Lv 4
Travis asked in Science & MathematicsWeather · 2 months ago

Why is the Western US drier than the Eastern US ?

6 Answers

  • 2 months ago
    Favorite Answer

    Not exactly true, because the Northwest is actually quite wet (some of the rainiest and/or cloudiest cities in the country).  The southwest is dry because it is in the latitude range that involves formation of High Pressure systems (descent of the Hadley Cell and Mid-latitude Cell air systems that bring dry air from up high down to surface).  This is similar to the reason that the Sahara is dry and Australia is (mostly) dry. Same basic global air circulation patterns.

    The equivalent latitudes to the east in the US are provided with ample moisture from the Gulf of Mexico, where the water is very warm.  Water along the west coast of the US is cycling down from the north so is "cold" and thus not a good source of moisture.  In effect, the US southeast gets a lot of moisture that it would not get if that warm Gulf of Mexico water body wasn't there to provide it.

    Both the southwest and southeast US get "Monsoons" or hurricanes, big rainstorms that form near the equatorial regions and migrate northward.  the southwest gets less but still gets plenty of rain in the wet season.  The dry season, though, is really dry.  the gulf region (southeast) is notable for its extremely high humidity, and that humidity comes from evaporation of the warm waters in the Gulf of Mexico, in large part.  The Rocky Mountains also play a role in blocking the migration of dryer air from the west into the southeast, so the air in the southeast tends to come more form the south than the west.

    It is a bit complicated in detail, but the general issue is simply one of global wind and water current patterns.  20-40 degrees North, and 20-40 degrees South are regions that get lots of dry air from above and are thus dry regions.  Look at a map of the world and check it out.

  • bill
    Lv 5
    2 weeks ago

    Lake effects have a lot to do with it in the great lakes area.

  • TQ
    Lv 7
    2 months ago

    More evidence Asker's are incapable of evaluating answers.


    Here are the real reasons why it rains on the east on most land masses in the Northern Hemisphere. They're related directly to the semi-permanent areas of HIGH pressure located offshore of both east and west coast.

     On the east coast .. the clockwise circulation are the HIGH transports warm moist tropical air into the mid-latitudes. The northerly motion away from lower latitudes increases deep vertical layer rotation (vorticity through the conservation of angular momentum) producing upward vertical motion.

     Upward vertical motions leads to condensation ... clouds ... and precipitation.

    The final reason is related to the westerlies (sometimes referred to incorrectly as the jet stream). The westerlies accelerate as they move off land and out over open water which favors the development of a long wave trough of LOW pressure where upward vertical motions are also present.

  • 2 months ago

    It is all to do with the earth's weather patterns as a whole!

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  • 2 months ago

    For one thing the off coast ocean current is cold on the west coast, so it doesn't put as much moisture into the air.

    Secondly the Gulf of Mexico feeds moisture north and up the Mississippi river.

    Thirdly the Great Lake effects. Which are better known for generating blizzards but also generate rain storms.

  • 2 months ago

    Because of the height of the Rocky Mountains.

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