Fun Weather Facts: Warm Air Rises and Cold Air Sinks---Doesn't it? Part II

Fun Weather Facts: Warm Air Rises and Cold Air Sinks---Doesn’t it? Part II

Last time you learned that through everyday experience you already know that air can expand and compress, and that expanding air cools and compressing air heats. 

There is one other thing that you already know about the atmosphere that is needed to complete the explanation.  Remember all those movies where a small hole is put in the cabin of a passenger jet flying at 30,000 ft and the 300 lb man sitting next to the hole is amazingly sucked (actually pushed by the normal pressure in the cabin) out of it?  Or----how hard it is to catch a breath while climbing at 10,000 ft, but much easier at the surface?

So--- you already knew that air pressure in the atmosphere goes down from being highest at the surface of the earth to nothing at the edge of the atmosphere. Now you are ready to move into that rarified group of people who actually know why the cold air above us doesn’t sink and turn us into ice cubes.

Pretend you are a nice hunk of air sitting on the surface of the earth minding your own business when all of a sudden the sun comes out from the clouds and starts heating you up.  You look around, and when you find you are warmer than the air around you that is still in the shade, you start to rise.  Up you go, but since air pressure is decreasing as you go up, you have to expand.  It feels good to expand, but now you know that by expanding you have to cool.  As you continue to rise you continue to cool from expanding.

As long as you are warmer than the air around you, you will keep rising, but since all good things always come to an end, eventually you will be the same temperature as the air around you and will stop rising.  You are definitely cooler than the air below you, but will you sink? 

Suppose you want to get back to the surface and try to come back down.  As soon as you try to descend (unfortunately for you) the air pressure increases as you go down and you are compressed.  Uh oh, bad news----now you are warming from compression and finding yourself warmer than the air around, and you go back up.

Even if someone pushed you all the way to the surface, when you got there you would have warmed up from compression.  So much for your evil plans of turning everyone into ice cubes.

On days where the air temperature rapidly goes down with height, bubbles of warm air from the surface can travel quite high into the atmosphere and even cause severe weather.  On days where the temperature increases with height (an inversion) bubbles of warm air can’t rise very far at all.

Why is a room in a house different?  The distance warm air rises in a room is too short to have any effect on cooling of the air from expanding.  Not like in the atmosphere where the distances are much greater.