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difficult to breathe, even at these "modest" mountain altitudes. As an example, Denver is at an altitude of 1.6 km, meaning that its pressure is only 83% that at sea level. It is also interesting to compare pressure changes with altitude with those associated with weather changes on the Earth. Severe storms are usually associated with regions of low pressure. However, even the most severe storms only have a drop in pressure to about 90% of normal pressure. More typically, the weather changes at sea level produce pressure changes of only a few percent. So these changes are much smaller than the changes that one encounters by climbing mountains. This is why we often use pressure meters as altimeters, that is, devices that tell us how high above sea level we are. Another important constituent in the Earth's atmosphere is water vapor. Equation (23.13) is not a good description of its distribution. This is because the atmospheric temperature is close to that at which water condenses. The water vapor may have a normal distribution at low altitudes. However, it may be almost totally absent at the cooler, higher altitudes.

23.3.2 Temperature distribution

The temperature distribution with altitude is shown in Fig. 23.12. Notice that it is more complicated than the pressure distribution. The complexity in the temperature distribution reflects the variety of mechanisms by which energy

H Temperature vs. altitude in the Earth's atmosphere. Layers are divided according to important energy balance mechanisms, so temperature behavior differs from layer to layer.

H Temperature vs. altitude in the Earth's atmosphere. Layers are divided according to important energy balance mechanisms, so temperature behavior differs from layer to layer.

enters the atmosphere. The ultimate source of energy for the atmosphere is the Sun. However, it is not the direct source of heat over most of the atmosphere.

Most of the Sun's visible energy passes through directly to the ground, and is not absorbed by the atmosphere. This is true even if there are clouds. The clouds tend to scatter (rather than absorb) the visible radiation. This scattered light can either be directed back into space, and have its energy lost, or it can bounce around in the clouds, and eventually reach the ground. This explains why it is still light on a cloudy day, but not as light as on a clear day. The visible solar radiation reaches the ground, and some is reflected back (mostly from the oceans), and the rest is absorbed.

The heated ground then gives off radiation characteristic of its temperature, so the radiation from the ground is mostly in the infrared. Infrared radiation from the ground is then trapped in the lower atmosphere. Thus, the ground is the immediate source of energy for the lower atmosphere, explaining why the temperature in the lower atmosphere decreases as one moves farther from the ground. As the ground heats the air just above it, that air expands and rises. This convection is another means of energy transport from the ground to the lower atmosphere. (The above description is very simplified. In certain situations, "temperature inversions" are present in which warmer air is on top of cooler air, and there is little convection. With little convection, pollution can build up.)

The Sun's radiation that is absorbed goes into heating the ground. This process is shown in Fig. 23.13. If there were no atmosphere, the ground would heat up to the equilibrium temperature that we discussed above. However, the calculation of that equilibrium temperature was done on the assumption that all of the energy radiated by the Earth escaped into space. Since the Earth is at a temperature in the range 250-300 K, it is much cooler than the Sun, and its blackbody spectrum peaks at longer wavelengths. So, while the Sun gives off most of its energy in the visible part of the spectrum, the Earth gives off most of its energy in the infrared part. Many of the molecules in the lower atmosphere, especially

Fig 23.13.

(a) Diagram showing the greenhouse effect. (b) Global (meaning averaged over all measuring stations) tropospheric deviations from monthly average temperatures, from 1979 to 2000. [(b) NASA]

the water vapor, carbon monoxide and carbon dioxide, are very efficient at absorbing that infrared radiation. Therefore, instead of being radiated into space, some of the energy given off by the Earth is trapped in the lower atmosphere. This results in the surface of the Earth being hotter than if there were no atmosphere.

This effect, in which the visible light from the Sun heats the ground, and the infrared radiation from the ground heats the air above the ground, is called the greenhouse effect. The name results because this effect is similar to the way that greenhouses work. In a greenhouse, the window replaces the Earth's atmosphere. The window lets the visible radiation through, heating the ground. The ground then gives off infrared radiation, which is trapped by the windows, and the air inside the greenhouse is hotter than if there were no glass. (In real greenhouses, the blocking of the wind is also important.) This effect also works in your house. Sunlight can pass through the windows, heating the interior, which produces infrared radiation, which is trapped by the windows. This explains how you can have useful solar heating, even in the winter.

On the Earth, the greenhouse effect is modest. It raises the temperature by about 25 K. We will see in the next chapter that the presence of large amounts of carbon dioxide on Venus has produced an extreme greenhouse effect on that planet. This leads us to worry that a similar thing could happen on Earth, if we build up the concentrations of gases that trap the infrared radiation near the ground. That is why atmospheric scientists are concerned over the by-products of human activity, from fires, to automobile and factory exhausts,

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