Air is made up of a mixture of gases that is subject to adiabatic heating when it is compressed and adiabatic cooling when it is expanded. As a result, air rises seeking a level where the pressure of the body of air is equal to the pressure of the air that surrounds it. There are other ways air can be lifted, such as through the thermodynamic pro-cesses of a thunderstorm or mechanically, such as having colder, denser air move under it or by lifting as it flows up over a mountain slope.

As the air rises, the pressure decreases allowing the parcel of air to expand. This continues until it reaches an altitude where the pressure and den-sity are equal to its own. As it expands, it cools through a thermodynamic process in which there is no transfer of heat or mass across the bound-aries of the system in which it operates (adiabatic process). As air rises, it cools because it expands by moving to an altitude where pressure and den-sity is less. This is called adiabatic cooling.

When the process is reversed and air is forced downward, it is compressed, causing it to heat. This is called adiabatic heating, (See fig. 2-4-1.) Remember, in an adiabatic process an increase in temperature is due only to COMPRESSION when the air sinks or subsides. A decrease in temperature is due only to EXPANSION when air rises, as with convective currents or air going over mountains. There is no addition or subtrac-tion of heat involved. The changes in temperature are due to the conversion of energy from one form to another.

The atmosphere has a tendency to resist ver-tical motion. This is known as stability. The normal flow of air tends to be horizontal. If this flow is disturbed, a stable atmos-phere resists any upward or downward dis-placement and tends to return quickly to normal horizontal flow. An unstable atmos-phere, on the other hand, allows these upward and downward disturbances to grow, result-ing in rough (turbulent) air. An example is the towering thunderstorm that grows as a result of a large intense vertical air current.

Atmospheric resistance to vertical motion (stability), depends upon the vertical distribution of the airs weight at a particular time. The weight varies with air temperature and moisture content.

Figure 2-4-1.Adiabatic cooling and heating process.

As shown in figure 2-4-2, in comparing two parcels of air, hotter air is lighter than colder air; and moist air is lighter than dry air. If air is relatively warmer or more moist than its surroundings, it is forced to rise and is unstable. If the air is colder or dryer than its surroundings, it sinks until it reaches its equilibrium level and is stable. The atmosphere can only be at equilibrium when light air is above heavier air-just as oil poured into water rises to the top to obtain equilibrium. The stability of air depends a great deal on temperature distribution and to a lesser extent on moisture distribution. Since the temperature of air is an indication of its density, a comparison of temperatures from one level to another can indicate how stable or unstable a layer of air might bethat is, how much it tends to resist vertical motion.

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