A foehn wind is the result of high pressure inland, often enhanced by low pressure over coastal watersCold 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.