The Washington times., 1901-08-12
WHY IT RAINS.
Variety of Causes which Lead Up to
Light or Heavy Showers

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The distribution of rainfall is the sub
ject of very interesting monograph
written by DR. A J. Herbertson and pub
lishod the other day by Mr.. John Murray
The amount of the rainfall depends on a
variety of causes. First among them are
heat and cold. All the rivers. says the
preacher, run into the seas, yet the sea
is not full. With good reason. because
the sun pumps the water up into the arr
from which it is again sent back by the
cold of space; as the air coois II De-
comes less capable of retaining vapor so
the water reappears. first as clouds, and
then as rain Thus the amount of ram
is often greatest in tropical countries, at
any rate near an ocean. while in polar
regions it is comparatively small-and
often comes down as snow. But the con-
tinents and oceans. the dominant winds,
which are partly dependent on them. and
the differences of level in the ground. all
affect the rainfall of distgict. both as
regards its total amount and its distrl-
button over the year. 1t may range from
nothing to a yard, or even more
The Desert OF Cobb, zone in Northern
Africa-including the Sahara, and extend
ing eastward across Arabia toward Al
ghanistan-and a narrow strip of South
America. west of the Peruvian and Chill
can Andes. are rainless; while the other
extreme is represented in the hhasla
Ghauts, where the annual fall some
times reaches 500 inches, the greater
portion of which comes down in about
half the year. In our own country
we think four to five inches in twenty
four hours a record. but at one station in
these Ghauts nearly fOrty-one inches fell
on a June day in 1876. In our islands the
eastern coasts are much drler than the
western. and the loWlandS than the up
lands Winds blowing from the Atlantic
come to our shores laden with vapor, and
on these it is condensed.
Rising ground hastens the process; the
air is cooler there for the thermometer
falls about one degree for each 800 feet OF
ascent and the hills themselves, like
big stone in the path of stream. force
the vapor-laden currents up INTO YET colu-
er regions. Owing to the fiIst cause.
while about two feet of rain falls annual
ly in London, almost half as much again
is measured on the Lancashire coast. Fur
the second reason. the fall is about hfty-
two inches at the Woodhead reservoirs,
and almost fifty-five at those on RlYinB-
ton Pike, whence Manchester and Liver
pool once drew their chief supplies But
the wettest spot in the British Isles Is
near the head of Borrowdaie. where about
$50 inches are registered every year.
Apart from these variations, the rainfall,
DIR. Heibertson tells US. Is distributed IN
equator is a wet belt, which is bounded
by two tropical dry belts: the temperate
zones are wet, and are succeeded by dry
polar caps.
But this system as might be anticipated
is not fixed in position, for to certain
extent, as seen from the excellent maps
with which<his volume is supplied. ii fol-
lows the sun. For instance. July is a wet
month over Central America, in the north
ern part of the southern continent, in
Africa between the equator and the Sa-
hara, over all India, except the extreme
south Burma. the southern half of China.
with the Malay archipelago and adjoining
islands to some little distance south Of the
equator. In that month. also much of
North America, Central and Northern Eu-
rope. and a broad belt extending through
Central Asia are moderately rainy, the
latter fact being indirectly connected with
the former. But in January all this re-
gion. with the exception of eastern North
America. is comparatively dry. while the
more rainy region includes much Of South
America and of Africa south of the Equa-
tor, the northern part of Australia, and
the islands as before. Not only are the
facts stated. but the scientific eNplana-
lions of them are also given in this pleas
antly learned essay.-London Standard