The Baltimore County union. [volume] (Towsontown, Md.) 1865-1909, 1905-04-01
RAINFALL IN THE UNITED STATES.


By Henry Gannett, Geographer Of the United
States Geological Society.
The average rainfall of the United
States, including Alaska, is about
thirty inches. That is, if it rained
the same amount every where in the
country, rain and melted snow would
fall each year to depth of two and
one-half feet. But the rainfall is not
distributed uniformly by any means.
Some regions receive vastly more than
is needed, while others receive so lit
tie that only desert vegetation can
grow. The eastern half of the coun-
try is we'll watered, while most of the
western half is insuf6ciently watered.
On the coast of the Gulf of Mexico
FIVE, sometimes six feet fall in year !;
upon the northwest corner of our
country, about Puget Sound, rain
commonly falls to depth exceeding
six feet, and ten feet have been known
to fall in single year. On the other
hand, in the deserts of Utah, Nevada.
Arizona and eastern Californa years
have passed without any rain, and the
average of many years is between FIVE
and ten inches.
To understand this difference in the
supply of rain to different parts of the
country, it is necessary to understand
why it rains-What conditions must
be present.
All air contains vapor of water in
solution sometimes their is more,
sometimes less. Air OF certain tenn.
perature can dissolve only certain
amount of moisture. When the limit
is reached, the air is said to be satur-
ated, just as water may be saturated
with salt in solution.
This limit is much higher with
warm air than with cold air, that is,
warm air can dissolve more water
than cold air can. Now If warm air
which is saturated or partly satur-
ated with moisture be cooled, it can
not hold so much moisture as before.
A part of it must be given out, and
this produces rain.
Two conditions must therefore be
present-moisture in the air, and
means of cooling the air below the
point of saturation. Neither of these
conditions will alone produce rain.
The air may be very moist, as it is
upon the coast of southern California
in the summer, but since there is no
way of cooling it, itnever rains there
in this season.
WHAT THE AIR-CURRENTS DO.
On the other hand, dry air may be
reduced greatly in temperature with
out producing rain, as occurs where
the air-currents blow over the Colo
rado mountainsin the winter, because
these air-current, containing little
moisture, are not cooled to the point
of saturation.
Air is commonly warmest at the
earth's surface, and becomes colder
as the elevation increases. ItisalsO,
of coures, warm near the equator and
and becomes colder northward and
southwark. Ascending currents,
therefore, become colder, and in the
northern hemisphere air currents from
the south are cooled as they move
| southwark. AS air-currents move up
ward or northward, all the moisture
beyond that amount which they, in
! their cooled condition, can hold in
solution is precipitated as rain or
snow.
The water which our country re-
ceives comes mainly from three
sources. the Atlantic Ocean, the Gulf
of Mexico and the Pacific Ocean, and
of these by far the largest amount
comes from the Gulf. The contribu-
tions of the Great Lakes and other
bodies of water are small in compari-
4 son. :.
The area supplied by the Gull
stretches across the entire breadth Of
the oountry from north to south and
from central Texas, eastern Kansas
and Nebraska. eastward to the Appa-
lachian Mountains.
The Atlantic ocean supplies the
strip of country east of these noun.
tains, and the Pacific supplies mois-
ture in varying amount oyer the
whole western country, its influence
extending as far east as the eastern
boundary of the Dakotas, Kansas
Nebraska, Oklahoma and central
Texas. OF course these limits are i'll
defined. They overlap one another ant
and they shift about with the seasons
but on the whole they are about it al
stated.
All storms are essentially winds
which may or may not be accompanied
bv rain or snow. The air moves in {
spiral course, in a direction in the
| northern hemisphere contrary to the
movements of the hands of a watch
about the centre of the storm. Th'
little dust-whirl, which is often seei
in the road on a summer day, is
miniature of the great whirls which
4 cover half continent.
FROM WEST TO EAST.
Such storms in this country common
| 1y travel from west to east, across tn
continent, and as they move the.
draw in air from all sides, carrying
mp into the higher atmosphere. Tn
Influence ola storm may be felt day
4,.eora ;ts centre arrives. The win