The National tribune., 1891-10-22
MsntiOu The National TrfsunA
THE WEATHER.
Warm Periods in Oregon and Washington
BY LIEUT.. ]. p. FINLEY. U. s. SIGNAL SERVICE

Correspondence Invited. suggestions desired.
and Information cheerfuily furnished. Every e.
fort employed To make this Burenu Of practical
To renders. All communications should be adj
dressed Weather, THE NATIONAL TRIBUNE Office.
Washington, ID. CT
Generally speaking the atmosphere is heated
In three ways. I. By the direct rays Of the
sun. 2. By contact with the warmer earth.
3. By the radiation and reflection Of heat from
the earth.
The atmosphere over the North Pacific States
is subject to periods Of high temperature, both
in Summer and Winter. In the former season
they may DO called warm waves, while in the
latter they are known locally as Chinook
winds Both are dependent upon the eondi-
tons of atmospheric pressure prevailing over
Oregon, Washington, Idaho. Montana and Brit
iSh Columbia. The Chinook appears with a
barOmetric Low passing eastward over British
Columbia. barOmetric HIGH is then central
over the Middle Plateau region Nevada. Utah
and western COloradO). This system of atmOs-
pheric circulation gives rise to southwesterly
winds over the North Pacific States and the
Northern Plateau region. This air is both dry
and warm, because 10 comes from the desert
regions Of southeastern Oregon, Nevada and
western Utah. It has absorbed the heat of
those regions where the percentage Of atmOs-
pherie moisture is the lowest in the United
States. Transported over regions covered with
snow the air laps up every yestige of it with
surprising rapdity. The humidity increases
the air becomes softer and more comfortable,
the sun shines with greater warmth, and
sort Of Indian Summer prevails on hill and in
daie. Such condition lasts until the winds
change TO the northwest. north and northeast,
the barometric Low has passed eastward 20 the
Mississippi Valley and HIGH now covers the
North Pacific States, accompanied with cold,
dry air and clouds Of snow. Winter again
resumes her sway. The hot waves Of Summer
appear with barOmetric HIGH over British
Columbia and Montana, and q barometrio
LOW over the ocean near Vancouver Island.
Under this condition Of atmospheric pressure
tho circulation gives rise 10 nOrtheasterly
winds over Oregon and Washington, carry
ing over these States the intensely heated
air from the lava beds and desert plains
Of Idaho, Montana and British Columbia.
During the Summer this region becomes abb
normally heated under the quiet and heavy air
Of barOmetric HIGH. The earth where de-
void Of vegetation absOrbs the heat quickly and
the air resting upon il is kept at high tenn
perature. When this air is transported over q
region having rank vegetation like northern
Oregon and Washington, and OF course very
moist, the effect is 10 hasten evaporation, raise
the temperatures exceedingly, develop clouds
aud give rise TO oppressive, sultry weather.
These hot waves usually last two or three days
and may continue fora week. There is very little
change at night, owing to the large amount Of
moisture in the air. aud therefore during these
tOrrid spells there is much discomfort to man
and beast, and some injury TO plant growth
| where the warm waves are long continued or
quickly succeed each other.
An important lesson to learn from this dis
custom is the fact that the temperature changes
over any region, and at any season of the year
depends upon the topography Of surrounding
regions and upon the changes Of atmospheric
pressure over them.