Lucy Anne Mason, age 13, of Mullins, South Carolina, for her question:
What is meant by adiabatic change?
The word "adiabatic" is used by chemists and physicists who work with systems and experiments involving heat. They often interchange it with the word "isothermal," meaning same heat. Meteorologists also talk of adiabatic changes and processes that occur in everyday weather events.
The basic meaning of adiabatic might be expressed as the olden sentryman's cry, "Thou shalt not pass through." It is a word of boundaries and barriers coined by scientists to describe some of the tricky behavior of heat. Chemical activities always require energy to perform their miraculous changes. The energy is in the form of heat and a chemical change either absorbs or gives up a quota of heat energy. Heat may be absorbed from warmer surrounding objects, making them cooler, or given up to cooler surrounding objects, making them warmer. Heat tends to spread and share its energy and it does so by several methods.
The unusual occurs when a chemical activity keeps its heat exchange to itself. Some type of barrier prevents changing temperatures from involving their surroundings. This is an adiabatic, or self contained, change. When gas is compressed, it becomes warmer and the heat tends to spread. If the gas is sealed in a metal tank and compressed, the heat cannot spread and we have an adiabatic change.
The weathery atmosphere is a great mixer and mingler. We would not expect it to produce any adiabatic changes. But it does. Masses of air gain heat and lose heat in adiabatic changes. When air is warmed by the sun baked ground, its molecules use the extra heat energy to spread out. The air expands, becomes thinner and rises aloft. The expansion automatically causes it to lose heat and become cooler. An airy activity involving a gain and loss of heat takes place within a closed, adiabatic system.
Dramatic adiabatic heating and cooling occur when a windy air mass blows up, over and down a mountain. As warm air rises up from the sunny valley, it expands and the pressure around it decreases. It loses heat at an average rate of ten degrees Fahrenheit for each 2,000 feet it rises. This is an adiabatic cooling process. The cool air moves up and over the hump. Descending down the other side, the process is reversed. The air compresses and this makes it warmer. Coming down the mountain, the air warms up ten degrees through each 2,000 feet it descends. The warming adiabatic change equals the rate of the cooling adiabatic change. The airy maneuver was a barrier system in which no outside heat is added or subtracted from the cooling and warming air mass.
A lot of adiabatic heating and cooling goes on throughout the weathery atmosphere. The surface air gets its original warmth from the sunny land and s a. Warm air tends to soak up moist water vapor and carry it aloft. But cooler air can hold less vapor. A mile above the ground the air may drop 20 degrees. This cooler air may have to shed some surplus water vapor. The gaseous vapor then changes into liquid droplets of mist. This kind of adiabatic cooling creates most of our high flying, clouds.