Adiabatic processes are those in which there is no net heat transfer between a system and its surrounding environment (e.g., the product of pressure and volume remains constant). Because it is a gas, air undergoes adiabatic heating and cooling as it experiences atmospheric pressure changes associated with changing altitudes. Increasing pressure adiabatically heats air masses, falling pressures allow air to expand and cool.
Adiabatic heating and cooling is common in convective atmospheric currents. In adiabatic heating and cooling there is no net transfer of mass or thermal exchange between the system (e.g., volume of air) the external or surrounding environment. Accordingly, the change in temperature of the air mass is due to internal changes.
In adiabatic cooling, when a mass of air rises—as it does when it moves upslope against a mountain range—it encounters decreasing atmospheric pressure with increasing elevation. The air mass expands until it reaches pressure equilibrium with the external environment. The expansion results in a cooling of the air mass.
With adiabatic heating, as a mass of air descends in the atmosphere—as it does when it moves downslope from a mountain range—the air encounters increasing atmospheric pressure. Compression of the air mass is accompanied by an increase in temperature.
Because warmer air is less dense than cooler air, warmer air rises. Counter-intuitively, moist air is also lighter than less humid air. The water , composed of the elements of oxygen and hydrogen is lighter than dominant atmospheric elements of oxygen and nitrogen. For this reason, warm moist air rises and contributes to atmospheric instability.
In the lower regions of the atmosphere (up to altitudes of approximately 40,000 feet [12,192 m]), temperature decreases with altitude at the atmospheric lapse rate . Because the atmosphere is warmed by conduction from Earth's surface, this lapse or reduction in temperature normal with increasing distance from the conductive source. The measurable lapse rate is affected by the relative humidity of an air mass. Unsaturated or dry air changes temperature at an average rate 5.5°F (3.05°C) per 1,000 feet (304 m). Saturated air—defined as air at 100% relative humidity—changes temperature by an average of 3°F (1.66°C) per 1,000 feet (304 m). These average lapse rates can be used to calculate the temperature changes in air undergoing adiabatic expansion and compression.
For example, as an air mass at 80% relative humidity (dry air) at 65°F (18.3°C) rises up the side of a mountain chain from sea level it will decrease in temperature at rate of 5.5°F (3.05°C) per 1,000 feet (304 m) until the changing temperature changes the relative humidity (a measure of the moisture capacity of air) to 100%. In addition to cloud formation and precipitation , the continued ascension of this now "wet" or saturated air mass proceeds at 3°F (1.66°C) per 1,000 feet (304 m). If the saturation point (the point at which "dry" air becomes "wet" air) is at 4,000 feet (1,219 m), the hypothetical air mass starting at 65°F (18.3°C) would cool 22°F (12.2°C) to 43°F (6.1°C) at an altitude of 4,000 feet (1,219 m). If the air ascended another 6,000 feet (1,829 m) to the top of the mountain chain before starting downslope, the temperature at the highest elevation of 10,000 feet (3,048 m) would measure 25°F (−3.9°C). This accounts for precipitation in the form of snow near mountain peaks even when valley temperatures are well above freezing . Because the absolute moisture content of the air mass has been reduced by cloud formation and precipitation, as the air moves downslope and warms it quickly falls below saturation and therefore heats at the dry lapse rate of 5.5°F (3.05°C) per 1,000 feet (304 m). A dry air mass descending 10,000 feet (3,048 m) would increase in temperature by 55°F (30.6°C). In the example given, the hypothetical air mass starting upslope at 65°F (18.3°C), rising 10,000 feet (3,048 m) and then descending 10,000 feet (3,048m) would measure 80°F (26.7°C) at sea level on the other side of the mountain chain.
Although actual lapse rates do not strictly follow these guidelines, they present a model sufficiently accurate to predict temperate changes. The differential wet/dry lapse rates can result in the formation of hot downslope winds (e.g., Chinook winds, Santa Anna winds, etc).
See also Air masses and fronts; Land and sea breeze; Seasonal winds