If warm air molecules are spread far apart, then why high in the sky are cooler ones spread far apart too? I have recently read a book that had said, molecules in warm air are spread farther apart than cool air molecules. And that the higher you get, the cooler it gets. So up higher in the air where it is coolest, you would assume that the air molecules would be closer together. Yet in this book it also said the higher you get the more spread out the air molecules get. And if it is cold air how can this be?

Ian, you're confusing two different density effects (or perhaps your teacher is).

A gas, in principle, expands to fill any container of any size, no matter what its temperature is (until it liquifies). It's liquids and solids that change density with temperature. What gases do is change pressure with temperature (and they also change temperature with pressure: compressing a gas heats it up, and allowing it to expand--thus reducing the pressure--cools it down). A small sample of a gas in a closed, rigid container will not shrink if you cool it down; the molecules will not get "closer together." However, the pressure the gas exerts on the container will get smaller.

The thing that makes pressure fall off with distance from the Earth's surface is not temperature, but gravity. As you get further away from the Earth's center, the gravitational force falls off too, and the air is less tightly held--so it has a lower pressure (the molecules are further apart). Furthermore, air at the surface is "pushed down on" by the weight of all the air above it, and this increases the pressure. As you go up, there is less and less air "pushing down" on the air you are looking at, so again the pressure falls off.

The temperature gradient as you go up in the atmosphere is partly an effect of falling pressure--but not entirely. There are "hot zones" in the upper atmosphere, where the temperature is higher than it should be on strictly pressure considerations. This is caused by absorption of hard ultraviolet light from the sun by the ozone layer, among other things.

Dan Berger