Why are eastern Oregon, southeastern Washington and southern Idaho so arid, as compared to the humid and lush western Oregon and Washington? In Oregon and Washington there is quite an abrupt boundary around the 121st longitude. Is the Cascade Range really that high to block most rain clouds from going further east? I've got the impression that only exceptional mountains of the range (volcanoes such as Mt Hood and Mt Adams) would be high enough for that.
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There are mountain ranges (Coast and Cascade Range) perpendicular to the main wind direction. Mountains don't "block", they force the air on the windward side up, thus cooling it, and the water it contains to condensate end eventually rain out. That makes the windward side much wetter than the leeward side, where the air then arrives much drier, and in cases also warmer.
See also: https://en.wikipedia.org/wiki/Climate_of_Oregon and the same for Washington.
There's more: When conditions are just right an effect called foehn develops, with a characteristic wall over the main ridge and Altocumulus lenticularis on the leeward side, with hot, very dry and windy conditions there.
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$\begingroup$ Why doesn't the same happen in eastern North America around the Appalachians? Both sides of the Appalachians are humid and lush. $\endgroup$– user29677Sep 22 at 12:11
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$\begingroup$ That's a new question :-) $\endgroup$– user29219Sep 22 at 13:20
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$\begingroup$ Note that "rain shadow" is a much more common term among non-meteorologists (at least in AME). $\endgroup$ Sep 22 at 19:36
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$\begingroup$ @Hannes not familiar with the geography you mention, but in fluid dynamics one compute the Burger number to evaluate if a fluid flow will circualte around or above an obstacle. Similarly, see this figure from a publication I did not read: ars.els-cdn.com/content/image/3-s2.0-B0122270908002906-gr4.jpg (check the threshold line flow over/flow around) $\endgroup$– EarlGreySep 24 at 20:47
