The National tribune. [volume] (Washington, D.C.) 1877-1917, 1890-10-16
by LIEUT.. J. p. FINLEY, U. s. SIGNAL SERVICE.

| BY LIEUT.. J. p. FINLEY, U. s. SIGNAL SERVICE.
| It Is moro difficult to treat of the distribu-
ton of rainfall than of temperature and baro-
metric pressure, whore in the latter case cor
rections of more or less exactitude can be ap-
| piled to reduce readings to sea-level. In the
| construction Of monthly rainfall charts large
regions must be ignored where observations
| have been impraeticable, owing to the difficulty
| of opening and maintaining stations.
In the Northern Hemisphere there are prac-
tically no stations in the interior and east of
Asia, the whole of British America, except
Canada, the entire Attic regions and the At
lantiC and Pacific Oceans.
Rainfall ia more subject to local variations
than any other meteorological element and
therefore requires very careful observation at
large number of places in any region. Not
only must the number of stations DO large but
the observations must continue at any station
over period of several years. tho longer the
better, before reliable results can be obtained
Those rigid requirements prevent satisfactory
records at sea in uninhabitcd countries.
Bearing in mind the difficulties attending the
observation and record Of rainfall we are better
able to appreciate the uncertainty of many
statements made by people and the false i'm
piCSSlOHo OHCISlIOul.
The distribution in vertical hight of the
amount of rain varies in different ATTITUDES
and with differences in the position of the
mountain chains, with regard to the prevailing
wind so that two stations situated close to-
gether may differ widely in the amount and in
the seasonal distribution of their rain Tho
three great agencies which are efIicacious in
bringing about precipitation OF moisture are
~ ascending currents; ~ the contact of warm
air with the cool surface of the ground 81 the
mixture of masses of air of different tempera-
.ures.
Winds moving from high to low ATTITUDES
are generally dry, while those moving in the
opposite direction are generally damp. Winds
blowing off the shore on to the sea-surface, cs-
pecialy if the coast be bold and the land ele-
waited, so that tho wind is forced 10 descend
are essentially dry. The wettest districts of
the globe are ~ the calm belts near the equa-
tor; t2) certain localities where damp winds
meet mountain ranges and are forced upward
1t is very generally stated in text books of
physical geography that the trade wind ro-
rions are quite rainlcss. Maury advanced this
Wow very positively in his Sailing Directions
and we know from observation that the trade
wind regions of tho ocean, beyond the imme-
date vicinity of the land, are for the most part
rainless regions.
Almost all of tho Tainless regions on land
owe their aridity. in great measure. to the
presence of mountainous belts which drain the
winds of their moisture. The precipitation
takes place on the mountain tops either as rain
or snow, and the valleys and low lands suffer
from drouth. Wherever the trade-winds
blow there is comparatively little rain, unless
where the wind blows on TO q mountainous
coast. With the periodical shifting of the
trade-wind areas the dry areas shift likewise,
and the descent OF the return trade. which is
really the overflow from the calm belt, brings
uIgagN Il ~
this condition prevails extends approximately
from latitude 800 to 40 north, in both henis-
phercs, and is the region of Winter rains, which
occur when the sun Is lowest The rains of
low ATTITUDES are essentially Summer rains, a.
they occur principally when the sun ia the
highest. In the Pacific Coast States and the
plateau regions adjoining. in the United States,
the rainy season occurs in Winter aud the dry
season in Summer. This applies also to Mexico,
where the wet season of Winter comes when
the sun is highest.
In this Western country the Winter precipi-
tation Is heaviest on the North Pacific coast,
and IS due TO cyclonic action. The storm belt
of tho Pacific Ocean lies just south of the
Aleutian Islands in the path of the kura Siwo,
or,Japan Current, body of water similar to
tho Gulf Stream Of the North Atlantic. The
storms from this region are more active, and
move further south in Winter than in Summer
consequently the heavier rains in the North
racine states, extending southwark into Cali
fornia. On the mountains the precipitation
falls as snow and disappears very slowly until
the heat Of Summer arrives. when the evapOra-
tion aud condensation from tho vast snow de-
posts give rise to the clouds and rains OF
Summer. Heavy snow in Winter is followed
by frequent showers in Summer and rice persa
The Winter storms are the result of cyclonic
action, that IS, the development aud movement
OF vast atmospheric disturbances while in
Summer there ARE no storms at all, simply in
frequent local disturbances, the result of heat
aud mountain snow. This explains the occur
renee OF wet and dry seasons in the Pacific
Coast States and adjoining plateau regions