Why is the western US so dry, but humid again at and beyond the Rockies?


Looking up several deserts in the US, such as Great Basin, Death Valley, and Mojave, wikipedia says they’re caused by the rain shadow effect. However as you move east into the Rockies, you get rain again. Where does this water come from if it’s been drained out by previous mountain ranges? And why do the Rockies themselves not cause nearly as severe of a rain shadow? The regions east of the Rockies from Canada to the US are not desert anymore.

I like climate science and can’t wrap my head around this one yet.

tl;dr why is the western US so dry but rainy again at and past the Rockies?

In: Earth Science

That’s because of the [Shadow Rain effect](https://www.treehugger.com/thmb/uw6aG-qv4JErLadhu98yjHgV5mQ=/1732×1732/filters:no_upscale():max_bytes(150000):strip_icc()/GettyImages-1184250907-fddb0d4c371641f681733c8f513e400d.jpg). The rotation of the earth create prevailing winds, when that wind go toward a mountain it drag the rain with it. The mountain will stop the clouds who will accumulate and drop the rain on the region. This leave little humidity for the other side of the mountain.

You can see the direction of the prevailing wind on the earth with this. If you look at a satellite map of the earth and look at South America. You can see that in Northern Chili the humid side is East, but in Southern Chili it’s the west side that is humid. The prevailing wind change direction and so not the same side of the mountain range stop the humidity.

There’s a big puddle of water nearby called the Gulf of Mexico. If you [go here](https://earth.nullschool.net/#current/wind/surface/level/orthographic=-93.18,35.60,1353), you can see a wind map and see air flowing north from the gulf towards Colorado and points north. There’s currently a high pressure system sitting there now that’s holding it back, but a low pressure system would have winds in the opposite direction and pull that air north..

Due to the rotation of the earth, wind in the northern hemisphere tends to go from the west to the east.

So the winds move over the pacific ocean absorbing water. When they get to land, the wind starts pushing up the Rocky Mountains. This causes the wind to cool down as the altitude increases. Cold air can absorb less water, so it rains. When it reaches the peak and begins moving down the other side of the mountains, it warms up again and can now absorb more water, but it has dropped the water it was carrying on the other side of the mountain. So now it sucks water up off the ground and does not rain.

Result – the western side of the mountains are wet. The Eastern side is a desert.

Picture it like a sponge, the windsponge is wrung out as it goes up one side of the mountains and sucks up water as it goes down the other side.