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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 (CO₂, CH₄, 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 ).
