Even on the shadow-side of the Valley the frost
is never very sharp. The lowest temperature I ever observed during four
winters was 7 degrees Fahrenheit. The first twenty-four days of January
had an average temperature at 9 A.M. of 32 degrees, minimum 22 degrees;
at 3 P.M. the average was 40 degrees 30', the minimum 32 degrees. Along
the top of the walls, 7000 and 8000 feet high, the temperature was, of
course, much lower. But the difference in temperature between the north
and south sides is due not so much to the winter sunshine as to the heat
of the preceding summer, stored up in the rocks, which rapidly melts the
snow in contact with them. For though summer sun-heat is stored in the
rocks of the south side also, the amount is much less because the rays
fall obliquely on the south wall even in summer and almost vertically
on the north.
The upper branches of the Yosemite streams are buried every winter
beneath a heavy mantle of snow, and set free in the spring in
magnificent floods. Then, all the fountains, full and overflowing, every
living thing breaks forth into singing, and the glad exulting streams
shining and falling in the warm sunny weather, shake everything into
music making all the mountain-world a song.
The great annual spring thaw usually begins in May in the forest region,
and in June and July on the high Sierra, varying somewhat both in time
and fullness with the weather and the depth of the snow.
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