Dylan said:Actually, warm air doesn't (actively) rise, but rather, COMPARATIVELY warm air is DISPLACED (within confined or semi-confined spaces) vertically by HEAVIER (denser) cool air which is descending by virtue of gravity. Cool air is more BUOYANT than is warm air.
Ever ridden in a cold-air balloon?
Hot air balloons are based on a very basic scientific principle: warmer air rises in cooler air. Essentially, hot air is lighter than cool air, because it has less mass per unit of volume. A cubic foot of air weighs roughly 28 grams (about an ounce). If you heat that air by 100 degrees F, it weighs about 7 grams less.