The humid east vs the dry west. Why is the west so dry?

Published: Oct. 24, 2021 at 9:28 PM MDT
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TWIN FALLS, Idaho (KMVT/KSVT) — If you were to take a trip from Southern Idaho to the east coast, the first part of the trip-through Idaho, Utah, and Wyoming-would look rather dry with lots of dry brush, and very little trees.

As you get into Nebraska however, the landscape will start to change. Dry brush will start looking greener and trees will begin to spring up. Why is this though?

The main reason is due to differences in annual precipitation in the United States. To the west of the Great Plains, areas only typically see about 5 to 10 inches of rain per year. Moving east, precipitation becomes greater – reaching 20 to 30 inches per year.

This is due in part to the increase of humidity in the air located to the east of a boundary separating dry air from humid air also known as a dryline. Why does this dryline exist though?

The main factor is the geographic features of the United States. For instance, the Gulf of Mexico and the lack of mountains in the southern states. This allows moisture from the Gulf to stream up into the eastern part of the country.

To the west, there are really two things that prevent that moisture. Mexico leaves us landlocked from any moisture streaming up into the area from the south.

Another thing we have to deal with is the Sierra Nevada mountains over California, Oregon, and Washington. This mountain range acts as a wall from any pacific moisture from streaming in.

As warm air ascends over the mountain from the west, any remaining moisture from the Pacific Ocean is condensed out into clouds. Because of this, the air dries out as it passes over the mountain, keeping air to the east of the mountain dry.

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