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As an air mass moves in, once it hits some type of barrier it will cause friction and lift. The air is forced to rise. As it rises, it cools and the humidity increases. Colder air can hold less moisture than hot air. So as it rises it cools and then the moisture will fall out.
Typically that side of the mountain gets the precipitation and then on the other side as the air descends the exact opposite process occurs, descending air will become warmer and drier. That's why often times that side of the mountain will be more desert like.
Oh, I definitely understand the process as the air rises up and over a mountain range. But my question pertains to areas with only a small change in elevation experiencing rather substantial increases in precipitation.
For example, in So. Cal, the city of Pasadena averages nearly 22” of annual precipitation; nearby Los Angeles, by contrast, averages less than 15” — and there is only about 500 feet difference in elevation between the two cities and probably 12-15 air miles separating the two locales. And that situation applies to almost every other foothill community in the area. A difference of 500-700 feet seems to produce stark rainfall differences, but that’s not very much lifting at that level. I can see how air being lifted several thousand feet up and over a mountain produces increased rainfall, but I can’t see how that process happens down at the bottom of a mountain, where elevation changes are minimal.
I’m a bit of a geography as well as a weather/climate buff, and this one particular situation has always baffled me.
The east side of the highest ridges will get less precipitation than the west side, and FAR less than the higher peaks themselves.
Think of an air mass heading east on the prevailing winds, dumping some excess moisture along the way. The air mass hits the high mountains and has to rise to get over them. Rising air is going to cool meaning it can hold less moisture, so it rains or snows quiet heavily at times.
Once the air mass is past these high ridges, there is less moisture left in the system. Often times the storm systems themselves get broken up on the high ridges and either don't reform, or reform once they are 30-40 miles or so past the rain shadow.
Thanks for the informative weather/precipitation lesson.
Oh, I definitely understand the process as the air rises up and over a mountain range. But my question pertains to areas with only a small change in elevation experiencing rather substantial increases in precipitation.
For example, in So. Cal, the city of Pasadena averages nearly 22” of annual precipitation; nearby Los Angeles, by contrast, averages less than 15” — and there is only about 500 feet difference in elevation between the two cities and probably 12-15 air miles separating the two locales. And that situation applies to almost every other foothill community in the area. A difference of 500-700 feet seems to produce stark rainfall differences, but that’s not very much lifting at that level. I can see how air being lifted several thousand feet up and over a mountain produces increased rainfall, but I can’t see how that process happens down at the bottom of a mountain, where elevation changes are minimal.
I’m a bit of a geography as well as a weather/climate buff, and this one particular situation has always baffled me.
I assume it is still orographic lift.
Exactly where and why each spot gets the exact amount of precipitation is does is a bit beyond my knowledge, but I would guess local effects matter, like temperature being effected by mountain breezes maybe, the mountains themselves being so massive they slow the passage of air allowing the rain to "backfill" further down the mountains, etc.
Exactly where and why each spot gets the exact amount of precipitation is does is a bit beyond my knowledge, but I would guess local effects matter, like temperature being effected by mountain breezes maybe, the mountains themselves being so massive they slow the passage of air allowing the rain to "backfill" further down the mountains, etc.
Sounds like a worthwhile explanation. Makes sense to me.
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