Why does warm air rise & cool as it expands?

Written by tyler lacoma
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Why does warm air rise & cool as it expands?
Diagram of rising air forming clouds, courtesy www.windows.ucar.edu

To understand why warm air expands and becomes cooler as it rises, it is necessary to understand some of the properties of air. Scientists call this study of air temperature and altitude the study of adiabatic processes, and refer to changes in temperature as adiabatic heating or cooling. This means that although the air moves, expands, and becomes warmer or cooler, no energy is gained or lost throughout its cycles.

Think of a pocket of warm air close to the earth's surface. This pocket of air is usually called a "parcel" by meteorologists, and it can be warmed by the sun, by the heat radiating from the earth, or by heat caught in specific areas by winds or tides. This thermal energy is transmitted through the parcel of air as kinetic energy, causing all of its molecules to move faster, vibrating and bouncing off of each other. Cooler air moves more slowly, and since its molecules jostle each other less, they can linger closer together. The molecules of hot cannot have such close quarters, however, and in their rapid movement they make the parcel of air less dense. This less-dense air becomes buoyant compared to its surroundings, and rises.

Rising Air

Now the molecules of the heated air have room to move around more freely, and continue to do so as long as the heat gives them extra kinetic energy. The higher the hot air floats, the more room there is for its molecules to move about, and so it continually expands, its energy pushing it out and up. Here the air runs into a balancing problem due to the laws of thermodynamics. The energy from all of its molecules can be used to propel the molecules out at greater distances so it can expand more, or the energy can be used to keep the air hot, but it cannot do both at the same time.

The energy of the parcel is always the same, no matter what the air is doing. If this energy is stretched out over a wide area, it cannot create heat by being concentrated in a single spot. The farther away from each the molecules moves, the lower the temperature of the parcel will become.

Rising Air and Weather Patterns

This means that in the end, the rising air will eventually reach the same temperature as all the air around it, and will cease to rise. Now part of the cooler air higher in the atmosphere, it will stay there until the cool temperatures and changing climate conditions move it back down, and the process is repeated. This rising and expanding of air is an extremely important part of all weather patterns, from the formation of hurricanes to the dry and wet seasons of alternating climates. Scientists have created formulas that predict how much heat the air will lose based on its moisture level and how high it is rising. It should be noted that this cooling effect is not the same as the natural cooling of air higher in the atmosphere, due to its distance from the earth's surface.

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