The issue of rain on the windward side of mountains arose in my trip last week. My dad attributed the phenomenon to cooling of the rising air mass. The reasoning is that air is cold at high elevation, so as the air gets higher, it cools, reaches dewpoint, and starts releasing water. After further study, fundamentally, it is correct that the air cools and reaches dewpoint. However, the cause for the cooling is kinda ... cool. It's called orographic lifting. Basically, the air expands as it goes to high altitude. As it expands, the air does work on the air that it displaces. The work energy had to come from somewhere. It came from the energy of the air molecules. Temperature is, basically, a measure of the average speed of the air molecules.
For further reference, see
http://en.wikipedia.org/wiki/Orographic_lift
On that page, there is a beautiful picture of clouds being formed at the crests of waves, called gravity waves. Orographic lifting can cause air at the crests of the waves to reach dewpoint and form clouds.
Clouds are created at the crests of gravity waves due to orographic lifting. These waves are created as air flows over an island in the southern Indian Ocean.
For more on gravity waves, see
http://en.wikipedia.org/wiki/Gravity_wave
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3 comments:
Interesting. When we see clouds on the sides of mountains, I guess these are crests of gravity waves?
Hmmm. Well, gravity waves are something more unusual than your run of the mill clouds on the sides of mountains. Run of the mill mountain clouds are caused by orographic lifting (the same *cause* as gravity wave clouds, but a little different scenario).
as the air is forced higher over mountain ridges gravity is reduced and as the air expands the energy in the air is diluted and must be cooler as a result. but the water to air molecule ratio remains the same, so as the air cools clouds must form.
could it be the air is actually just on a free ride and not actually doing "work".
perhaps the air traveling up a mountain ridge is being "crowded" by the air behind which is where the work is coming from.
steve
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