Page 252 - NS-2 Textbook

METEOROLOGY 247

IONOSPHERE posed of three atoms of oxygen per molecule (rather than
the two in a normal oxygen molecule), absorbs the ultra-
Above the stratosphere lies an area of electrically
violet rays.
charged particles called ions. This ionosphere begins per-
There has been much concern in recent years that the
haps as low as 30 or 40 miles up and extends to about 500
ozone layer is being slowly depleted by fluorocarbon gas
miles. Disturbances on the Sun, such as sunspots, change
reacting with the ozone. Fluorocarbons have been widely
the ionosphere's form, and it is turbulent with magnetic
used as propellants in aerosol cans for such products as
and electrical storms. It is in the ionosphere that the
hair spray and spray paint, and also as refrigerants in air
Northern Lights create their colorful display.
conditioning systelns in cars, homes, and businesses. As
It is possible to send ordinary radio waves around
a result of tlns dangel; many spray can and air condi-
the world by bouncing them off ionospheric layers. In
tioning manufacturers in the United States and else-
other words, the ionosphere will reflect radio waves of
where have switched to other substances, but these alter-
certain frequencies. By determining the best frequencies
natives tend to be nlore expensive. Fluorocarbons are
and times of day to transmit them, communications are
also released by the burning of styrofoam.
greatly enhanced. This phenomenon is one with which
every Navy and ham radio operator is familiar.
The mesosphere, the lowest level of the ionosphere, EXOSPHERE
extends from about 30 to 50 miles above the Earth. nris
The topmost layer or outer fringe of the atmosphere is
layer is one of extreme temperature changes. At the low-
called the exosphere. It begins about 500 uriles above the
est part of the layer the temperature may be as high as 32 Earth's surface and continues out to about 18,000 miles.
degrees F. But it will drop to below -100 degrees F at the
Only the light hydrogen and helium atoms exist in the
mesosphere'S upper limits. It will then start to rise again
area-in atomic form because of the intense cosmic radi-
above 70 miles, as one moves into the thermosphere.
ation. Temperatures may be as high as 4,500 degrees F in
The thermosphere is the highest level of the iono-
daylight and may drop to near absolute zero (-460 de-
sphere. The air is extremely thin and the particles are ion-
grees F) at night.
ized, or electrified, by loss of their electrons. This ioniza-
Within the exosphere are the intense radiation areas
tion is caused by the constant bombardment of cosmic
called the Vall Allen radiation belts. These belts encircle
rays from outer space. It is in the thermosphere that the
Earth in two segments. One is about 400-3,400 miles
principal radio-reflecting layers of the ionosphere are lo- above Earth, while the outer belt extends from about
cated. Extremely high temperatures exist in this layer.
8,000-40,000 miles out. The inner belt contains high-
Recent information shows that temperatures in the ther-
energy protons, and the outer belt contains high-energy
mosphere can approach 1,700 degrees F at a 300-mile al- electrons. Manned space missions are intentionally
titude. flown well beneath the lower limits of the Van Allen
Another very important transition zone within the
belts, and satellites operating in these regions must be
broad ionospheric region, but actually starting well below shielded against the radiation encotmtered there. The
it, is the citel110spilere or ozone layer. Beginning at an alti-
exosphere is the end of our air ocean. Beyond it is outer
tude of about 15 miles, this layer shields Earth from the
space.
harmful ultraviolet rays of the Sun. Ozone, a gas com-
ATMOSPHERIC PRESSURE
The layer of atmosphere that surrounds us exerts a pres-
sure of nearly 15 pounds per square inch (14.696 psi, or
5
about 1 x 10 Pascals) at sea level. The weight of the at-
mosphere varies from place to place, depending on the
amount of water vapor present, the temperature, and the
height above Earth's surface. Variations in atmospheric
pressure are measured by a baroilleter.
The Navy uses two types of barometers: mercurial
and aneroid. Usually the aneroid type is employed
aboard ship. The mercurial type consists of a calibrated
glass tube filled with mercury. It is used at shore activi-
ties to check aneroid barometers for accuracy. The
The ionosphere is a radio reflector for shorter wavelengths. They re- aneroid} or dry, barometer contains a small metallic cell
bound from it and make long-range radio communication possible.
Longer radio waves are not reflected by the ionosphere and pass that contracts ·when atnlospheric pressure increases and
through it. expands when pressure decreases. The cell is connected