Solar Radiation
● The earth’s surface receives most of its energy in short wavelengths.
● The energy received by the earth is known as incoming solar radiation which in short is termed as insolation.
● As the earth is a geoid resembling a sphere, the sun’s rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun’s energy.
● On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.
● The solar output 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 its revolution around the sun, the earth is farthest from the sun (152 million km on 4th July). This position of the earth is called aphelion.
● On 3rd January, the earth is the nearest to the sun (147 million km). This position is called perihelion.
● Therefore, 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 in the solar output is masked by other factors like the distribution of land and sea and the atmospheric circulation.
● Hence, this variation in the solar output does not have great effect on daily weather changes on the surface of the earth.
Variability of Insolation at the Surface of the Earth
The amount and the intensity of insolation vary during a day, in a season and in a year.
The factors that cause these variations in insolation are : Factors Controlling Temperature Distribution
(i) the rotation of earth on its axis
The fact that the earth’s axis makes an angle of 66½ with the plane of its orbit round the sun has a greater influence on the amount of insolation received at different latitudes.
Length of the Day in Hours and Minutes on Winter Solstice (December 22) and Summer Solstice (June 21) in the Northern Hemisphere:
1 . Latitude : 0°
December 22 : 12h 00m
June 21 : 12 h
2 . Latitude : 20°
December 22 : 10h 48m
June 21 : 13h 12m
3 . Latitude : 40°
December 22 : 9h 8m
June 21 : 14h 52m
4 . Latitude : 60°
December 22 : 5h 33m
June 21 : 18h 27m
5 . Latitude : 90°
December 22 : 0
June 21 : 6 months
(ii) the angle of inclination of the sun’s rays
The second factor that determines the amount of insolation received is the angle of inclination of the rays. This depends on the latitude of a place.
The higher the latitude the less is the angle they make with the surface of the earth resulting in slant sun rays.
The area covered by 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.
Moreover, the slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion.
(iii) the length of the day
(iv) the transparency of the atmosphere
(v) the configuration of land in terms of its aspect.
The last two however, have less influence.
The Passage of Solar Radiation through the Atmosphere
● The atmosphere is largely transparent to short wave solar radiation.
● The incoming solar radiation passes through the atmosphere before striking the earth’s surface.
● Within the troposphere water vapour, ozone and other gases absorb much of the near infrared radiation.
● Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface. This process adds colour to the sky.
● The red colour of the rising and the setting sun and the blue colour of the sky are the result of scattering of light within the atmosphere. https://ncert.nic.in/ncerts/l/kegy209.pdf
Spatial Distribution of Insolation at the Earth’s Surface
● The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles.
● Maximum insolation is received over the subtropical deserts, where the cloudiness is the least.
● Equator receives comparatively less insolation than the tropics.
● Generally, at the same latitude the insolation is more over the continent than over the oceans.
● In winter, the middle and higher latitudes receive less radiation than in summer.
Must read: Heat Budget of the Planet Earth
PRACTICE QUESTIONS
QUES . On June 21 every year, which of the following latitude(s) experience(s) a sunlight of more than 12 hours? UPSC 2024
1 . Equator
2 . Tropic of Cancer
3 . Tropic of Capricorn
4 . Arctic Circle
Select the correct answer using the code given below:
(a) 1 only
(b) 2 only
(c) 3 and 4
(d) 2 and 4
Ans (d)
