s
certain, although, owing to the slight snow accumulations of the former,
its streams are decidedly variable. A heavy rain-belt, with a normal
fall of more than 40 in., covers all the northern half of the Sierra and
the north-west counties; shading off from this is the region of 10-20
in. fall, which covers all the rest of the state save Inyo, Kern and San
Bernardino counties, Imperial county and the eastern portion of
Riverside county; the precipitation of this belt is from 0 to 10 in. In
excessively dry years the limits of this last division may include all
of the state below Fresno and the entire Central Valley as well. In the
mountains the precipitation increases with the altitude; above 6000 or
7000 ft. it is almost wholly in the form of snow; and this snow, melting
in summer, is of immense importance to the state, supplying water once
for placer mining and now for irrigation. The north-west counties are
extremely wet; many localities here have normal rainfalls of 60-70 in.
and even higher annually, while in extreme seasons as much as 125 in.
falls. Along the entire Pacific Coast, but particularly N. of San
Francisco, there is a night fog from May to September. It extends but a
few miles inland, but within this belt is virtually a prolongation of
the rainy season and has a marked effect on vegetation. Below San
Francisco the precipitation decreases along the coast, until at San
Diego it is only about 10 in. The south-east counties are the driest
portions of the United States. At Ogilby, Volcano, Indio and other
stations on the Southern Pacific line the normal annual precipitation is
from 1.5 to 2.5 in.; and there are localities near Owen's lake, even on
its very edge, that are almost dry. For days in succession when it
storms along the Southern California coasts and dense rain clouds blow
landwards to the mountains, leaving snow or rain on their summits, it
has been observed that within a few miles beyond the ridge the contact
of the desert air dissipates the remaining moisture of the clouds into
light misty masses, like a steam escape in cold air. The extreme heat of
the south-east is tempered by the extremely low humidity characteristic
of the Great Basin, which in the interior of the two southernmost
counties is very low. The humidity of places such as Fresno, Sacramento
and Red Bluff in the valley varies from 48 to 58. Many places in
northern, southern, central, mountain and southern coastal California
normally