Climatology/Insolation and Heat Budget
Solar irradiance (SI) is the power per unit area (watt per square metre, W/m2), 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.
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 AtmosphereOur
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.