Weather Forecast: Kitchener, Ontario - The Weather Network Relative Humidity Relative humidity is a percentage of water vapour per area at a specific temperature. Relative humidity is calculated at all times of the year, but the humidex is usually only given in the warmer months from May to September. Relative humidity can be calculated using various methods and instruments. It is a calculation to find out how many grams of water vapour can be held at a given temperature. Usually, the warmer the air, the higher its capacity for holding water vapour. Each specific temperature has a holding limit for water, and the actual amount of water held in the air at the time of measurement can be represented as a percentage. For example, if a parcel of air at 20ï¿½C has a limit or capacity of 16 grams of water vapour and 16 grams of water vapour is measured, the air's holding capacity is at its maximum and the relative humidity (RH) of the air is said to be 100%. Take the same air with 8 grams of water vapour and your RH is 50%. Dew Point The dewpoint is a measure of atmospheric moisture.

It is the temperature at which air must be cooled in order to reach saturation (assuming that air pressure and moisture content are constant). As the surface of the earth cools at night, warm moist air near the ground is chilled and water vapour in the air condenses into droplets on the grass and other objects. Dew is particularly heavy on clear nights, when the earth cools rapidly. When a blanket of cloud insulates the earth, the cooling rate is slower. The greater the difference between the temperature and the dew point, the drier the air. The amount of water vapour in the air can be quantified in three different ways: Relative humidity Relative Humidity (RH) is the most common measure of humidity. It measures how close the air is to being saturated - that is how much water vapour there is in the air compared to how much there could be at that temperature. Warmer air can hold more water vapour because there is more energy available. If the RH of the air is 100% then it is fully saturated. In warm temperatures, air with very high RH is very uncomfortable as the saturated air affects our body's cooling mechanism.

The air cannot easily contain anymore water as a vapour and so cannot effectively evaporate the sweat from our skin. In cool temperatures, air with very high RH can make us feel cooler. This is because there is more water vapour close to our skin and since water is a much better conductor than dry air, the cold temperature of the air is conducted to our skin, making us feel cooler. Specific humidity Specific humidity and the mixing ratio measure the actual amount of water vapour in the air as a weight in grams. They are very similar but the specific humidity is the weight of water vapour for every kilogram of air (including water vapour) whereas the mixing ratio is the weight of water vapour for every kilogram of dry air (not including water vapour). Both the specific humidity and the mixing ratio are highest around the equator at around 20 g kg-1, where the air is warm and can hold more water vapour, and lowest (near zero) in the cold polar regions and high in the atmosphere. Thermal humidity 'Dew point temperature' and 'wet bulb temperature' are also measures of humidity. These are both measures of how close the air is to being saturated.

If they are equal to the actual air temperature then the air is saturated and RH is 100 %. The wet bulb temperature is the traditional way of measuring humidity. It is measured by allowing the air to cool a thermometer exposed to water by evaporation. The dew point temperature is measured by cooling a surface to the point at which the air condenses out some water vapour - this is the temperature at which the air has become saturated and is akin to the dew seen on grass in early mornings when the temperature has dropped over night. Last updated: Aug 10, 2016 1:20 PM USATODAY.com Find a forecast: Understanding humidity Humidity is the amount of water vapor in the air and can be described in different ways. The term that you'll hear most often to describe the humidity is relative humidity. This also happens to be, in my opinion, the measurement of humidity that's hardest to understand. Another common measurement of humidity is the dew point, which most of us also also have to work, at least a little, to understand.

Unless you are a weather forecaster, or a scientist or engineer involved in work involving the amount of water vapor in the air, you can easily go through life without understanding relative humidity. On the other hand, it is nice to know what a TV meteorologist means by dew point. These and other terms used to in relation to humidity are defined at the bottom of this page. If after glancing at them, you still don't quite understand what's going on, try going to the USATODAY.com Getting a handle on humidity page. Why humidity is worth worrying about The humidity combined with hot temperatures makes the heat more dangerous. (Related: The dangers of heat and humidity ) Indoor, as well as outdoor humidity is important, and humidity that's too high or too low can be dangerous for both you and your house. (Related story: Humidity inside your home ) Humidity is a key player in the weather The U.S. Geological Survey estimates that the Earth has about 326 million cubic miles of water. This includes all of the water in the oceans, underground and locked up as ice.

Only about 3,100 cubic miles of this water is in the air, mostly as water vapor, but also as clouds or precipitation, at any one time, the Geological Survey estimates. While this is a small share of Earth's water, our planet would be very different without it. If Earth's air didn't contain as much humidity as it does, our weather would be like that of Mars: No clouds (except dust), no rain, sleet or snow, no thunder and lighting, no fog. And, without all of this water in all of its forms, Earth's life, if there were any at all, but be as hard to find as life on Mars is to find even signs of. (Related: Understanding water in the atmosphere ) Humidity can be less than 100% when it's raining Humidity is a measure of the amount of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. This normally happens when the air rises and cools.

Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. Some water from the rain evaporates into the air it's falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%. Humidity is important in climate change Water vapor in the air, the humidity, plays an important part in global climate. Like carbon dioxide, water vapor is a greenhouse gas. Climate scientists have found that carbon dioxide human activities is adding to the air is causing the Earth's average climate to warm. (Related graphic: The greenhouse effect ) This, in turn, is almost surely affecting other aspects of global climate, and could have even bigger effects in the future. (Related: Understanding climate change science ) As the climate warms, the humidity can increase. Since water vapor is a greenhouse gas, it should increase warming as it increases. Measuring how much global humidity is increasing and figuring out how it will increase, and the effects of such an increase are major challenges for climate scientists.

(Related: NASA report on warming and humidity ) Answers to questions about humidity If you have questions about humidity, there's a pretty good chance you'll find the answers in our answers to previous questions, or in our FAQ on humidity. FAQ on humidity and water in the atmosphere Definitions of humidity-related terms Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter Actual vapor pressure: The partial pressure exerted by the water vapor present in a parcel. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric air. Vapor pressure is also measured in millibars. Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapor to liquid water. Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The dewpoint temperature assumes there is no change in air pressure or moisture content of the air. Dry bulb temperature: The actual air temperature. See wet bulb temperature below. Freezing: The phase change of liquid water into ice. Evaporation: The phase change of liquid water into water vapor.

Melting: The phase change of ice into liquid water. Mixing ratio: The mass of water vapor in a parcel divided by the mass of the dry air in the parcel (not including water vapor) Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent. Click here for text explaining this in more detail. Saturation of air: The condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air. Saturation vapor pressure: The maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature. Saturation vapor pressure varies with atmospheric pressure. When a given atmospheric pressure is steady, then the saturated vapor pressure is directly proportional to the temperature.

Specific humidity: The mass of water vapor in a parcel divided by the total mass of the air in the parcel (including water vapor) Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into ice or the chance of ice directly into water vapor. Chemists, and sometimes meteorologists, refer to the vapor to solid phase change as "deposition." Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity Measuring Weather | Temperature | Pressure | Humidity Physical Attributes of Weather Why measure the weather? In everyday language weather means such qualities as wet or fine, warm or cold. For most people, such descriptive terms are adequate. However, many industries today require more quantitative assessments of the weather, with the use of standardised terms measured by suitably designed instruments.

The science of the study of weather is called meteorology; the meteorologist measures temperature, rainfall, pressure, humidity, sunshine and cloudiness, and makes predictions and forecasts about what the weather will do in the future. Temperature The hotness or coldness of a substance is called its temperature and is measured with a thermometer. The ordinary thermometer consists of a hollow glass bulb attached to a narrow stem with a thread-like bore. The bulb is filled with liquid, usually mercury, but also alcohol when very low temperatures need to be measured, which expands when the temperature rises and contracts when the temperature falls. The amount of expansion and contraction is measured by a calibrated scale. Whilst thermometers are really measuring their own temperature, they are usually needed to measure the temperature of the surrounding air. To ensure that the temperature of the surrounding air is the same as the thermometer, it must be shaded from sunlight and be exposed to adequate ventilation. These conditions are provided by enclosing the thermometer within a wooden box with louvred sides, called a Stevenson screen. The Stevenson Screen Most temperature scales today are expressed in degrees Celsius (�C), although one will sometime see Fahrenheit (�F) in use.

The Celsius scale is fixed by two points, the freezing and boiling point of water, which at normal atmospheric pressure are 0�C and 100�C respectively. The scale is then divided into 100 units. 0�C is equivalent to 32�F and 100�C to 212�F. The Kelvin temperature scale is the absolute temperature scale. Absolute zero, the coldest temperature possible in the universe is 0K or -273�C. Because one Kelvin is equivalent to one degree Celsius, 0�C is the same as 273K. 15�C is the same as 288K. Special thermometers are used to indicate the maximum and minimum temperatures reached over a period, usually one day. For the amateur, a popular combined maximum and minimum thermometer is the U-shaped thermometer. Thermometers are also used to measure the temperature of the ground at night, which may fall several degrees below that of the air above. Figure 2.2. The maximum-minimum thermometer Pressure Like all fluids, the air exerts a pressure on everything within and around it, although we are not aware of it.

Pressure is a force, or weight, exerted on a surface per unit area, and is measured in Pascals (Pa). The pressure exerted by a kilogram mass on a surface equals 9.8 Pa. The pressure exerted by the whole atmosphere on the Earth's surface is approximately 100,000 Pa. Usually, atmospheric pressure is quoted in millibars (mb). 1mb is equal to 100 Pa, so standard atmospheric pressure is about 1000mb. In fact, actual values of atmospheric pressure vary from place to place and from hour to hour. At sea level, commonly observed values range between 970 mb and 1040 mb. Because pressure decreases with altitude, pressure observed at various stations must be adjusted to the same level, usually sea level. Atmospheric pressure is measured by a barometer. A mercury barometer measures the pressure by noting the length of mercury which is supported by the weight of the atmosphere. One centimetre of mercury is equal to 13.33 mb, so normal atmospheric pressure can support a column of mercury about 75 cm (or 30 inches) high.

An aneroid barometer is a more compact instrument for measuring pressure. It consists of a box of partially exhausted air which expands and contracts as the pressure falls and rises. The box is connected through a system of levers to a pointer which, in conjunction with a dial, indicates the pressure. Variations in atmospheric pressure lead to the development of winds that play a significant role in shaping our daily weather. The mercury barometer Humidity Some water in the form of invisible vapour is intermixed with the air throughout the atmosphere. It is the condensation of this vapour which gives rise to most weather phenomena: clouds, rain, snow, dew and fog. There is a limit to how much water vapour the air can hold and this limit varies with temperature. When the air contains the maximum amount of vapour possible for a particular temperature, the air is said to be saturated. Warm air can hold more vapour than cold air. In general the air is not saturated, containing only a fraction of the possible water vapour. The amount of vapour in the air can be measured in a number of ways.

The humidity of a packet of air is usually denoted by the mass of vapour contained within it, or the pressure that the water vapour exerts. The variation of absolute humidity (vapour content of saturated air) with temperature is shown in the figure below. Variation of absolute humidity with temperature Relative humidity is measured by comparing the actual mass of vapour in the air to the mass of vapour in saturated air at the same temperature. For example, air at 10�C contains 9.4 g/m3 of water vapour when saturated. If air at this temperature contains only 4.7 g/m3 of water vapour, then the relative humidity is 50%. When unsaturated air is cooled, relative humidity increases. Eventually it reaches a temperature at which it is saturated. Further cooling leads to condensation of the excess water vapour. The temperature at which condensation sets in is called the dew point. The dew point, and other measures of humidity can be calculated from readings taken by a hygrometer. A hygrometer has two thermometers, one dry bulb or standard air temperature thermometer, and one wet bulb thermometer.

The wet bulb thermometer is an ordinary thermometer which has the bulb covered with a muslin bag, kept moist via an absorbent wick dipped into water. Evaporation of water from the muslin lowers the temperature of the thermometer. The difference between wet and dry bulb temperatures is used to calculate the various measures of humidity. The hygrometer Wind The air is nearly always in motion, and this is felt as wind. Two factors are necessary to specify wind, its speed and direction. The direction of wind is expressed as the point of the compass from where the wind is blowing. Air moving from the north-east to the south-west is called a north-east wind. It may also be expressed in degrees from true north. A north-east wind would be 45�. A south-west wind would be 235�. The wind speed can be expressed in miles or kilometres per hour, meters per second, knots or as a force on the Beaufort scale.

The Beaufort Scale Unit 4: Temperature-Moisture Relationship — Free Online Course Materials — USU OpenCourseWare Temperature & Heat :: Temperature & Altitude :: Atmospheric Moisture :: Relative Humidity :: Clouds :: Exercises Atmospheric Moisture You should realize that moisture in the atmosphere can appear in three states--solid, liquid, and a gaseous vapor. It is very rare when the air does not contain some water vapor. When the air is cooled to its saturation point, condensation occurs in the form of clouds and perhaps precipitation. At very high altitudes where the air is very cold, clouds consist of tiny ice crystals. And, of course, precipitation can occur in the form of snow and hail. Natural forms of moisture in the atmosphere An important point to remember is that each time water changes state it either gives off or takes on heat energy. When ice melts it requires about 144 BTUs of heat to change 1 pound of ice to water with a temperature of 32°. This is called the heat of fusion. It takes additional heat to raise the temperature of the water to reach its boiling point. This is about 180 BTUs for 1 pound of water.

To change the liquid state into water vapor, it requires an additional 972 BTUs per pound of water. By changing state twice; that is, ice to water to vapor, approximately 1,300 BTUs of heat energy per pound of water has been stored in the moisture which is bound to the air. Any time condensation and freezing occurs, that amount of heat energy will be released, thus raising the temperature of the air. It is possible in the atmosphere for ice crystals to go directly into water vapor, or water vapor directly to ice crystals. This process is called sublimation. The amount of heat involved in sublimation equals the sum of the heat of fusion plus the latent heat of vaporization. Energy requirements to convert ice to liquid and liquid to vapor When discussing moisture in the atmosphere, it's desirable to have some points of reference or means of qualifying the amounts present at any one time or place. The two most common points of reference used are dew point and relative humidity. Dew Point : The temperature to which a parcel of air must be cooled to reach its saturation point. A dew point is a useful reference because it tells at which temperature clouds and precipitation will occur.

Does dew point change with changes in temperature or relative humidity? First, you should understand that the temperature of the air influences the amount of water vapor that can be bound to the molecules of air. Water vapor capacity increases with temperature increase. There are three parcels of air, each in separate containers. Only relative values of moisture and temperature are shown. At low temperatures, the air will hold only two parts of water vapor. As the temperature increases, it will hold 6, then 12 parts of water vapor. If the very warm air on the right is cooled, it must lose some of its bound water vapor. This will occur through condensation. In any one stationary parcel of air, the dew point of that air will remain the same as will the amount of moisture in that parcel, regardless of air temperature. But as air temperature increases, so will the capacity of that air to hold more water vapor. The higher the air temperature, the stronger the bond between water and air molecules, and the drier the air seems to be. To determine how dry or wet the air is at any given temperature, we use a unit of measure called relative humidity.

Relative Humidity : The ratio of the actual amount of water vapor in a given volume of air to the amount which could be present if the air were saturated at the same temperature. It's commonly expressed as a percentage. Since warm air will hold more moisture than cold air, the percentage of relative humidity must change with changes in air temperature. The graphic below illustrates this relationship. Again we have three parcels or containers of air. The number of water vapor molecules is the same in each container. At 40° air temperature, the parcel is saturated and will hold no more molecules of water vapor. The relative humidity is 100 percent. If the temperature of that air parcel is raised by 20°, it will hold about twice as many water molecules to reach saturation. Thus, the new relative humidity is now 48 percent. If the temperature is raised another 20°, it will again double its capacity to hold water vapor molecules. The relative humidity is only 24 percent. Relative humidity decreases as temperature increases The importance of air temperature to moisture is obvious. At 80°, the air has a relatively low humidity and is relatively dry.

As it cools, the humidity increases, reaching its saturation point at 40°. Now the air is very moist, and clouds will form. The dew point of the air is 40° in all three containers in the illustration. The example in the graphic below shows a relatively moist airmass with a dew point of 60°. A relatively dry airmass has a dew point of 38°. This illustration shows that a cool, dry airmass may actually have a higher relative humidity than a warm, moist airmass. Relative humidity alone can be misleading when comparing atmospheric moisture conditions. A cool, dry air mass may actually have a higher relative humidity than a warm, moist air mass. Rule of Thumb : Relative humidity doubles with each 20 degree (Fahrenheit) decrease, or halves with each 20 degree increase in temperature. Generally, as temperature goes up, relative humidity goes down and vice versa. Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 27). Unit 4: Temperature-Moisture Relationship.

Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License USATODAY.com FAQ on humidity and water in the atmosphere Q: How can relative humidity be low, yet it still feels uncomfortable? A: The old adage is, "It's not the heat, it's the humidity." However, sometimes it is just plain hot. Relative humidity is just a measure of the amount of water vapor actually in the air compared to the amount of water vapor that could be in the air at a given temperature and pressure. Since warm air has a greater capacity for water vapor, desert climates with dry air can have single-digit relative humidities since the air is very far away from reaching saturation. While it is a "dry" heat, 120°F in Death Valley is still hot. There's plenty more about humidity on this USA TODAY resource page . (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, August 20, 2007) Q: During which time of year is dew most likely to form?

A: Dew forms when a surface, such as a blade of grass or car hood, cools to the dew point of the air with which it comes into contact. If the dew point temperature is above freezing, water vapor in the air condenses onto the surface in the form of dew. If the dew point is below freezing, the result is frost. Warmer air has more capacity for water vapor, so cool nights following warm days (typical of late spring and early fall) promote dew formation. During relatively short summer nights, surfaces sometimes don't cool enough for dew to form. Learn more about humidity on this USA TODAY resource page . (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, May 16, 2007) Q: Is the relative humidity percentage the amount of water that's in the air at that time? A: The amount of water in the air is given by absolute humidity, typically measured in grams per cubic meter. Relative humidity is a ratio of how much water vapor is in a given volume of air compared with the amount of water vapor that would saturate the air at a given temperature and pressure. Because its a ratio, relative humidity is given as a percentage.

When the air is saturated and contains the maximum amount of water vapor, the relative humidity is 100%. There's plenty more about humidity on this USA TODAY resource page . (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 16, 2007) Q: Is relative humidity the only type of humidity? A: In common usage, humidity typically refers to relative humidity. However, relative humidity is just a ratio of how much water vapor is in a given volume of air compared with the amount of water vapor that would saturate the air at a given temperature and pressure. The measure of the water vapor content in the air is known as absolute humidity. Absolute humidity is typically given by grams of water vapor per cubic meter of air, while relative humidity is simply a percentage value. There's plenty more about humidity on this USA TODAY resource page . (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 1, 2007) Q: I know that humid air makes people feel hotter in the summer, how does humidity affect the "feels like" temperature in the winter?

A: Some people claim that a "wet" cold feels colder than a "dry" cold. One possible reason for this is that we wear insulating clothing in the winter. The dead air spaces within the fill materials of a jacket or within the fibers of a sweater limit the loss of body heat, due to the low thermal conductivity of air. Thermal conductivity increases as relative humidity increases, so body heat is more quickly lost in more humid conditions, making one feel colder. All other conditions being equal, a cold day with rain and/or fog feels colder than a dry day. If moisture is on the skin, heat will be lost through conduction of heat from the body to the water as well as the evaporative cooling effects of the water. There's plenty more about how the body regulates temperature on this USA TODAY resource page . (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, December 13, 2006) Q: What is the relationship between air temperature and relative humidity? A: Relative humidity is a percentage that reflects how close the air is to saturation, where 100% relative humidity indicates fog or rain and single-digit relative humidity indicates extremely dry air.

For a given amount of water vapor in the air, relative humidity will decrease as air temperature increases and increase as air temperature decreases. Typically, relative humidity at a location is highest around daybreak when temperatures are coolest and lowest during the heat of the afternoon. Of course, there are exceptions to the rule as the amount of water vapor in the air doesn't remain constant throughout the day. In addition, temperatures not always lowest in the morning and highest in the afternoon. Learn more about humidity on this USA TODAY resource page . (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, November 19, 2006) Q: Can the dew point change quickly over the course of a few hours? A: Yes, it can change rapidly, especially if a front moves through. The dew point indicates the amount of moisture in a given air mass. A front, be it cold, warm, stationary or occluded, denotes a boundary between two significantly different air masses. Although the main difference between two air masses is usually the air temperature, sometimes, especially in summer, there may not be much of a temperature difference across a front, but there can be a significant change in the dew point.

Learn more about dew point and humidity on this USA TODAY resource page . (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 1, 2006) Q: Can you explain in simple terms what dewpoint is? Here's a very simple look at dew point. For more details and a more scientific look at humidity, go to Understanding humidity page . Humidity refers to water vapor in the air; that is water in the form of an invisible gas. The warmer the air, the more water vapor it can have in it. When the air can no longer "hold" all of the water vapor in it, the vapor begins condensing into ordinary, liquid water. If the air is in the sky, the vapor condenses into cloud drops. If the air is right above the ground the vapor condenses to make fog (which is nothing but a cloud that's on the ground.) If the air is touching things such as grass or a car's windshield, it will condense to make dew. If air cools enough it will eventually reach a point at which the water vapor begins condensing.

Dew point is really a measure of how much water vapor is in the air, but it's given as the temperature at which that particular bunch of air has to cool to for the water vapor to begin condensing. For example, if a weather report says that the temperature is 70 degrees and the dew point is 55 degrees, you know that if the air cools to 55 degrees the water vapor in it will start condensing into dew on the ground or fog right above the ground or clouds in the sky. Q: Who invented the hygrometer? A: Horace B�n�dict de Saussure, a Swiss physicist and geologist, invented the first hygrometer to measure humidity in 1783. He used human hair as a critical part of his hygrometer after noticing that it becomes longer in air with high humidity and shorter with low humidity. Horse hairs, metallic and infrared sensors can substitute for human hairs. Hygrometers and other weather instruments are explained on the Measuring weather page. Q: What is the relationship between current temperature, relative humidity, and dew point?

A: The warmer air is, the more water vapor it can "hold." Dew point is a measure of how much water vapor is actually in the air. Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at the temperature it happens to be when you measure it. To see how this works, let's use the chart below, which is adapted from a good introductory college meteorology textbook, Meteorology Today by C. Donald Ahrens, published by West Publishing. Air temperature in degrees C Water vapor air can hold at this temperature. 30 degrees 30 grams per cubic meter of air 20 degrees 17 grams per cubic meter of air 10 degrees 9 grams per cubic meter of air These numbers, which apply to air at sea level pressure, are based on measurements over the years. They are basic physical facts. Now, let's see how dew point and relative humidity work. Imagine, that at 3 p.m. you measure the air's temperature at 30 degrees and you measure its humidity at 9 grams per cubic meter of air. What would happen if this air cooled to 10 degrees with no water vapor being added or taken away?

As it cools to 10 degrees the air becomes saturated; that is, it can't hold any more water vapor than 9 grams per cubic meter. Cool the air even a tiny bit more and its water vapor will begin condensing to form clouds, fog or dew - depending on whether the air is high above the ground, just above the ground, or right at the ground. Back at 3 p.m., when we made the measurements, we could say that the air's dew point is 10 degrees C. That is, if this particular air were cooled to 10 degrees at ground level, its humidity would begin condensing to form dew. How about relative humidity? At 3 p.m. the air has 9 grams of water vapor per cubic meter of air. We divide 9 by 30 and multiply by 100 to get a relative humidity of 30% In other words, the air actually has 30% of the water vapor it could hold at its current temperature. Cool the air to 20 degrees.

Now we divide 9, the vapor actually in the air, by 17, the vapor it could hold at its new temperature, and multiply by 100 to get a relative humidity of 53% (rounded off). Finally, when the air cools to 10 degrees, we divide 9 by 9 and multiply by 100 to get a relative humidity of 100% - the air now has all the vapor it can hold at its new temperature. The Understanding water in the atmosphere page has links to a lot more information about humidity and its role in weather. Q: What does dew point tell us about the weather now and in the next few hours? A: For most people, the air feels humid when the dew point is above 60 degrees and uncomfortably hot and sticky when it goes above 70 degrees. The dew point is a good indication of how low the temperature can fall overnight unless a cold front arrives with a different air mass . For example, if the afternoon temperature is 80 degrees and the dew point is 70 degrees, the temperature isn't going to fall below 70 that night.

When it reaches 70, the water vapor will begin condensing into fog or dew and such condensation releases heat. If the dew point and temperature are within only a few degrees of each other, low clouds and fog are likely as the air cools. Low clouds are likely because the air, which cools as it rises, won't have to rise as far for condensation to begin as drier air would. This is why aviation weather reports give the dew point and temperature. Pilots know that if the dew point and temperature are close to each other they should be prepared for low clouds and fog. Q: Is it possible for relative humidity to be higher than 100%? A: Yes, it is possible. In Elements of Meteorology the authors Albert Miller and Jack C. Thompson explain that in extremely clean air which contains no "foreign" particles, water droplets, or ice crystals it is extremely difficult for condensation to occur even when supersaturated conditions are present. Supersaturated conditions refer to relative humidities much greater than 100%. C.T.R. Wilson invented an instrument called a cloud chamber in which clouds can be manually made.

He discovered that in pure, clean air the relative humidity must reach 800% before water drops form. Realistically, air is not pure and clean. There are many particles that are small enough to remain suspended in the atmosphere such as salt particles from sea water, combustion products, meteoritic dust, volcanic material, and soil. These particles, prominent as they are, act as nuclei for water to condense upon and, hence, are an easier means of condensation. This means that condensation in real air can usually produce humidities just barely over 100%. Q: How can a location with a lower relative humidity (RH) have a higher actual water vapor content than a location with a higher relative humidity? A: The answer lies in the definitions of relative humidity and actual water vapor content. Relative humidity is the ratio of the amount of water vapor present in the air to the maximum amount of water vapor that can evaporate into the air before condensation begins to occur. It is usually expressed as a percent. Water vapor content is the quantitative amount of water vapor present in a parcel of air. This is expressed as a unit of weight such as grams.

The warmer the air temperature, the more water that will be able to evaporate into the air. The main reason for this is that warmer air temperatures mean more kinetic energy is available to keep water molecules in a vapor state. As a result, cold air that is saturated (RH=100%) often has less water vapor in it than very warm, unsaturated air. For example, one cubic meter of saturated air at 32 degrees Fahrenheit has about 5 to 6 grams of water vapor. Unsaturated air at 86 degrees Fahrenheit with a relative humidity of only 50% contains about 14 to 15 grams of water vapor. The saturated cold air in this example contains less than half of the water vapor that is in the warm air at only 50% relative humidity even though the cold air's relative humidity is 100% compared to 50% for the warm air. Q: What is the "wet bulb"? Is it related to the dew point? A: Both are measures of humidity, but they are different. "Wet bulb" is the temperature that a thermometer reads when its bulb is covered with a wet cloth and air is blown onto it.

This causes the water to evaporate, cooling the thermometer. Our humidity definitions page explains more about this. Dew point is explained in an answer above. Q: If it's raining, how can the humidity be less than 100%? A: First, humidity is a measure of the amount of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. This normally happens when the air rises and cools. Often, rain will be falling from clouds where the humidity is 100% into air with a lower relative humidity. Some water from the rain evaporates into the air it's falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%. Our water in the atmosphere index has more information on other aspects of this. Q: Is humid air more or less dense that dry air? A: Contrary to what "common sense" might tell you, humid air is less dense than dry air. The Understanding air density page explains why.

Q: Does humid air make cool weather feel cooler? A: The amount of humidity in the air by itself doesn't make any difference in how cold the air "feels." Sometimes you will hear that the water vapor in the air carries heat away, cooling you faster. At first glance that might seem to make sense because most people have heard of victims of boating accidents dying because they weren't pulled from chilly water before dying of hypothermia -- a cooling of the body's core temperature. In these cases the water does quickly carry heat away from the body and people with no protection can die quickly in 40-degree water. But, even on the most humid of days, especially when the air is chilly, the percentage of water molecules in the air as water vapor is very small. Even if water molecules did carry heat away from your body, there are too few to noticeably cool you. Also, it turns out that water vapor molecules are no better at conducting heat than the nitrogen and oxygen molecules that make up most of the air. What I have to say here is based on the "Water Vapor Mysticism" chapter in a delightful book, What Light Through Yonder Window Breaks: More Experiments in Atmospheric Physics by Craig F.

Bohren, published by John Wiley & Sons, Inc. in 1991. In it, he not only gives the theoretical arguments in the paragraph above, but also describes some simple, home experiments to show things don't cool faster in cool but humid air than in cool but dry air. Why do people think wet cold is more uncomfortable than dry cold? Bohren's answer makes sense to me. Humid places are often cloudy. A cold, sunny day doesn't feel as chilly as a cold, cloudy day because even as the air is cooling us, direct sunlight is warming us. The USATODAY.com Understanding water in the atmosphere page has links to a lot more on how water in all of its forms affect the weather. Atmospheric moisture Relative humidity Dew point Absolute humidity: is the mass of water vapor in a given volume of air (usually grams/meter3) Since temperatures and pressures change constantly and they change from place to place, meteorologists usually prefer mixing ratio. Mixing ratio: mass of water vapor in a unit of air compared to dry air. E.g., mass of water vapor (in grams)/mass of dry air (kilograms). Not affected by changes in temperature or pressure. Also known as specific humidity.

Pressure Pressure is defined as a force per unit area. That force is caused by air molecules striking your skin because they are in motion. That's why the air pressure is lower at great altitudes - fewer air molecules. Air pressure is dependent on the kinetic energy of the air molecules (temperature), the mass of the molecules, and gravity. In meteorology we use units of millibars to measure air pressure. Sea level is ~1013 mb. Vapor pressure: that part of the total atmospheric pressure attributable to water vapor. In chemistry we would call this the partial pressure. We could also measure, or calculate, the partial pressures of all the other gasses in the atmosphere (e.g., N2, O2, Ar, CO2, etc.) - they should all add to the total atmospheric pressure. This is temperature dependent. We can actually measure the number of molecules (by way of the pressure) leaving water, or entering water. When the number of molecules leaving water is the same as the number entering, we call that saturation, and the pressure is the saturation vapor pressure. Relative humidity: the ratio of the air's actual water vapor content compared with the amount of water vapor required for saturation (at that temperature and pressure).

In effect, its a measure of how close to being saturated the air is. The amount of water vapor required to saturate air at various temperatures is variable. That's why on a very cold day the relative humidity may be close to 100 percent but it doesn't feel like a muggy 70 percent relative humidity day when it's 95°F outside. If the amount of water vapor is constant: Increasing temperature decreases RH If temperature is held constant: Adding water vapor increases RH Removing water vapor decreases RH This is why we usually see higher relative humidity at night. Since relative humidity is easily changed by temperature or amount of water vapor, we'd like to find another variable that isn't so easily changed. Dew Point: temperature to which the air would have to be cooled, without changing pressure or moisture content, to reach saturation. The drier the air, the more is must be cooled to reach saturation. High dew points indicate a large amount of moisture in the air. Low dew points indicate small amounts of moisture in the air. Frost (hoar frost, white frost) is caused when the temperature is below freezing and the air is saturated with moisture.

Dew points are directly related to the amount of moisture in the air, so is a much better indicator of humidity. Dew point depression, Tdd = T - Td Small Tdd indicate high RH Large Tdd indicate low RH. Cool the air without adding or removing moisture. Add water vapor without changing the temperature. Measurement So we now know the various measurements of the amount of water vapor in the atmosphere, but how do we actually measure it? The most common technique is to use the sling psychrometer. The psychrometer has two thermometers that are twirled around on a sling. One thermometer is covered with a wet cloth wick, so that as it is twirled it is cooled through evaporation, hence the name sling psychrometer (sling, psychro- cold, and -meter, to measure). The lowest temperature recorded is called the wet bulb temperature. We then look up the wet bulb depression (difference in temperatures between the wet bulb thermometer and the dry thermometer) versus the dry temperature to get the RH.

There are other ways to measure relative humidity: Electronic (semiconductors) In order to make a cloud we need: Moisture Nuclei on which to condense (cloud condensation nuclei, CCN, or ice nuclei, IN) A method of cooling the air to saturation If heat (energy) is neither added nor removed from the atmosphere, but the temperature changes because its volume changes we call it an adiabatic temperature change. (Think of pumping up a bicycle pump) As air ascends, the pressure decreases allowing the volume of air (a parcel) to increase. As the volume increases, its temperature decreases - we call this adiabatic cooling. Unsaturated air will cool 1°C/100 meters of ascent, or it will heat that amount upon descent. This is called the dry adiabatic rate: 1°C/100 meters (10°C/km) If this parcel of air cools enough it will reach its dew point and condensation, in the form of cloud development, will occur. As air is compressed is heats up - adiabatic warming. As air expands it is cooled - adiabatic cooling.

Question: What common term refers to the amount of water vapour that is held in the air?
Answer: Humidity