Describe adiabatic temperature changes and explain why the wet adiabatic rate of cooling is less than the dry adiabatic rate.

Recall that condensation occurs when sufficient water vapor is added to the air or, more commonly, when the air is cooled to its dew-point temperature. Condensation may produce dew, fog, or clouds. Heat near Earth’s surface is readily exchanged between the ground and the air directly above. As the ground loses heat in the evening (radiation cooling), dew may condense on the grass, while fog may form slightly above Earth’s surface. Thus, surface cooling that occurs after sunset produces some condensation. Cloud formation, however, often takes place during the warmest part of the day—an indication that another mechanism must operate aloft that cools air sufficiently to generate clouds.

The process that generates most clouds is easily visualized. Have you ever pumped up a bicycle tire with a hand pump and noticed that the pump barrel became very warm? When you applied energy to compress the air, the motion of the gas molecules increased, and the temperature of the air rose. Conversely, if you allow air to escape from a bicycle tire, the air expands; the gas molecules move less rapidly, and the air cools. You have probably felt the cooling effect of expanding propellant gas as you applied hair spray or spray deodorant. The temperature changes just described, in which heat energy is neither added nor subtracted, are called adiabatic temperature changes: that is, temperature changes that result from changes in pressure rather than changes in heat energy. When air is compressed, it warms, and when air is allowed to expand, it cools.

Adiabatic Cooling and Condensation

To simplify the discussion of adiabatic cooling, imagine a volume of air enclosed in a thin balloon-like bubble. Meteorologists call this imaginary volume of air a parcel. Typically, we consider a parcel to be a few hundred cubic meters in volume, and we assume that it acts independently of the surrounding air. It is also assumed that no heat is transferred into or out of the parcel. Although this image is highly idealized, over short time spans, a parcel of air behaves much like an actual volume of air moving up or down in the atmosphere.

Recall from Chapter 1 that atmospheric pressure decreases with height. Any time a parcel of air moves upward, it passes through regions of successively lower pressure. As a result, ascending air expands and cools adiabatically. Unsaturated air cools at a constant rate of 10°C for every 1000 meters of ascent (5.5°F per 1000 feet). Conversely, descending air undergoes increasing pressure and is compressed and heated 10°C for every 1000 meters of descent (Figure 4.15). This rate of cooling or heating applies only to unsaturated air and is known as the dry adiabatic rate (“dry” because the air is unsaturated).

If an air parcel rises high enough, it will eventually cool to its dew point and trigger the process of condensation. The altitude at which a parcel reaches saturation and cloud formation begins is called the lifting condensation level (LCL). At the lifting condensation level, an important change occurs: The latent heat that was absorbed by the water vapor when it evaporated is released as sensible heat—energy that can be measured with a thermometer—as condensation takes place. Although the parcel will continue to cool adiabatically, the release of latent heat slows the rate of cooling. In other words, when a parcel of air ascends above the lifting condensation level, the rate at which it cools is reduced. This slower rate of cooling is called the wet adiabatic rate (also commonly termed the moist or saturated adiabatic rate).

The amount of latent heat released depends on the quantity of moisture present in the air (generally between 0 and 4 percent). Therefore, the wet adiabatic rate varies from 5°C per 1000 meters for air with a high moisture content to 9°C per 1000 meters for air with a low moisture content. Figure 4.16 illustrates the role of adiabatic cooling in the formation of clouds.

• adiabatic temperature change: The cooling or warming of air caused when air expands or is compressed, not because heat is added or subtracted.

• dry adiabatic rate: The rate of adiabatic cooling or warming in unsaturated air. The rate of temperature change is 1°C per 100 meters.

• lifting condensation level (LCL): The height at which rising air that is cooling at the dry adiabatic rate becomes saturated and condensation begins.

• parcel: An imaginary volume of air enclosed in a thin elastic cover. Typically a parcel is considered to be a few hundred cubic meters in volume and is assumed to act independently of the surrounding air.

• wet adiabatic rate: The rate of adiabatic temperature change in saturated air. The rate of temperature change is variable, but it is always less than the dry adiabatic rate.

• When air expands, it cools, and when air is compressed, it warms. Temperature changes produced by means of changes in air pressure, in which thermal energy is neither added nor subtracted, are called adiabatic temperature changes.

• When air rises, it expands and cools adiabatically. If air is lifted sufficiently, it will cool to its dew-point temperature, and clouds will develop.