth. It acts perpendicular to the axis of the earth.It is determined by the mass of the body and its rate of rotation. The earth rotates from west to east on its axis. Hence, the Coriolis force operates in north-south direction. The Coriolis force is zero at the equator and maximum at the poles. This concept was first explained by French engineer G. G. Coriolis in 1835, thus it is known by his name . One rotation takes about 24 hours. The velocity of the rotation of the earth is 1670 km/hour (the circumference is about 40000 km) along the equator which comesdown to half (835 km/hour) along 600 north and south latitudes.We know that the earth is not a non-rotating body but, it rotateson its axis. When the air moves equatorward on the spinning earth in the northern hemisphere, the air is deflected to the right as the surface itself moves forward. The same thing also happens when the air is moving from equator to pole, i.e., rightward turning in the northern hemisphere.

### Frictional Force

Air friction is the resistance to motion of air in relation to the surface roughness and irregularity through which the wind is blowing. Frictional force reduces the velocity of the wind near the ground. Therefore, wind is most affected by the friction andit is maximum near the ground. Above the ground, the friction is reduced very drastically.The surface irregularity and undulation is impacting the blowing winds. Within a height of about one km from the ground, the wind is creating eddies and straight blow is heavily affected. After that the velocity is increasing. Right moving arrows shows the direction as well as the velocity of the winds.

### Centripetal Force

Centripetal force operates at right angles to the blowing wind. It is the inward pulling force i.e. towards the centers of rotation. Vertically above about a height of around 5.5 km, the air pressure is approximately 500 mb. By this height, the friction force is almost terminated except over the highlands and mountains. Coriolis force is equal to centripetal force plus pressure gradient force. But in case of high pressure where anticyclone is developed in upper troposphere, pressure gradient force is equal to Coriolis force plus centripetal force. In these conditions, the winds are blowing parallel to the isobars. It is called geostrophic winds by which the jet stream is developed.

## Classification of Wind

They are classified in two broad categories:-
* Permanent Winds or Invariable Winds or Planetary Winds
* Variable Winds

### Permanent Wind

Direction of winds remains more or less same throughout the year though their area change seasonally, such winds are called permanent winds these are related to thermally and dynamically induced pressure belts and rotation of the earth ,hence they are called planetary winds. these winds include trade winds,Westerlies and polar winds.

#### Winds in the Tropical Region

Areas extending between 30°N and 30°S latitude are include in tropical zone.trade winds blow from the subtropical high pressure belt to the Equatorial low pressure belts.there is a belt of calm or doldrum characterized by feeble air circulation.

##### Equatorial Westerlies

The westerlies, anti-trades,prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30° and 60° latitude. They originate from the high-pressure areas in the horse latitudes and trend towards the poles and steer extratropical cyclones in this general manner.
The westerlies are strong, especially in the Southern Hemisphere, in areas where land is absent, because land amplifies the flow pattern, making the current more north-south oriented, slowing the westerlies. The strongest westerly winds in the middle latitudes can come in the roaring forties, between 40° and 50° latitude. The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse.

##### Doldrums

The doldrums, usually located between 5° north to 5° south of the equator, are also known as the Intertropical Convergence Zone or (ITCZ ). The trade winds converge in the region of the ITCZ, producing convectional storms that produce some of the world's heaviest precipitation regions.The ITCZ moves north and south of the equator depending on the season and solar energy received. The location of the ITCZ can vary as much as 40° to 45° of latitude north or south of the equator based on the pattern of land and ocean. The Intertropical Convergence Zone is also known as the Equatorial Convergence Zone or Intertropical Front.

##### Trade winds

Trade winds blow in a belt lying between5°N-30°N in the northern hemisphere and 5°S-30°S in the southern hemisphere.As we all know that air travels through high pressure to low pressure. There is low pressure on the equator , while high pressure at the sub tropics. So, air moves from tropics toward equator. Due to corolis force and rotation of earth winds move toward left hand side in southern hemisphere and right hand side in northern hemisphere.

##### Polar Winds

Polar winds begin near the North and South Poles. Frigid air in the winter sinks toward the ground creating a high pressure area at the poles. These winds occur in both hemispheres.The polar easterlies Winds are also called Polar Hadley cells. they are dry, cold prevailing winds ,blow from the high-pressure areas of the polar highs at the North and South Poles towards low-pressure areas within the Westerlies at high latitudes.

### Variable Wind

It is type of local winds.Local Winds are produced due to local variability in temperature and pressure condition. Thus, they are more localised in their extent and cover limited horizontal and vertical dimensions and confined to the lower levels of the troposphere.

#### Surface winds

Surface friction plays an important role in the speed and direction of surface winds.it moves over the ground, wind blows across the isobars into a center of low pressure and out of a center of high pressure.the effects of surface friction, the winds, locally, do not always show the speed and direction that would be expected from the isobars on the surface weather map. These variations are usually due to geographical features such as hills, mountains and large bodies of water.Except in mountainous regions, the effect of terrain features that cause local variations in wind extends usually no higher than about 2000 feet above the ground.

#### Land and sea breezes

It is caused by the differences in temperature over land and water. The sea breeze occurs during the day when the land area heats more rapidly than the water surface. This results in the pressure over the land being lower than over the water. The pressure gradient is often strong enough for a wind to blow from the water to the land.The land breeze blows at night when the land becomes cooler.Then the wind blows towards the warm, low-pressure area over the water.Land and sea breezes are very local and affect only a narrow area along the coast.

#### Mountain Winds

Hills and valleys substantially distort the airflow associated with the prevailing pressure system and the pressure gradient. Strong up and down drafts and eddies develop as the air flows up over hills and down into valleys.lines of hills and mountain ranges will act as a barrier. If there is a pass in the mountain range, the wind will rush through this pass as through a tunnel with considerable speed. Daytime heating and nighttime cooling of the hilly slopes lead to day to night variations in the airflow. At night, the sides of the hills cool by radiation. The air in contact with them becomes cooler and therefore denser and it blows down the slope into the valley. This is a mountain winds, also called katabatic wind or mountain breeze. If the slopes are covered with ice and snow, the katabatic wind will blow, not only at night, but also during the day, carrying the cold dense air into the warmer valleys. The slopes of hills not covered by snow will be warmed during the day. The air in contact with them becomes warmer and less dense . This is an anabatic wind or valley breeze. In mountainous areas, local distortion of the airflow is even more severe. Rocky surfaces, high ridges, sheer cliffs, steep valleys, all combine to produce unpredictable flow patterns and turbulence.

#### Hot Local Winds

Hot Local winds are produced generally by the mechanism of downslope compressional heating also known as adiabatic heating. The examples of the Hot Local Winds include Chinook, Harmattan, Foehn, Sirocco, Norwester,, Khamsi, Loo etc.

##### Chinook

These are warm and dry winds blowing on the eastern slopes (leeward side) of the Rocky Mountain. They are the result of adiabatic heating which occurs due to downslope compression on the leeward side, as the mountain barrier creates frictional drag which tends to pull the air from the higher level down on the leeward and air forced down is heated adiabatically and at the same time its relative humidity is also lowered.The temperature in Chinook is so warm that it can remove the underlying snow cover/ice and sometimes these winds are so dry.

##### Foehn

Foehn is dry and warm wind resulting due to adiabatic heating on the leeward side of the Mountain range.These winds are more common on the northern side of Alps in Switzerland and with the arrival of these winds, there is a rapid rise in temperature. The Foehn winds are present throughout the winter and due to the presence of such winds the temperature increases, and valleys of Switzerland are called Climatic Oasis during the winter season.

##### Harmattan

These hot and dry wind originate from the Sahara Desert and blow towards the Guinea coast of Africa.Due to their journey over the Sahara Desert, these winds become extremely dry and as they pass over the Sahara Desert they pick up more sand especially red sand and turn dusty.these winds arrive in the western coast of Africa, the weather which is warm and moist before its arrival, turns into pleasant dry weather with low relative humidity, thus bringing great relief to the people. Due to this reason, they are also known as doctor winds in the Guinea coast area of Western Africa.

##### Sirocco

Sirocco is the warm, dry, dusty local wind which blows from the Sahara Desert over the central Mediterranean, southern Italy and Spain etc.the Sirocco descend through the northern slope of the Atlas Mountain they become extremely warm and dry. These winds carry red sand particle from Sahara due to which they cause red colour rainfall in southern Europe and this rainfall is called as Blood Rain.Sirocco winds are known as Levech in Spain and Leste in Morocco and Madeira, Gibli in Lybia, Chilli in Tunisia.

##### Loo

It originates from the Thar desert and has north-westerly to a westerly direction. They dominate during early summer in the months of March to May and create heat waves like condition in Northern India and adjoining parts.They have desiccating effects and are considered as environmental hazards.

#### Cold Local Winds

Cold local winds are dust-laden winds and they have a temperature below freezing point, they create Cold Wave condition.examples of Cold Local winds are - Mistral, Bora, Blizzard, Purga, Laventer, Pampero, Bise etc.

##### Mistral

It is a cold and dry wind which blows in the Spain and France from North-west to South-East direction, mostly occur during winter months.Due to the presence of the Rhome River, these winds are channelized into the Rhome valley due to which they become extremely cold. they pass through the narrow Rhome Valley, they turn into stormy northerly cold winds and their average velocity which is 55-65 km/hr to 128km/hr.

##### Boro

These are cold and dry north-easterly winds which blow from the mountains towards the eastern shore of Adriatic Sea.Bora is more effective in North Italy ,it descends the southern slopes of the Alps, although due to descend it gets adiabatically heated still its temperature is very low in comparison to the coastal area and these are the typical example of fall winds.These winds blow with strong gusts with velocities in the range of 128-200 km/hr.

##### Blizzard

Blizzard is cold, violent, powdery polar winds (pick dry snow from the ground). They are prevalent in the north and south polar regions, Canada, USA, Siberia etc. Due to the absence of any east-west Mountain barrier, these winds reach to the southern states of USA.

##### Buran

It is an extremely cold north-easterly or easterly wind which blows in the central Siberia and eastern Russia.

### Jet Stream

Jet Stream are defined as swift geostrophic air streams in the upper troposphere that meander in relatively narrow belts.It exist in the higher levels of the atmosphere at altitudes ranging from 20,000 to 40,000 feet or more. A jet stream in the mid latitudes is generally the strongest.Jet Streams extend from 20 degrees latitude to the poles in both hemispheres.jet Streams develop where air masses of differing temperatures meet.So, usually surface temperatures determine where the Jet Stream will form.Greater the difference in temperature, faster is the wind velocity inside the jet stream.

#### Types of Jet Streams

There are three types of jet stream:-

##### Sub Tropical Jet Streams

It is developed in winter and early spring. Their maximum speed approaches 300 knots which are associated with the merger with polar-front jets. A subsidence motion accompanies subtropical jets and gives rise to predominantly fair weather in areas they pass over. Sometimes they drift northward and merge with a polar-front jet.

##### Tropical Easterly Jet Stream

It occurs near the tropopause over Southeast Asia, India, and Africa during summer. This jet implies a deep layer of warm air to the north of the jet and colder air to the south over the Indian Ocean.

##### Polar-Night Jet Stream

It meanders through the upper stratosphere over the poles. They are present in the convergence zone above the sub polar low pressure belt.

#### Characteristics of Jet Streams

* Its genesis is associated with the thermal contrast of air cells, for example Hadley cell, Ferrel cell.
* The meandering or the whirl movement of the Jet Stream is called Rossby Wave.
* Equatorial extension of the Jet Stream is more in winter because of the southern shift of the pressure belts.
* During winters, the thermal contrast increases and the intensity of the high pressure centre at the pole increases. It intensifies the formation of Jet Streams, its extension as well as its velocity.

# The Monsoon

The word ‘monsoon’ has been derived from the Arabic word Mausim which means season. The climate of India is described as the monsoon type. The monsoon word was used by Arab traders to describe a system of seasonal reversal of winds along the shores of the Indian Ocean. Monsoons are especially prominent within the tropics on the eastern sides of the great landmass, but in Asia, it occurs outside the tropics in China, Korea and Japan. In Asia, this type of climate is found mainly in the south and the southeast.

## Origin of Monsoon

Monsoon is a complex meteorological phenomenon. Experts of meteorology have developed a number of concepts about the origin of the monsoon:

### Thermal Concept

Halley proposed the thermal concept. According to this hypothesis, monsoons are the extended land breeze and sea breeze on a large scale. During winter the huge landmass of Asia cools more rapidly than the surrounding oceans with the result that a strong high pressure centre develops over the continent. The pressure over adjacent oceans is relatively lower. As a consequence the pressure-gradient is directed from land to sea. In summer the temperature and pressure conditions are reversed. The huge landmass of Asia heats quickly and develops a strong low pressure centre.
Criticism of Halley's hipothesis includes:
* It fails to explain the intricacies of monsoon such as sudden burst of monsoon, breaks in monsoon, spatial and temporal distribution of monsoon.
* The low pressure areas are not stationary.
* The rainfall is not only convectional but a mix of orographic, cyclonic and convectional rainfall.

### Regional aspects of monsoons

The regional aspects of monsoons are as follows:
* Summer monsoon or SW monsoon in India and Southeast Asia.
* The Australian NW monsoon.
* The West African monsoon.
* The winter monsoon over Malaysia and Indonesia.

### Recent Concept of the Origin of Indian Monsoon

After the Second World War, the upper atmospheric circulation has been studied significantly. It is believed that the differential heating of sea and land alone can’t produce the monsoon circulation. Recent concepts of monsoons heavily on the role of:
* Himalayas and Tibetan plateau as a physical barrier and a source of high-level heat.
* Circulation of upper air jet streams in the troposphere.
* Existence of an upper air circumpolar whirl over north and south poles in the troposphere.
* The occurrence of ENSO (El-Nino and Southern Oscillation) in the South Pacific ocean.
* Walker cell in Indian Ocean.
* Indian Ocean Dipole.

### The Onset of the Monsoon

Out of a total of 4 seasonal divisions of India, monsoon occupy 2 divisions:
* The southwest monsoon season: Rainfall received from the southwest monsoons is seasonal in character, which occurs between June and September.
* The retreating monsoon season: The months of October and November are known for retreating monsoons.

#### Onset of the South-West Monsoon

The Monsoon, unlike the trades, are not steady winds but are pulsating in nature, affected by different atmospheric conditions encountered by over the warm tropical seas. The duration of the monsoon is between 100 and 120 days from early June to mid-September. The location of ITCZ shifts north and south of the equator with the apparent movement of the Sun. During the month of June, the sun shines vertically over the Tropic of Cancer and the ITCZ shifts northwards. The southeast trade winds of the southern hemisphere cross the equator and start blowing in southwest to northeast direction under the influence of Coriolis force. These winds collect moisture as they travel over the warm Indian Ocean. In the month of July, the ITCZ shifts to 20°-25° N latitude. It located in the Indo-Gangetic Plain and the south-west monsoons blow from the Arabian Sea and the Bay of Bengal. These winds approach the land, and their southwesterly direction is modified by the relief and thermal low pressure over northwest India.
The monsoon approaches the Indian landmass in two branches:
* The Arabian Sea branch - The monsoon winds originating over the Arabian Sea.
* The Bay of Bengal branch - The Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent. The monsoon, therefore, enters West Bengal and Bangladesh from south and southeast instead of from the south-westerly direction.

#### Retreating Monsoon Season

The retreating southwest monsoon season is marked by clear skies and rising temperatures.The land is still moist.the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the October heat. In the second half of October, the mercury begins to fall rapidly, particularly in northern India. The weather in the retreating monsoon is dry in north India but it is associated with rain in the eastern part of the Peninsula. Here, October and November are the rainiest months of the year. The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea. These tropical cyclones are very destructive. A bulk of the rainfall of the Coromandel Coast is derived from these depressions and cyclones.

# Humidity
## Humidity

Water vapour present in the air is known as Humidity.At any specific temperature amount of water vapour that can be held by air has a definite limit known as saturation point.Air at saturation point is known as saturated air.Temperature at which saturation occurs is known as Dew point. Capacity of air to absorb water vapour increases with increase in temperature. There are three type of humidity:-
* Absolute Humidity
* Relative Humidity
* Specific Humidity

### Absolute Humidity

The actual amount of the water vapour present in the atmosphere is known as the absolute humidity.It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre.The absolute humidity differs from place to place on the surface of the earth.

### Relative Humidity

The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity.
Relative Humidity = [Actual amount of water vapor in air (absolute humidity)/humidity at saturation point (the maximum water vapor air can hold at a given temperature)] X 100
The change of air temperature, the capacity to retain moisture increases or decreases, the relative humidity is affected. Relative humidity is greater over the oceans and least over the continents (absolute humidity is greater over oceans because of greater availability of water for evaporation).The relative humidity determines the amount and rate of evaporation.Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’. The temperature, air cannot hold any additional amount of moisture.relative humidity of the saturated air is 100%.If the air has half the amount of moisture that it can carry,it is unsaturated and its relative humidity is only 50%.

### Specific Humidity

It is expressed as the weight of water vapour per unit weight of air.it is measured in units of weight (usually grams per kilogram), the specific humidity is not affected by changes in pressure or temperature.Absolute Humidity and Relative Humidity are Variable whereas Specific Humidity is a constant.

## Condensation

Transformation of water vapour into water, caused by loss of heat when moist air is cooled.Cooling may reach a level when air’s capacity to hold water vapour , then excess of water vapour condenses into liquid form.If water vapour directly condensed into solid form, it is known as sublimation.In free air, condensation results from cooling around very small particles termed as condensation Particle of dust, smoke & salt from oceans are particularly good nuclei as they absorb water (Hygroscopic nuclei).

### Form of Condensation
#### Dew

The temperature at which saturation occurs in a given sample of air is known as dew point.Dew forms when moisture is deposited in form of water droplets on cooler surfaces of solid objects such as stone, glass, blades, plant leaves etc.It forms when temperature of air falls below dew point but above freezing point.Dew point occurs when Relative Humidity = 100%.

#### Frost

Frost forms on solid surfaces when condensation takes place below freezing point i.e. 0 °C means dew point is below freezing point.

## Evaporation

Evaporation is the process by which matter changes from liquid to a gaseous or vapour state. The atmospheric moisture or humidity is nothing but water vapour which has escaped from oceans, rivers, lakes, ponds, and humans into the atmosphere.Heat energy is required for evaporation to take place.In case of atmospheric moisture, the energy is provided by solar radiation. The water molecules, supplied with this energy, get the required motion to escape and conserve this energy as latent heat of vaporisation. when the vapours get condensed into water drops, this energy is released in the form of latent heat of condensation.evaporation is associated with a cooling effect.

### Factors Affecting Rate of Evaporation

Several factors affecting the rate of evaporationare following:-
* Amount of water available:-Rate of evaporation is greater over the oceans than over the continents.
* Temperature:-high temperature implies greater availability of energy for evaporation.the rate of evaporation is directly proportional to temperature of the evaporating surface.
* Relative humidity:- the moisture-holding capacity of air at a given temperature is limited, dry air,evaporates more water than moist air.evaporation is greater in summer.
* Area of evaporating surface:-larger surface area exposed to heat implies enhanced evaporation.
* Air Pressure:-It exerted on the evaporating surface.Lower pressure over open surface of the liquid results in a higher rate of evaporation.
* Composition of water:-Evaporation is inversely proportional to salinity of water. Rate of evaporation is always greater over fresh water compare to salt water.

# Precipitation

Precipitation is the falling of water from the sky in different forms. They all form from the clouds which are located about 8 to 16 kilometers above the ground in the earth’s troposphere. The process of precipitation represents falling down of this condensed material in the form of rain, snow, hail, sleet or some other forms.

## Forms of Precipitation

Precipitation occurs in many forms or phases such as :-
* Liquid precipitation (Drizzle and rain),
* Freezing precipitation (freezing drizzle, freezing rain and sleet) and
* Frozen precipitation (snow, ice pellets and hail etc.).

### Rain

Rain is any liquid that drops from the clouds in the sky, in the form of liquid.There size described as water droplets of 0.5 mm or more than 0.5mm.Rainfall rates vary from time to time.tropical areas, cumulus clouds rain is the result ofcollision and coalescence process. In temperate areas, most rainfall is produced by nimbostratus clouds. The vertical clouds, cumulonimbus produce heavy rainfall with lightning and thunder.

### Drizzle

Drizzle represents light liquid precipitation in the form of uniform water droplets of diameter less than 0.5 mm.It associated with stratus, nimbostratus and stratocumulus clouds.Precipitation in the form of drizzle continues for several hours or occasionally for days.It is the most frequent form of precipitation over subtropical Oceans.

### Snow

Snow represents precipitation in the solid form of water.It is precipitation in the form of virga or flakes of ice water falling from the clouds.Snow is normally seen together with high, thin and weak cirrus clouds.Snow has fluffy, white and soft structure and its formation is in different shapes.

### Sleet (Ice Pellets)

when snow falls into a warm layer then melts into rain and then the rain droplets falls into a freezing layer of air that is cold enough to refreeze the raindrops into ice pellets.sleet is defined as a form of precipitation composed of small and semitransparent balls of ice.

### Freezing Rain

Freezing rain occurs when raindrops pass through the subfreezing air near the surface, the raindrops become supercooled and freeze when they strike on surface features.

### Snow Pellets

It consist of small, white and opaque compact grains of ice. The grains are mostly spherical and have a diameter of 2–5 mm. Snow pellets are also known as soft hail. the grains are brittle and bounce and break after striking surface.

### Hail

Hailstones are big balls and irregular lumps of ice that fall from large thunderstorms.mostly diameter between 1 cm and 5 cm. Its size depends on the content of ice and snow it acquires while falling down. Hailstones are common in violent summer thunderstorms.hailstones are formed from super-cooled droplets that slowly freeze and results in sheet of clear ice.

# Fog and Clouds
## Fog

Fog is basically a cloud with its base at or very near to ground.It is formed when warm & cold currents meet.Fogs are of different kinds depending upon the nature of the cooling process.Fog is not the same thing as mist. Fog is denser than mist.It means fog is more massive and thicker than mist. Fog happens when it is very humid. In order for fog to form, dust, water vapour or some kind of air pollution needs to be in the air.

### Types of Fog

There are several different types of fog, including radiation fog, advection fog, valley fog, and freezing fog, etc.

#### Radiation Fog

It forms in the evening when heat absorbed by the Earth’s surface during the day is radiated into the air.heat is transferred from the ground to the air, water droplets form. its also called ground fog. Ground fog does not reach as high as any of the clouds overhead. It usually forms at night. Fog that is said to “burn off” in the morning sun is radiation fog.

#### Advection Fog

It forms when warm, moist air passes over a cool surface is called advection.When the moist, warm air makes contact with the cooler surface air, water vapor condenses to create fog. Advection fog shows up mostly in places where warm, tropical air meets cooler ocean water. The Pacific coast of the United States, from Washington to California, is often covered in advection fog. The cold California Current, which runs along the western coast of North America, is much cooler than the warm air along the coast.

#### Valley Fog

It forms in mountain valleys, usually during winter. Valley fog develops when mountains prevent the dense air from escaping. The fog is trapped in the bowl of the valley. In 1930, vapor condensed around particles of air pollution in the Meuse Valley, Belgium. More than 60 people died as a result of this deadly valley fog.

#### Freezing Fog

It happens when the liquid fog droplets freeze to solid surfaces. Mountaint that are covered by clouds are often covered in freezing fog. As the freezing fog lifts, the ground, the trees, and even objects like spider webs, are blanketed by a layer of frost. The white landscapes of freezing fog are common in places with cold, moist climates.

## Clouds

Clouds as aggregates of water droplets and ice crystals are a form of condensation and a precondition for precipitation. It represent a visible aggregate of droplets of water, or tiny ice crystals or a mixture of both. Clouds are usually product of condensation or sublimation caused by lifting process.
It play a important role in heat budget and cause cloud are devided into three layers.
* High Clouds
* Middle Clouds
* Low clouds

### High Clouds

It usually found in the altitude of 6 to 12 km from the earth surface.High family clouds composed of mainly ice crystals because at greater heights amount of moisture is less and dew point is achieved at low temperatures in supersaturated situation. high clouds are generally associated with fair weather conditions.these are three types:-

#### Cirrus

These are detached clouds composed of white, delicate, ice filaments.It has a fibrous appearance like 'mares tails' or a silky sheen or both.

#### Cirrostratus

It represent a thin, milky or whitish sheet of fibrous appearance covering the sky totally or partially. It composed of tiny ice crystals.They indicate about approaching storm.

#### Cirrocumulus

These clouds are patches of small white flakes or small globules which are arranged in ripples or wavelike form.Wavelike regular pattern forms a mackerel sky.These clouds are least common in high clouds.

### Middle Clouds

Middle clouds are in the altitude range of 2 to 6 km and have prefix alto in their names.They are composed of water droplets, ice crystals or both things. Two types are middle clouds:-

#### Altocumulus

It consist of layer or patches of white or grey clouds It generally arranged in fairly regular pattern of lines, groups or waves.It composed of supercooled water droplets.Altocumulus clouds differ from cirrocumulus as, cirrocumulus clouds are smaller and less dense.globular group within altocumulus clouds is known as sheep clouds or wool pack clouds.

#### Altostratus

Altostratus clouds appear as fibrous sheet of gray or blue-grey, covering the sky totally or partially.They are thicker than the higher cirrostratus.clouds are associated with infrequent precipitation either in the form of light snow or drizzl.

### Low Clouds

Cumulus and cumulonimbus clouds have their base in the range of low clouds, but they extend upward into the middle or higher altitude.

#### Stratus

It generally represent grey coloured uniform layer covering much of the sky, composed of many uniform layers. These are dense and low-lying fog-like clouds.

#### Stratocumulus

These are large globular masses or rolls of grey or whitish or both.It arranged in lines, groups or waves.It usually associated with fair weather but occasionally rain and snow may occur.

#### Nimbostratus

The Latin word nimbus means ‘rain cloud’ and stratus stands for layered.These are rainy clouds. clouds bring light to moderate rainfall for long durations over widespread areas.The rain, snow and sleet are associated with these clouds but never accompanied by thunder, lightning or hail.

#### Cumulus

these are dense, widespread, dome-shaped (resembling a cauliflower) and have flat bases.

#### Cumulonimbus

clouds show great vertical extent, from a few hundred meters above the ground upward to 14 to 18 km. These clouds are common in equatorial low pressure belts and in tropical cyclones.

# Cyclones

cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. around a low pressure center. There are two types of cyclones:
* Tropical Cyclone
* Temperate cyclone or Midlatitude or Extratropical Cyclones

## Tropical Cyclone

A rotating system of clouds and thunderstorms that originated over tropical or sub-tropical areas.The major tropical-cyclone basins include the North Atlantic (including the Caribbean),Eastern Pacific, Western Pacific, North Indian Ocean, Southwest Indian Ocean, Southern Pacific and Australian region.It develop within 5 and 30 degrees of latitude.In the North Atlantic and Caribbean as well as the northeastern Pacific, they called hurricane. In the Northwest Pacific – the most active tropical-cyclone basin in the world is typhoons.but in the Indian Ocean and South Pacific they are called“tropical cyclones” or “cyclones.”

## Temperate cyclone

The cyclonic storms that develop along frontal boundaries in the middle latitudes called “extratropical cyclones”or “midlatitude cyclones”. The polar front when the temperature difference between two air masses is large. These air masses blow past each other in opposite directions.Coriolis Effect deflects winds to the right in the Northern Hemisphere, causing the winds to strike the polar front at an angle.The temperate cyclones occur mostly in winter, late autumn and spring. They are generally associated with rainstorms and cloudy weather.

### Characteristics of Temperate Cyclones
#### Size and Shape

* The temperate cyclones are asymmetrical and shaped like an inverted ‘V’.
* They stretch over 500 to 600 km.
* They may spread over 2500 km over North America (Polar Vortex).
* They have a height of 8 to 11 km.

#### Wind Velocity And Strength

* The wind strength is more in eastern and southern portions, more over North America compared to Europe.
* The wind velocity increases with the approach but decreases after the cyclone has passed.

#### Structure

* The north-western sector is the cold sector and the north-eastern sector is the warm sector.

#### Orientation And Movement

* Jet stream plays a major role in temperate cyclonogeneis.It also influence the path of temperate cyclones.

# Further Reading

* Hartmann, D. Global Physical Climatology
* Peixoto & Oort, Physics of Climate
* Richard Heinberg. "The Party's Over: Oil, War and the Fate of Industrial Societies". ISBN 0865714827. Interesting philosophical outlook on history of technological civilization and it's possible fate.
* Human Footprint video by National Geographic Channel
* Elizabeth Royte. "Bottlemania: How Water Went on Sale And Why We Bought It". ISBN 1-59691-371-1. History and problems of bottled water with comprehensive review of issues with tap water and additional filters.
* "Food, Inc. How Industrial Food Is Making Us Sicker, Fatter, and Poorer — And What You Can Do About It". ISBN 978-1-58648-694-5. Great insight into modern industrial food production system.

# External Links

* [http://www.exploratorium.edu/climate/ Introduction to climate change... good for younger audience too.
* Global Warming
* Local effects of climate change
* RealClimate
* Climate Science Special Report – U.S. Global Change Research Program
* The Earth's climate – Centre national de la recherche scientifique (CNRS – France)
* Climatology News Daily publication with news in all areas of climatology plus free news feeds for webmasters.
* Climate Prediction Center
* KNMI Climate Explorer The Royal Netherlands Meteorological Institute's Climate Explorer graphs climatological relationships of spatial and temporal data.
* Climatology as a Profession Amer. Inst. of Physics account of the history of the discipline of climatology in the 20th century
* Sunpreview annual and global weather project

# FAQs
### What is the greenhouse effect?

Greenhouse gases contribute to global warming, which is caused by the greenhouse effect. The greenhouse effect can be demonstrated in this way: suppose you are driving in a car on a hot summer day. The sun's rays shine down into your car through the windows. However, the heat cannot get out of the car because the windows reflect the heat trying to get out back towards the interior of the car. This causes the car to continue to heat up until the thing blocking the heat from getting out is removed or a cooling element of some type is activated in the car.
In the subject of global warming, the greenhouse gases are the "windows" and the Earth is the "car." The sun's rays are somewhat deflected or absorbed through their travel through space and the atmosphere, but most of the energy still reaches the Earth's surface. This energy is reflected back toward space by the Earth, but because of the greenhouse gases it cannot go back into space. Therefore, the entire Earth warms up.
The global temperature has only risen a few degrees since the start of the Industrial Revolution. However, one must remember that we are talking about a global temperature increase, not just in one relatively small location. One would expect the temperature in one location to be varied, but the forces required to raise the average temperature on the entire planet would have to be immensely strong.

### Why are greenhouse gases detrimental to the environment?

Granted, greenhouse gases (GHG) occur naturally in the environment, but since the Industrial Revolution, humans have intensified this process by emitting more GHG into the atmosphere than our environment can handle. The main contenders are the automobile makers, industries such as coal (fossil fuel) burning power plants, and even landfills which release large amounts of hazardous methane (from decomposition). Some parties assert that this actually helps plant life to grow, on account of the fact that plants absorb carbon dioxide as a part of their photosynthesis. While this may be true, when the plant decays, the stored carbon is released back into the air. Scientists are afraid that if the dumping of greenhouse gases continues at the present rate, plant life may not be able to keep up.

## Further reading

To read the Articles listed under the Kyoto Protocol, please visit the United Nations Framework Convention on Climate Change: