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Cloud Characteristics, Continued
Fog
Fog at sea is frequently formed through the process known as advection
(the transport of an atmospheric property solely by the mass motion of
the atmosphere). If warm air that passed over warm water moves to an
area where the water is colder, fog is likely to develop in the latter
region. The temperature of seawater is fairly uniform within a large
area and accounts for fog that often lasts for many days and nights at
sea.
The great fog banks of the North Atlantic, as well as those around the
Aleutians, demonstrate what can happen when two adjacent bodies of
water have markedly different temperatures. In the vicinity of
Newfoundland, warm air that has passed over the warm Gulf Stream
quickly turns to fog when it strikes the inshore current of very cold
water that flows southward along the coastline. Off Alaska, the same
situation prevails when the air from over the warm Japanese stream
(Kuroshio) comes in contact with the cold southward-flowing waters of
the Bering Sea (Oyashio).
Along coastlines special conditions may exist. Onshore winds blow
warm, moist air inland from the ocean. The waters adjacent to the coast
are sometimes colder than those farther offshore. At night an onshore
wind lays down a thick blanket of fog that often extends some distance
inland. The fog hangs on until the Sun heats up the land enough to
evaporate the droplets or until an offshore wind drives the fog blanket
away.
How can you tell when a fog is on the way or in the process of
formation? The difference between the temperature shown by the wet
bulb and the dry bulb of the psychrometer, called wet-bulb depression, is
your fog indicator.
In general, fog forms when the depression is 4° or less. A continuous
record of the wet-bulb depression serves as a fairly reliable predictor of
fog.
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