Huntsville gazette., 1882-12-16
The Terrestrial "Dry Zones.'

The theory of the dry zones Of the
globe propounded by Prof. Guyot before
the National Academy of Science is of
great Importance in modern climat-
ology. These zones extend, though
with very marked gaps, around the
earth between the twenty eighth and
thirtieth parallels, in both hemispheres.
According to Prof. Guyot, the cause of
these dry areas near the tropics is to be
| found In the fact that in them the preva-
lence OF descending currents is very
decided, and the dry upper trade winds,
blowing from tropical to extra tropical
latltudes, we know, are constantly tax
ing up much of the moisture in these
zones. Meteorological observations show
that these dry zones are occupied gener-
ally the year round by vast anti-cyclones
OF atmosphere under high barometric
pressure. As, by mechanical law, in
every anti-cyclone there must be de-
scending air currents in the central por-
tlons just as ascending currents must
always exist in the centre of a cyclone)
the rainlessness in these zones is intelli-
gible. For with every descent of a mass
of air five thousand feet its capacity to
absorb and sustain invisible water vapor
and hence to dissipate the rain clouds
is doubled, and the dry upper trade has,
in its passage from the Equator to the
"dry Zone," three or four times as much
descent.
The permanent zones of high barome-
ter, which coincide with the limits of
the "dry zones" as indicated by Pro-
fessor Guyot, are due to the effect
of the earth's rotation on its axis
and the consequent centrifugal forces
acting on the grand atmospheric cur
rents. If we could imagine the atmos-
| pheric ocean composed of layers equally
dense throughout, its surface would be
still uneven under the sway of these
forces. NJ. Peslin, the eminent French
engineer, has clearly shown that owing
to these forces the very "surface of the
sea, even in an ocean as open as the
Atlantic, is not vigorously horizontal, -
and ''that over very extended regions 1t
presents inclnations that can amount
to many seconds, and level differing
from the mean sea level by several me
tres" Fortunately for the United States
the continuity of the "dry zone" is
broken on our Gulf coasts, especially in
summer, by the arrest of the surface
trade winds by the Mexican mountain
range and the consequent diversion OF
these vapor laden, fertilizing wind;
northward over the Mississippi Valley,
whence they spread over the whole
country east of the Rocky Mountains
But on the North Atlantic the existence
Of the dry region, overhung by no cloud
canopy serving to screen the waters
from the suns rays, is made conspicn-
Gus by the extraordinary fact that il
September the average temperature OF
the ocean surface between the twenty
eighth and thirtieth parallels is consid-
erably higher than that of the surface
waters on the Equator itself The de-
termination of the mean limits in differ
ent seasons of these arid zones of high
air pressure on the oceans would be a
great practical boon to navigators; for,
since cyclones can not develop or be
supported in these regions of calm dry
air, they offer to ocean steamships, in
| dependent of the winds, the most storm
less tracks across the deep.-N. Y. Her
Gld