Solar Insulation is the amount of solar radiation incident upon a unit horizontal surface over a specified time for a given place. It mostly depends on the solar zenith angle and also on the ratio (d/dm) of the actual distance to the mean distance of the Earth from the Sun.
The surface of the earth receives solar radiation in form of short wavelengths. This energy received by the earth is known as Incoming Solar Radiation also called Insolation.
Insolation on Earth’s Surface
The earth gets a very small amount of solar energy. This is because the earth is a geoid that resembles a sphere. Due to this, the sun’s rays fall obliquely at the top of the atmosphere.
What time of year is insolation greatest?
The solar output received at the top of the atmosphere varies slightly in a year due to 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 the 4th of 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 atmospheric circulation.
Hence, this variation in the solar output does not have a 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 :
- The rotation of the earth on its axis
- The angle of inclination of the sun’s rays
- The length of the day
- The transparency of the atmosphere
- The configuration of land in terms of its aspect.
But the transparency of the atmosphere and configuration of the land has less influence.
Rotation of the Earth on its Axis
The fact that the earth’s axis makes an angle of 66 with the plane of its orbit around the sun has a greater influence on the amount of insolation received at different latitudes.
|December 22||12h 00 m||10h 48m||9h 8m||5h 33m||0|
|June 21||12h||13h 12m||14h 52m||18h 27m||6 months|
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 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 a greater depth of the atmosphere resulting in more absorption, scattering, and diffusion.
Transparency of 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 space and towards the earth’s surface. This process adds colour to the sky. The red colour of the rising and the setting sun and the blue
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. The Equator receives comparatively less insolation than the tropics.
Generally, at the same latitude, the insolation is moreover the continent than over the oceans. In winter, the middle and higher latitudes receive less radiation than in summer.
Why is less insolation received at the equator in June?
During June, especially around June 21 of the year, the position of the earth in its orbit is inclined directly towards the sun. This is called the Summer solstice in the Northern Hemisphere.
As a result during June, maximum insolation is received at the Tropic of Cancer, as Sun is directly perpendicular to the Tropic of Cancer. The Equator is below the Tropic of Cancer, and thereby receives less insolation at the equator in June.