HOT Weather from Cooler Air??

[asker]

For years and years I've wondered this...

If cooler air is heavier than warm air, why does it get so insanely hot here in the Calif. Central Valley from a high pressure system? Shouldn't it get cooler because the cooler air is on the bottom like when a cold front comes in?  I have read that it is something to do with air above sinking and compressing but I still don't get it.

[snow_wizard]

Just saw this one even though it was posted quite a while ago.  In a situation like the valleys of CA west of the Sierras are in, the cold air (which mainly slides down east of the mountains) creates higher pressure east of the mountains which forces the air to flow westward toward the valleys and the ocean.  When the air is forced over the mountains it compresses the air that is in place west of the mountains which warms and dries the air.  Even though the air begins cold it ends up warm at the surface in the valleys west of the mountains.  If the air mass east of the mountains is really cold then the valleys west of the mountains can also end up somewhat cold.

Even up in WA State, which is where I live, this same thing happens although the air is often a lot colder to begin with than what you experience down there.  In spite of that there is a still a lot warming as the air compresses west of the mountains.