Warm Air

The piece that follows recently posted to an AOL weather newsgroup. I think the author does a great job explaining why great thermals depend upon sinking air. I've been making the same pitch about this for years but probably never quite as succinctly as it's been done here.

"Myth: Warm air automatically rises, pulling in cooler air to take its place.

Reality: Nothing with mass (and all air has mass) will move against the force of gravity unless propelled by a stronger force. Warm air will simply stay put, slowly expanding and becoming less dense, until denser air gets underneath it and forces it up. When that happens, the warm air will indeed rise. It will continue to rise as long as it is being pushed up. When the upward force stops, the warm air will stop rising.

As that warm air rises, it must do work against the pull of gravity. This causes it to cool at the rate of 5.4¡F per thousand feet of atltitude. If it continues to rise, at some elevation its temperature will equal the dew point for that air mass. Condensation usually occurs when that happens, and a cloud will form. Cumulus clouds often have flat bottoms because the rising air mass that forms them reaches the dew point at the same altitude throughout. Clouds are the visible tops of invisible masses of rising air.

Although warm air will not rise spontaneously, cool air will sink spontaneously. This is because gravity is working with the movement, not against it. It works to pull denser air down toward the earth's surface. This is what causes cold air to move down hillsides and mountain slopes and collect in low places.