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The frozen planet
The frozen planet

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5 Energy from the Sun

Although the Earth as a whole intercepts more or less the same amount of solar radiation from day to day, the amount received at any single point on the Earth’s surface varies dramatically because the Earth rotates on its axis once every 24 hours. From the point of view of a person standing anywhere on the Earth’s surface, the Sun appears to rise, travel across the sky and set. As a result, the angle at which solar radiation hits the Earth’s surface at any one point is constantly changing. On a summer’s day it is cool at dawn, but very hot by the afternoon, before again cooling at the evening. The location on the Earth is also important. If the Earth presented a flat circle towards the Sun, then the energy would be equally shared over the area of the circle (Figure 9a). But of course the Earth is not flat – it is a sphere.

  • Looking at Figure 9b, why would the solar energy received at the surface be different at low latitudes compared with high latitudes?

  • At high latitudes the light will travel through more of the atmosphere and when it reaches the surface it will be received over a larger area. So overall, the intensity will be reduced.

Figure 9
Figure 9 (a) The amount of energy that passes through a circle is spread over a sphere. (b) A schematic diagram showing why there is a difference in the amount of solar radiation reaching the Earth at different latitudes.

At higher latitudes the energy is reduced because its path through the atmosphere is longer and it is spread over a larger area. As a consequence, the equatorial regions will always be warmer than the polar regions simply because they receive more solar energy over the same area, but this is before the higher albedo of ice-covered land is taken into account (Section 4.2). The uneven heating of the Earth’s surface drives the weather and ultimately the global climate system.