Biology Reference
In-Depth Information
Fog creeping into the bay from offshore. (Francis Parchaso)
the south. Warmer surface waters get pushed away, to be replaced by
deeper waters that are colder by as much as 15 degrees Fahrenheit. This
phenomenon gives rise to the fog so characteristic of Bay Area summers,
as evaporation from the ocean condenses above cold, upwelled waters.
The higher pressure of this cold air mass pushes a river of fog through the
Golden Gate and other Coast Range gaps toward the warmer, lower pres-
sure region of the Central Valley. As air over the Central Valley cools, ma-
rine air retreats, returning hot weather to the region again in a repeating
cycle.
When fall arrives, shorter days prevent Central Valley temperatures
from rising to summertime peaks. Diminishing temperature differences
between land and sea cause a series of new changes. The westerly winds
weaken, the fog machine stutters to a stop, and a period of warm, clear
days follows. This is why, for the Bay Area, September to November tend
to be the balmiest months of the year.
All too soon, Indian summer fades. The jet stream moves into Califor-
nia's latitudes, bringing with it cold polar air masses. Encounters with the
warmer southerly air above the Pacific often give birth to cyclonic storms
that drop rain over the Bay Area all winter long. In fact, about 80 percent
of the watershed's annual precipitation falls from November to March. The
heights of Mount Tamalpais ensure that the North Bay receives the lion's
