Solar Radiation

The earth’s surface receives most of its energy in short wavelengths. This incoming solar radiation is called insolation. Because the earth is a geoid — roughly spherical — the sun’s rays strike the top of the atmosphere obliquely, and the earth intercepts only a very small portion of the sun’s total energy output. On average, the earth receives 1.94 calories per square centimetre per minute at the top of its atmosphere.

The distance between the earth and the sun changes slightly over the course of a year, and this causes a small variation in the solar output received at the top of the atmosphere. The earth is farthest from the sun, at 152 million kilometres, on 4th July — a position called aphelion. It is nearest to the sun, at 147 million kilometres, on 3rd January — a position called perihelion. As a result, the annual insolation received on 3rd January is slightly more than that received on 4th July. However, this variation is masked by other factors such as the distribution of land and sea and the atmospheric circulation, so it has no great effect on daily weather changes at the earth’s surface.

Variability of Insolation at the Surface of the Earth

The amount and intensity of insolation vary during a day, across a season, and over a year. The factors causing these variations 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.

The last two factors have less influence than the first three.

The earth’s axis makes an angle of 66½° with the plane of its orbit around the sun, and this has a greater influence on the amount of insolation received at different latitudes. The angle of inclination of the rays is the second major factor, and it depends on the latitude of a place. The higher the latitude, the smaller the angle the rays make with the surface of the earth, resulting in slanting sun rays. Vertical rays cover a smaller area than slant rays; when a larger area is covered, the same energy gets distributed over it, so the net energy received per unit area decreases. Moreover, slant rays must pass through a greater depth of the atmosphere, which causes more absorption, scattering, and diffusion.

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 the visible spectrum, both towards space and towards the earth’s surface. This scattering adds colour to the sky. The red colour of the rising and setting sun and the blue colour of the sky are both results of the scattering of light within the atmosphere. …