The Mobridge news. [volume] (Mobridge, Walworth Co., S.D.) 190?-1917, 1912-07-26
IN the AIR
Curious rlay Of the Winds IN the
Yosemite Valley.
EBB AID FLOW WITH THE SUN

These Air Currents or Mir Falls Arc 60
Regular That They May Almost Be
Timed-Mirror Lake and The Spray
Combs at Bridal Veil Falls.
Did you know there are air falls in
the atmosphere just as real and ap-
parent as are the waterfalls you have
so often viewed with admiration and
delight because of their natural beau
ty? In the famed Yosemite valley the
most interesting feature is, to the sci
enlist perhaps. its winds.
The winds there are seldom more
than light zephyrs. moody and capri
cious to the ordinary tourist, but when
rightly understood n Of the wonders
Of the valley. These interesting facts
are told by Professor T.. Il. MattheS of
the United States geological survey in
the Sierra Club Bulletin.
In no other place in the entire world
perhaps are the air currents more syS-
tematic and regular than in the Yosem-
tie valley, he says. In the first place,
the sun naturally heats the ground
more rapidly than it does the air.
Thus every hillside basking in the sun
becomes a heat radiator and gradually
warns the air above it. so that the air,
becoming lighter, begins to rise.
But under these conditions the air
does not rise vertically because the air
directly over it is still cool and is press
ing downward. Therefore up the sides
Of the warm slope the heated air makes
its way. That is why the tourist max
ing his way up the mountain slope
with the sun on his back finds his own
dust travelling upward with him in IA
choking cloud.
But on coming down the same trail,
when the face of the slope is in the
shadow. the dust ever descendS with
the traveller in the same irritating
cloud. When the face of the mountain
is in the shade the air is cooling from
the face Of the slope and is pressing
its way down into the valley.
Just as soon rs the sun leaves the
slope Of the mountain the earth be-
gins to lose it's heat by radiation and
In u very short time is really cooler
than the air. The layer Of air next
the face Of the hillside chills by con
tact with the earth and. becoming
heavier as 1t condenses, begins to press
down along the slope. Thus there are
normally the warm up draft on the
sunny slope and the cold down draft
on the side in the shadow. In a wind
less region like the Yosemite this rule
may be depended upon at almost any
time.
But in the Yosemite. with its bold
cliff topography, these upward and
downward air currents are somewhat
interrupted. On every sunny slope
bold cliffs create shadows, and conse-
quently there are downward air cur
rents or local breeZes daily at regular
hours as the shadows come and go.
Glacier Point is one place in particu-
lar in which Professor Matthes says
this shadowy effect on the air cur
rents may readily be tested by casting
small bits of paper into the air. AS
the afternoon wears on and the shad-
W in the valley gather the cold draft
in the hills pours downward. forming
the valley like a great river and flow
ing On to the plains below. Every side
canyon and valley sends its reinforce
ments, like the tributaries of a great
river. to this general air current flow
ing onward to the plain.
With the return of the morning sun
the earth at the tops Of the hills is
warmed and the downward current in
the air is suspended. The up draft soon
begins as the sun shines into the val
legs. The air currents are so regular
that they may almost be timed.
Few realize. says the author of the
paper. that it is on these reversing air
currents that n of the chief attrac-
tons Of the Yosemite depends. Mir
ror lake, to be viewed at its best. must
be seen in the early dawn, when the
reflections are most perfect.
The lake is Stillest and it's surface
most mirror-like when the cold night
currents have ceased and the uprising
day currents Of air have not yet be-
gun. Yet unless n is punctual he
will miss the chief beauty of the place.
for this perfect stiffness is as brief as
the turn of the tide.
In the evening and during the night,
when the down draft of air from the
mountain sides Is strong. the stream