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Solar radiation is the Earth’s main energy source — but it never falls evenly. That unevenness sets the whole planet’s winds and currents in motion.
- Solar radiation reaches the Earth as electromagnetic (EM) waves that travel through vacuum at the speed of light, 3 × 10 8 m s −1 .
- The EM spectrum ranges from high-frequency, short-wavelength gamma rays and X-rays to low-frequency, long-wavelength infrared and radio waves.
- About 99% of the Sun’s energy reaching us is ultraviolet (UV), visible and infrared (IR); gamma rays and X-rays are mostly filtered by the upper atmosphere.
- Short-wavelength UV radiation is mostly absorbed by the ozone layer , protecting life and warming the upper atmosphere.
- Visible light reaches the surface and powers photosynthesis; it also partly warms the land and water.
- Infrared radiation warms the surface, which re-radiates heat; some of this outgoing heat is trapped by greenhouse gases (COâ‚‚, CHâ‚„, water vapour), keeping the Earth warm.
The UV rays lie in the wavelength range of 100 nm to 400 nm [1 nanometre (nm) = 10 −9 m] and carry much higher energy than visible light. Prolonged exposure can damage the eyes and skin and increase cancer risk, so use UV-protective glasses and sunscreen. UV rays are also useful for killing germs in water purifiers and for powering fluorescent lights.
- Insolation is the amount of the Sun’s radiation that reaches the Earth’s surface; it warms the surface and the atmosphere.
- The solar constant is the average solar energy received per unit time per unit area perpendicular to the Sun’s rays at the top of the atmosphere — about 1.4 kW m −2 (about 1400 J s −1 m −2 ).
- After absorption and scattering, the maximum insolation reaching the surface is lower — about 1 kW m −2 under clear skies.
Anna Mani , India’s pioneering atmospheric scientist, mapped solar insolation across India in the 1950s. With S. Rangarajan she published Solar Radiation Over India (1982), creating the country’s first insolation atlas . Her measurements revealed India’s vast solar potential, now being realised through large-scale solar power.
How much solar energy will be received by a 1 m 2 area in one hour, if the insolation on the surface of the Earth were 1 kW m −2 ?
Answer: E = Intensity × area × time
E = 1 × 1000 J s −1 m −2 × 1 m 2 × 3600 s
E = 3600000 J = 3.6 × 10 6 J
This is roughly the energy needed to melt 5 kg of ice and heat the water to 100°C — and equals about one unit of electricity used in a household.
An interesting estimation: how much of the Earth’s surface must be covered with solar panels to supply all the electric power India uses today? Find the numbers online, assume some insolation and a conversion fraction, and you will likely find that even a fraction of the Thar desert , if covered with solar panels, could meet India’s electricity needs.
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What is insolation?
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The amount of the Sun’s radiation that actually reaches the Earth’s surface. It warms the surface and the atmosphere above it. -
What is the value of the solar constant?
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About 1.4 kilowatts per square metre (1.4 kW m −2 ), or roughly 1400 joules per second per square metre — measured at the top of the atmosphere, before any absorption or scattering. -
Which surface has the higher albedo — snow or black soil?
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Snow. It reflects 0.80–0.90 of the sunlight falling on it, while black soil reflects very little and so heats up more. -
Which atmospheric layer holds nearly all weather phenomena?
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The troposphere (0–12 km), which is heated from the Earth’s surface and in which temperature falls with height.
- Electromagnetic (EM) waves — waves that travel through a vacuum at the speed of light, 3 × 10 8 ms −1 ; solar radiation reaches the Earth as EM waves.
- Electromagnetic spectrum — the entire range of electromagnetic radiation, from gamma rays and X-rays to microwaves and radio waves.
- Insolation — the amount of the Sun’s radiation that reaches the Earth’s surface.
- Solar constant — the average solar energy received per unit time per unit area perpendicular to the Sun’s rays at the top of the atmosphere — about 1.4 kW m −2 (1400 J s −1 m −2 ).
