How is it east United States is so humid and the west is so dry?
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The prevailing winds – the ones created by the effect of the earth rotating, go from west to east in the northern hemisphere.
The winds come over the Pacific Ocean picking up moisture, then hit to coast and climb the mountains. As the air rises, it cools because of the decreasing air pressure. As it cools, the amount of moisture it can carry decreases, and it rains. This is why the west coast is wet. By the time the air makes it over the first range and comes down the other side, it is bone dry – and we get dessert on the leeward side of the mountains.
As the air then moves across the continent it picks up what moisture there is, and the weather gets less and less dry the further east it goes.
All over planet earth, the windward side of mountains are wet, the leeward side is dry.
The prevailing wind over the US is west to east. The rocky mountains, however, block and divert that wind, causing most of it the moisture to fall as rain on the western side of the mountains. This is why much of the North West does get quite a bit of rain.
Those winds don’t really have a chance to reclaim any moisture until they start interacting with warmer, moister air coming up from Gulf and Atlantic oceans.
Rain shadows, they are caused by biggg mountain ranges https://en.m.wikipedia.org/wiki/Rain_shadow
Ocean currents. The Pacific ocean currents off the west coast flows from North to South hence cooler water along the coast. Since the ocean is cooler, less moisture makes it to the atmosphere overall. It’s the opposite in the East coast as the Atlantic ocean currents flow from the warm gulf of mexico and the equator up along the coast. In addition, there are relatively few cold fronts (mainly in the Southeast) so warmer air equals more moisture equals more humidity.
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