A foehn wind is the result of
high pressure inland, often
enhanced by
low pressure over
coastal waters.
Cold polar air settles in over the land, such as in the
Great Basin area of
North America, and begins spreading along the surface, since it is
dense. Guided southward by the
Sierra Nevada in the west and the
Rocky Mountains to the east, the
air mass picks up speed and also heats up due to compression as it flows to the lower altitudes of
Southern California, where it is known as the
Santa Anas. The air starts out dry, and as it ascends various
minor ranges along the way it tends to lose what
moisture it has. These winds tend to blow for days at a time (3-7 in the case of Santa Anas) because they are being driven by a huge air mass. Other foehn winds are due to similar mixes of
atmoshperic conditions and
geography.
The Santa Anas occur in the
autumn and
winter, when the land is cool relative to the ocean surface. The high pressure cells would persist for long periods if
low pressure systems didn't come barrelling down from the
Gulf of Alaska or the mid-
Pacific to break them up. In
spring and
summer the high is over the water and the low is over the land, since the land is then much warmer than the ocean surface, and the resulting
airflow is not
noteworthy enough to merit names.