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THE ADIABATIC PROCESS 

The adiabatic process is the process by which a gas, such as air, is heated or cooled, without heat being added to or taken away from the gas, but rather by expansion and compression. In the atmosphere, adiabatic and nonadiabatic processes are taking place continuously. The air near the ground is receiving heat from or giving heat to the ground. These are nonadiabatic pro-cesses.

However, in the free atmosphere somewhat

removed from Earth’s surface, the short-period processes are adiabatic. When a parcel of air is lifted in the free atmosphere, pressure decreases. To equalize this pressure, the parcel must expand. In expanding, it is doing work. In doing work, it uses heat. This results in a lowering of temperature as well as a decrease in the pressure and density. When a parcel of air descends in the free atmosphere, pressure increases. To equalize the pressure, the parcel must contract. In doing this, work is done on the parcel. This work energy, which is being added to the parcel, shows up as an increase in temperature. The pressure and den-sity increase in this case also.

Terms

In discussing the adiabatic process several terms are used that you should understand.

LAPSE RATE.— In general, lapse rate is the rate of decrease in the value of any meteorological element with elevation. However, it is usually restricted to the rate of decrease of temperature with elevation; thus, the lapse rate of the temperature is synonymous with the vertical temperature gradient. The temperature lapse rate is usually positive, which means that the temperature decreases with elevation.

INVERSION.— Inversions describe the at-mospheric conditions when the temperature in-creases with altitude, rather than decreases as it usually does. Inversions result from the selective absorption of Earth’s radiation by the water vapor in the air, and also from the sinking, or sub-sidence, of air, which results in its compression and, therefore, heating. Either effect alone may cause an inversion; combined, the inversion is stronger.

When air is subsiding (sinking), the com-pressed air heats. This frequently produces a subsidence inversion. When subsidence occurs above a surface inversion, the surface inversion is intensified. Such occurrences are common in wintertime high-pressure systems. The air in the inversion layer is very stable, and the cold air above the inversion acts as a lid trapping fog, smoke, and haze beneath it. Poor visibility in the lower levels of the atmosphere results, especially near industrial areas. Such conditions frequently persist for days, notably in the Great Basin region of the western United States.

An inversion is a frequent occurrence (espe-cially at night) in the Tropics and in the Polar regions. For night conditions all over the world, polar and tropical regions included, it may be said that low-level inversions are the rule rather than the exception.

ISOTHERMAL.— In the isothermal lapse rate, no cooling or warming is noted and the rate is neutral with height—no change in temperature with height.

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