The Roanoke times. (Roanoke, Va.) 1897-1977, 1897-11-19
RAINFALL IN THE UNITED STATES

The rainfall of the United States is the
title of n bulletin prepared by the weather
bureau and which is con piled from sta
tstics extending over many years. From
this it appears that rain falls in this
country ut one point or another every day
in the year. On very rare occasions the
observers report no precipitation at n
given hour, hut it should he Remembered
that their range of vision can cover at The
utmost much less than II one. hundredth
part of the whole area.
The theories Of rainfall given in the
books of twenty or thirty years ago are
not now accepted. There is one plinci-
pal, however, upon which no disagree
ment exists, namely that in order to pro-
duce abundant rail the temperature of
the air must be suddenly cooled below
the dew point When the air is thus
cooled n portion of the water vapor
which isssalways present in it is changed
to the liquid state and that which for
merlv existed as Il gas or vapor now be-
comes visible in the form of minute par
ticles of fog or mist. These particles NAV
float away with the wind or they may
increase in size. coalesce and fall to the
ground of their own weight. Whether
the conden sation of vapor thus outlined
results simply in cloud or whether rain
falls, depends on the magnitude of the
temperature changes that take place in
the air mass, whose vapor is being con
densed.
y rise in temperature is inimical to
further condensation aud fall is favura-
BILE to precipitation. The precise man
ner in which air is cooled sufficiently Ito
produce rain, whether by contact or mix
ing, is not clearly apprehended. Cooling
by expansion as all ascends is one Of che
most effective causes of rainfall. This
ascensional movemenG of air is brought
about III several ways, chief Of which are
these: The air may he forced up the side
of mountain into II region of diminished
pressure.and lower temperature, as hap
pens whenever n mountain range runs Mt
right angles to the prevailing winds; in
the warm season the lower layers of the
atmosphere under the effect of the solar
radiation and probably other causes, fire.
quently reach a state of unequal equiiib
rium thus inducing ascensionai currents,
summer thunder storms being la1 gely the
result of this process; while lastly, and
doubtlessly tue most important of all, is
the circulation of the air in cycionic
storms, namely, n radial inflow from all
sides and an ascensional movement in
the ce inter-. a very large percentage of
the rain in the United States is precipi-
tated in connection with storms OF the lat
ter class.
The conditions which fleet or control
the rainfall may be divided into TWO
classes. permane mt and variable The per-
manent conditions which contribute to
au abundant rainfall are chiefly nearpess
to the ocean, or other large body water,
II location within or near the track Of cy-
clone storms; mountain ranges, particu.
larry those running in direction at right
angles to the direction OF rain bearing
winds. These operations may operate
singly or In combination.
The main facts regarding the geo
graphic distribution of precipitation in
the United states are as follows: There
is a narrow belt of very heavy rainfall on
tho northwest coast. extending from
Cape Flattery On the north to midway OF