# Introductory Agrometeorology/Solar radiation
## Objectives of this chapter

* To understand nature and properties of solar radiation
* To know about "Solar Constant"
* To know effects of solar radiation in atmosphere and in crop production
* To know measurement of solar radiation

## Solar radiation
### Solar radiation

* Radiation is energy from the source that travels through some materials and some space; heat, light, and sound are types of radiation.
* There are two types of radiation:
** Ionizing radiation (produced by unstable atoms; can produce charged particles called ions)
** Non-ionizing radiation (sound, heat, visible light)
* Light is ionizing radiation
* The flux of radiant energy from the sun is solar radiation.
* Radiation is temperature dependent (the higher the temperature, the greater the radiation). At higher temperature, a body emits a shorter wavelength:
          i.e. λ = c / f
          where, λ = wavelength
          c = velocity of light
          f = frequency/vibration per second
* Heavenly bodies emit – short wave radiation
* Near surfaces including Earth emit - long wave radiation

### Radiation flux

The amount of radiant energy emitted, received, transmitted across a particular area is known as radiant flux.

### Radiant flux density

The radiant flux divided by the area across which the radiation is transmitted is called radiant flux density.

### Emissive power

The radiant flux density emitted by a source is called its emissive power.

### Latent heat

A part of the incident radiation on the surface is absorbed, while a part is reflected and the remainder is transmitted.

### Absorptivity

Absorptivity of a substance is defined as the ratio of the amount of radiant energy absorbed to the total amount incident upon that substance. The absorptivity of a black body is unity. Natural bodies like the Sun and the Earth are near perfect black bodies.

### Reflectivity

Reflectivity is defined as the ratio of the radiant energy reflected to the total incident radiation upon that surface. If it is expressed in percentage it becomes albedo.

### Transmittivity

Transmittivity is defined as ratio of the transmitted radiation to the total incident radiation. upon the surface.

### Emissivity

Emissivity is defined as the ratio of the radiant energy emitted by a given surface to the total heat energy emitted by a black body. The emissivity of a black body is unity.

### Black body radiation

A black body is defined as a body, which completely absorbs all the heat radiations falling on it without reflecting and transmitting any of it. It means reflectivity and transmittivity become zero. When such a black body is heated it emits radiation of all wavelengths depending upon its temp.

### Radiation balance

The difference between all incoming and outgoing radiation at the Earth’s surface and top of the atmosphere is known as radiation balance at the Earth’s surface.

### Solar constant

Solar constant is the energy received on a unit area at the outer most boundary of the Earth (atmosphere) surface held perpendicular to the Sun’s direction, at the mean distance between the Sun and the Earth. Solar constant is not a true constant. It fluctuates by as much as ± 3.5 % about its mean value depending upon the distance of the earth from the sun.

### Albedo

This is the percentage of reflected radiation to the incident radiation.

## Laws of Radiation

a. Plank’s Law:
    The electromagnetic radiation consists of a stream or flow of particles or quanta. Each quantum having energy content.
          E = h ϑ
          where, h = plank’s constant (6.625 × 10-27 ergs sec-1)
          ϑ = frequency of electro-magnetic length. (f = 1 / λ)
    The greater the frequency (i.e. shorter the wavelength) the more the energy content.
b. Stefan Boltzmann’s Law:
    The intensity of radiation emitted by a radiating body is proportional to the fourth power of its absolute temperature.
          E = σT$^4$
          where, E = emissivity of the black body
          T = absolute temperature
          σ = Stefan Boltzmann’s constant
c. Wein’s displacement law:
    The wavelength of maximum intensity of emission is inversely proportional to the absolute temperature of that body. i.e. wavelength of maximum intensity of emission (λmax α 1/T), in which λmax = 2897 T-1
d. Kirchoff’s law:
    Any object (other than perfect black body) receive radiation will absorb, reflect, and transmit a fraction of total radiation depending upon its radiational properties. This law states that absorptivity of an object for radiation of a specific wavelength is equal to its emissivity for the same wavelength (λ).
          i.e. a(λ) = e(λ)
          where, a = absorptivity (nil for white body, one for black body)
          e = emissivity.
          Total incident radiation (I) = absorptivity (Ia) + transmissivity (It) + reflectivity (Ir)
    Solar radiation
          2
          Or, I = Ia + It + Ir
          Or, 1 = Ia/I + It/I + Ir/I
          Therefore, 1 = α + r + t

### Energy balance or heat balance

The net radiation is the difference between total incoming and outgoing radiations and is a measure of the energy available at the ground surface. It is the energy available at the Earth’s surface to drive the processes of evaporation, air and soil heat fluxes as well as other smaller energy consuming processes such as photosynthesis and respiration

## Effects of atmosphere in incoming solar radiation

Radiant energy from the Sun is the major source of energy for terrestrial life, practically all the energy for all the physical and biological processes occurring on the Earth arise in the form of solar radiation. Radiation is the ultimate of all the changes and motion of the atmosphere and it is the single most important control of climate. It is a meteorological element of highest important. Radiation from the Sun comes in forms of short-wave electromagnetic radiation. The shortwave radiation are referred to as short-wave incoming radiation. The outgoing radiation from the soil is called the long-wave terrestrial radiation from the soil is called the long-wave terrestrial radiation.

## Solar radiation and crop plants

Crop production is exploitation of solar radiation Three broad spectra 1. Shorter than visible range: Chemically very active
* When plants are exposed to this radiation the effects are detrimental.
* Atmosphere acts as regulator for this radiation and none of cosmic, Gamma and Xrays
reaches to the earth.
* The UV rays of this segment reaching to the earth are very low and it is normally
tolerated by the plants. 2. Higher than visible wavelength
* Referred to IR radiation
* It has thermal effect on plants
* In the presence of water vapour, this radiation does not harm plants, rather it
supplies the necessary thermal energy to the plant environment. 3. Visible spectrum
* Between UV & IR radiation and also referred as light
* All plant parts are directly or indirectly influenced by the light
* Intensity, quality and duration are important for normal plant growth
* Poor light leads to plant abnormalities
* Light is indispensable to photosynthesis
* Light affect the production of tillers, the stability, strength and length of Culms
* It affects the yield, total weight of plant structures, size of the leaves and
root development.

### Critical stages of plant growth for light

* Radiation intensity during the third month of Maize plant
* Rice – 25 days prior to flowering
* Barley – flowering period

## Measurement of Solar radiation

* Solar intensity measurement
* Pyranometer or solarimeter
* Bimetallic actinograph
* Bellani pyranometer
* Solar duration measurement
* Campbell stroke’s sunshine recorder and
* Sunlight quality measurement
* Spectrobolometer
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