Once the surface of the Earth becomes warmer than the interplanetary space surrounding it, its surface begins to emit radiation at a wavelength that is proportional to its temperature (see "Radiation from the Sun and from the Earth" on pages 128-134). Earth emits radiation in the infrared waveband, at wavelengths of approximately 3-30 pm ("pm" are micrometers, equal to millionths of a meter). Its radiation is most intense at about 10 pm. As the diagram on page 97 shows, water vapor absorbs radiation at wavelengths of 5.3-7.7 pm and above 20 pm. Carbon dioxide absorbs at 13.1-16.9 pm. Ozone absorbs at 9.4-9.8 pm. Between them, these gases would absorb all of the outgoing radiation if it were not for a "window," between 8.5 pm and 13.0 pm. There is no atmospheric gas that absorbs radiation at these wavelengths.
When molecules of water vapor, carbon dioxide, ozone, or one of a small number of other gases absorb outgoing infrared radiation they also begin to radiate and, because they are airborne, they radiate in every direction. Some of their radiation is directed upward, out of the atmosphere altogether. Some is directed to the sides, where it encounters other molecules and warms them, so they also start to emit radiation. Some is directed downward, to the surface, where it is absorbed. As this radiation bounces back and forth its wavelength changes. Whenever radiation at 8.5-13.0 pm moves upward, it escapes through the window and leaves the atmosphere. Eventually all of the radiation from the Earth's surface leaves Earth, but its departure is delayed.
During the day, the surface of the Earth absorbs solar radiation and grows warmer. It also radiates its heat away, but it continues absorbing
Radiation emission and absorption
heat faster than it loses it. Consequently, its temperature, and that of the air above it, rises, reaching a peak in the middle of the afternoon. As the Sun sinks lower in the sky the intensity of the sunshine decreases. The Earth's surface absorbs a decreasing amount of heat, but it continues to radiate its own heat. This continues through the night and the temperature falls as the Earth emits more radiation than it absorbs. The following morning the Sun rises, its radiation intensifies, and the balance shifts. The diagram shows how this daily cycle produces a period from about 9 a.m. to about 4 p.m. during which there is an energy surplus allowing the surface to grow steadily warmer. From 4 p.m. to 9 a.m. there is an energy deficit at the surface, so it becomes cooler. These times represent an average. Between the spring and fall equinoxes (in March and September, respectively, in the Northern Hemisphere), there are more hours of daylight than there are of darkness and so the surface absorbs more heat than it loses, with the positive heat balance peaking in the middle of summer. The opposite situation obtains between the fall and spring equinoxes, when there are more hours of darkness than of daylight and the surface loses heat.
Absorption of radiation by greenhouse gases. The radiation curve shows the radiation emitted from the Earth's surface, peaking at about 10 ^m wavelength. The shaded areas show the percentage of this radiation that is absorbed by particular gases at particular wavelengths.
Radiation and temperature during the day
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