Solar Radiation

Notes

The Sun's surface temperature is about 6,000°C (about 5,500°C). Insolation (incoming solar radiation) is the solar energy received by the Earth. The Earth intercepts only about two billionths of the Sun's output. The solar constant is about 1.94 cal/cm²/min (about 1,360 W/m²) at the top of the atmosphere.

Nature of radiation

The Sun emits short-wave radiation (visible light, 0.4-0.7 micrometres, ultraviolet and near infrared). The Earth emits long-wave (terrestrial) infrared radiation.
Heat transfer: conduction (contact), convection (vertical movement of air), advection (horizontal movement, e.g., winds) and radiation.

Factors affecting insolation

Angle of incidence (latitude): vertical rays concentrate energy; slanting rays spread it and pass through a thicker atmosphere.
Duration of day, transparency of the atmosphere (clouds, dust and water vapour reflect or absorb), altitude and the Earth-Sun distance.
Perihelion (about 3 January; about 147 million km) and aphelion (about 4 July; about 152 million km); the mean distance is about 149.6 million km.
Earth's tilt (23.5°) and revolution cause seasons: the Sun is overhead at the Tropic of Cancer on 21 June, at the Tropic of Capricorn on 22 December and at the equator on the equinoxes (21 March and 23 September).

Distribution of insolation

It is highest in the subtropics (about 20-30°) over deserts, because of clear skies, not at the equator where clouds reduce it. It is lowest at the poles.
Land receives more than ocean; summer more than winter.

Scattering and albedo

Scattering: Rayleigh scattering of short wavelengths gives the blue sky; red sunrise and sunset occur because blue light is scattered away over the long path.
Albedo: the fraction of insolation reflected. The Earth's mean albedo is about 35%; fresh snow 80-90%, thick clouds 50-80%, desert sand about 25-40%, forests 10-15%, and water 6-10% (low).

Key Points

Insolation is short-wave; terrestrial radiation is long-wave.
Solar constant is about 1,360 W/m².
Insolation depends on latitude (angle), day length, atmospheric transparency, altitude.
Maximum insolation in subtropical deserts; minimum at the poles.
Albedo: fresh snow high, water low; Earth about 35%.

MCQs

Q1. The incoming solar radiation is mainly:

(a) Long-wave
(b) Short-wave
(c) Ultrasonic
(d) Infrared only
Ans: (b).

Q2. Insolation received is highest over:

(a) The equator
(b) Subtropical deserts
(c) The poles
(d) Oceans at 60°
Ans: (b). Cloud-free skies.

Q3. Which surface has the highest albedo?

(a) Forest
(b) Fresh snow
(c) Water
(d) Dark soil
Ans: (b).