Humidity and Location in North America
Why is the east coast and mid-west of the US prone to
high humidity, and the west coast normally is not?
The part of the United States generally east of the Rocky Mountains
receives moisture from the Gulf of Mexico, particularly during the
warmer months of the year when the winds blow generally from the
south. The water temperature is fairly high in the Gulf of Mexico,
therefore the water evaporates more easily than Pacific Ocean water
does (which has a lower temperature). Furthermore, a more westerly
or northerly wind blows over the Pacific towards the west coast,
bringing drier air from the northern Pacific or the Arctic.
One exception to the "drier west coast" idea is the Pacific Northwest,
which during the cooler months of the year is usually humid, even rainy.
David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012