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 same effect while applying hair spray or spray deodorant: The propellant gas cools as it expands out of the nozzle. Temperature changes of this type, in which heat energy is neither added nor subtracted, are called adiabatic temperature changes. In the cases described, the change in temperature is caused by a change in pressure. 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 expandable membrane of a determinable size. 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. We can also assume that no heat is transferred into or out of the parcel. Although this model is highly idealized, over short time spans, actual parcels of air moving up or down in the atmosphere behave in much this way. (In nature, a rising or descending column of air is sometimes infiltrated by the surrounding air, a process called entrainment. For the following discussion, however, we assume that no mixing of this type occurs.)

Dry Adiabatic Rate

Atmospheric pressure decreases with altitude. Any time a parcel of air moves upward, therefore, the pressure surrounding it gradually decreases. As a result, it expands and cools adiabatically. Unsaturated air cools at a constant rate of 10°C for every 1000 meters of ascent (17.5°F per 1000 feet). Conversely, descending air comes under increasing pressure and is compressed and heated by 10°C for every 1000 meters of descent (▼). 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).

Wet Adiabatic Rate

If an air parcel rises high enough, it will eventually cool to its dew point, triggering the process of condensation. The altitude at which a parcel reaches saturation and begins to form clouds is called the lifting condensation level (LCL), or simply condensation level. 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. Although the parcel will continue to cool adiabatically as it rises, 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 referred to as the moist, or saturated, adiabatic rate (refer to ▲).

Because the amount of latent heat released depends on the amount of moisture present in the rising air (generally between 0 and 4 percent), the wet adiabatic rate varies from about 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.

To summarize, rising air expands and cools at the dry adiabatic rate from the surface up to the lifting condensation level, after which it cools at the slower wet adiabatic rate.

• Cooling of air as it rises and expands due to decreasing air pressure is the basic cloud-forming process. Temperature changes that result when air is compressed or when air expands are called adiabatic temperature changes.

• Unsaturated air warms by compression and cools by expansion at the rather constant rate of 10°C per 1000 meters (5.5°F per 1000 feet) of altitude change, a quantity called the dry adiabatic rate. When air rises high enough, it cools sufficiently to cause condensation and form clouds. Air that continues to rise above the condensation level cools at the wet adiabatic rate, which varies from 5°C to 9°C per 1000 meters of ascent. The difference between the wet and dry adiabatic rates is due to the latent heat released by condensation, which slows the rate at which air cools as it ascends.

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