What causes humidity to be so high in areas such as
Baltimore and lower in areas such as Maine or New England States?
The further north, the more likely that moist air
from the Gulf of Mexico or the gulfstream Atlantic
will not reach you. Baltimore and other coastal
locations are more likely to be humid because of
the proximity of the ocean (this can be true even
along the coastline of Maine during a Bermuda high
pressure event). However, high pressure areas
centered off of the east coast allow large amounts
of water vapor to accumulate, particularly in the
southeastern and middle Atlantic states, and along
the seaboard as wet air blows in off of the ocean.
More inland and northern areas receive mostly
continental air (from the landmass) and this air
has not normally accumulated as much moisture.
You can get an idea of the amount of moisture in
the atmosphere by looking at an infrared water vapor
satellite view at either the Intellicast or Weather
Channel web sites. The water vapor shown is
throughout the depth of the atmosphere, but most
water vapor is in the lowest level, the Troposphere,
where we live.
meteorologist at Argonne Nat. Lab.
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Update: June 2012