Terrestrial Radiation

The earth’s surface receives solar energy in short-wave form, but once heated, it does not keep that energy to itself. The surface itself becomes a radiating body and sends energy back out into the atmosphere — but now in long-wave form. This outward flow of energy from the heated earth is called terrestrial radiation. Because this radiation is long-wave, it behaves very differently from the incoming short-wave insolation, and this difference is the key to understanding how the atmosphere gets warm.

The atmosphere is not heated directly by the sun’s rays. Instead, it is heated from below — by the earth’s surface, which first absorbs solar energy and then re-radiates it upward. This long-wave radiation is readily absorbed by atmospheric gases, especially carbon dioxide and the other greenhouse gases. So the atmosphere is warmed indirectly, through the earth’s radiation, not directly by sunlight. Once warmed, the atmosphere itself radiates and transmits heat onward into space.

To see the full picture, the book lays out a precise accounting of energy in units, where the incoming solar radiation is taken as 100 units. Roughly 35 units are reflected back to space even before they reach the earth’s surface. Of these, 27 units are reflected from the top of the clouds, and 2 units from snow- and ice-covered areas of the earth. This reflected portion is called the albedo of the earth. The remaining 65 units are absorbed — 14 units within the atmosphere and 51 units by the earth’s surface.

Now the earth radiates back those 51 units as terrestrial radiation. Of these, 17 units go directly to space, and the remaining 34 units are absorbed by the atmosphere. That absorption happens in three ways: 6 units are absorbed directly by the atmosphere, 9 units through convection and turbulence, and 19 units through the latent heat of condensation. In total, the atmosphere absorbs 48 units — 14 from insolation and 34 from terrestrial radiation — and radiates all 48 back into space.

Adding it up: the radiation returning from the earth and the atmosphere together is 17 + 48 = 65 units, which exactly balances the 65 units received from the sun. This balance is called the heat budget or heat balance of the earth. It explains why the earth neither warms up nor cools down over time, despite the enormous transfer of heat constantly taking place. The amount of heat received from the sun is returned to space, and so a constant temperature is maintained at the earth’s surface and in the atmosphere.