Q1.
Does the solar radiation heat the Earth's surface evenly?
Answer
No. Solar radiation is spread very unevenly over the Earth, and this unevenness is what drives the winds, the ocean currents and the water cycle.
Three separate reasons, all worth knowing:
- The Earth is a sphere, so latitude matters. Near the equator the Sun's rays strike almost perpendicular and a beam is concentrated on a small area. Near the poles the same beam strikes at a slanting angle and is spread over a much larger area, so each square metre receives far less energy.
- Different surfaces respond differently — albedo. Snow reflects 0.80 – 0.90 of the radiation falling on it and stays cold; black soil and ocean water reflect very little, absorb most of it, and become relatively warm. Land also heats faster than water.
- The axis is tilted, and the Earth revolves. This changes which hemisphere leans towards the Sun, giving the seasons and changing the length of the day — so even one place does not receive the same insolation through the year.
Intensity received on a horizontal surface, I = I0 cos θ
(θ = angle between the Sun's rays and the vertical at that place)
At the equator with the Sun overhead, θ = 0°: I = 1 kW m–2 × 1 = 1 kW m–2
At 60° latitude, θ = 60°: I = 1 kW m–2 × 0.50 = 0.5 kW m–2
Near the pole, θ → 90°: I → almost 0
(θ = angle between the Sun's rays and the vertical at that place)
At the equator with the Sun overhead, θ = 0°: I = 1 kW m–2 × 1 = 1 kW m–2
At 60° latitude, θ = 60°: I = 1 kW m–2 × 0.50 = 0.5 kW m–2
Near the pole, θ → 90°: I → almost 0
Why this matters: uneven heating means uneven air pressure, and air always moves from high pressure to low pressure. So this single fact is the starting point for everything in Section 13.2 — local winds, planetary winds and ocean currents.