Evening star. [volume] (Washington, D.C.) 1854-1972, 1930-07-24

CConLlnue0 from F1fSt PageD
serious question, but the back of the
dry spell Is likely to be broken only
when enough moisture accumulates on
the surface of the land to fill the arr
through the process of evaporation
with water vapor to condense into rain
drops through the rather complicated
physical process that attends such
phenomena. And in order for enough
moisture to accumulate to cause rain
there must be rain.
The presentSc condition has crept on
the country almost unnoticed until
within the last few days.
Both wet and dry spells, explains
Prof. William J. Humphreys of the
Weather Bureau. generally speaking are
cumulative effects, which run in vicious
circles. The dryer 1t gets the dryer 1t
is likely to be. 1t doesn't rain in the
Sahara Desert because there isn't
enough rain to make 1t rain.
Brooks, rivers, lakes and ponds In
the affected area are so depleted and
the soil and vegetation SO dried out
that they have no moisture to provide
water vapor to make cumulus clouds
which eventually turn to rain. Before
there can be rain something must hap
pen to break the circle.
Rain Moves In Steps.
Rain, as explained by DR.. Humphreys,
tends to move In steps. First there Is
the evaporation of sea water which falls
as rain. Then part Of this Is taken
into the atmosphere again by evapOr-
alton and again come down as rain, the
process repeating itself over and over.
This process accounts for about two
thirds of all rain
Whether 1t rains or not, says Prof.
Humphreys, depends on q rather COM
plicated and delicate balance which
has been completely upset for this Sec
ton. There is a constant tendency fer
water in its liquid form to change lilo
gas, the light particles OF which escape
into the atmosphere, but whose molecules
do not enter a chemical combination
with other element of the air. The
capacity of given volume to hold
water vapor increases rapidly with the
temperature. Warm air, loaded with
this gas, is pushed up by the colder air
around it. AS 1t goes up 1t expands
because the pressure Of the surround
ing atmosphere is less. But, pushed
from behind, ut must push in turn
against this continually decreasing
pressure and in this pushing the air Is
doing work. Whatever does work loses
heat. The cooling OF the air as 1t goes
up is not due directly to the effect OF
the cooler regions Of the atmosphere.
but to the loss Of heat by work" against
the pressure of the upper air.
Cooler Air Less Favorable.
But, says DR.. Humphreys, the cooler
the air gets the less water vapor can
be in it. The surplus tends to con-
dense. In order to do so there must be
something for 1t to condense about.
Ordinarily the air is filled with fine
dust particles about which the water
vapor particles condense--a great part
of them fine particles of salt thrown up
by the sea spray. AS the condensation
goes on the water vapor joined with
dust or salt particles forms into a
cumulus cloud which grows larger and
eventually, the condensation proceeding
as more and more humid air aScends
comes down as rain.
The essential part of the whole
process, DR.. Humphreys stresses, is that
to have rain volumes of atmosphere
nearly saturated with water yapor must
go up. In order for warm air to go UP
there must be warmer and cooler aiI
currents, the latter pushing the former
Unfortunately, at this time of year
there is nowhere to get cold air from
1t is just about as hot in one part OF
the Northern Hemisphere as another
So the atmosphere, with what little wa
ter vapor it has accumulated, tends tc
remain in an equilibrium.
1t Is this equilibrium, coupled with
the prolonged dryness, that has crepi
up on the stricken region.
Explanation OF Source.
The great mass Of water yapor comes
DR.. Humphreys explained, from thu
place where there is the most water
the sea, and the ascending air cur
rents containing 1t follow rather def.
mite directions across the United States
Warm air laden with water yapor from
the Gulf Of Mexico sweeps northwarc
over the Gulf States and into the Easi
Central States. But as 1t hits lanC
most Of the water vapor Is forced ou'
and falls as rain. In order to bring
abundant rain further north and cas-
the lost water vapor must be picked uI
from the surface Of the land on the
way. Once a drought starts there Il
less and less of this water to be picket
up. So there Is less and less chanc'
! for rain. And the less rain there L
the less the supply Of water yapor u
make more rain.
The best hope, says DIR. Humphreys
seems to be for sufficient atmosphere
mass sufficiently saturated with watel
yapor to come up from the Gulf and
| through a fortunate but unpredictabl'
atmospheric combination. carry its LOAF
of rain further north, thus saturatinI
the surface and providing the materia
| for more water vapor. When this hap
/ pens-and it might happen any day
there will he a break In the drought