Solar Radiation and Its Impact on Earth’s Weather for GATE, NET, and GSI Exams

Solar Radiation

The weather results from the interaction of solar radiation with the earth’s atmosphere and surface. Earth’s rotation and revolution explain the changing elevation of the sun, along with the latitudinal and seasonal variations in day length, receipt, and escape of radiation, which together affect weather conditions.

The Sun as an Energy Source

The sun, with a temperature of about 6000 Kº, is the source of nearly all the energy we receive. The earth intercepts an extremely small portion of the sun’s output, about 10^-5%. Only part of the sun’s radiation reaches the earth’s surface as direct radiation, while the rest is reflected, absorbed, or scattered by the atmosphere. The maximum solar radiation emission occurs at short wavelengths within the visible spectrum, between 0.4 μm and 0.7 μm.

Solar Energy and the Solar Constant

The sun radiates approximately 56 × 10²⁶ cal of energy per minute. The energy per unit area incident on the earth is calculated as:

S ≈ 2.0 cal cm-2 min-1

The symbol S represents the Solar Constant, with 1.5 × 10¹³ cm being the mean distance of the earth from the sun. The Solar Constant is defined as the flux of solar radiation received at the earth’s outer atmosphere on a surface perpendicular to the sun’s direction. This energy is measured in Langleys (ly), where 1 ly equals 1 cal cm^-2. Hence, the solar constant for Earth is 2 Ly.

Global Energy Distribution

If we consider the earth as a non-rotating circular disc with radius r, equivalent to the earth’s radius, the amount of solar radiation intercepted by the sun-facing side is the same as the total intercepted by the rotating spherical earth. Since the surface area of a sphere is four times greater than the area of one side of a disc (4πr² vs. πr²), the global average energy received at the top of the atmosphere equals 0.5 ly min^-1, or one quarter of the solar constant.