MadSci Network: Earth Sciences

Re: Exactly what happens when a parcel of air rises?

Date: Sat Mar 17 17:21:21 2001
Posted By: Denni Windrim, Staff, science, Sylvan Learning Centre
Area of science: Earth Sciences
ID: 984446272.Es

Air rises (or falls) because of a difference in density between it and the 
surrounding air - for exactly the same reason that bubbles rise and rocks 
sink. Warm air is less dense than cool air; when air is warmed by passing 
over heated ground, it expands and becomes less dense. Cooler and therefore 
heavier air is able to move in underneath it and force it upward. Warm air, 
and bubbles, do not rise because they are pulled upward, rather because they 
are pushed upward from underneath. 

Compressing a gas forces its molecules closer together; the energy used in 
compressing the gas is transferred to the molecules, which results in 
heating. Conversely, as a gas is decompressed, it cools as its molecules 
move farther apart. Both these phenomena are easy to reproduce - if you've 
ever used a manual tire pump, you know how much the pump's cylinder warms up 
as you fill your tire. On the other hand, a compressed airflow from a spray 
cannister is always cool. Hairspray or spray deodorent, for instance, never 
feel warm when they contact your skin.

Depending on the moisture content of rising air, one of two things will 
happen: nothing, or condensation. A rising mass of warm, dry air will just 
rise. A rising mass of warm, moist air will form cloud. This happens because 
as the warm air rises and cools, it is not able to hold as much moisture in 
vapor form, and the vapor will condense out as water droplets. When moisture 
condenses, it releases heat and tends to make the air packet even less 
dense, providing additional buoyancy. In the presence significantly cooler 
air, such warm, moist packets result in summer thunderstorms.

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