Page 253 - NS-2 Textbook

248 NAUTICAL SCIENCES

The Van Allen radiation belts encircle the Earth somewhat like
doughnuts, with radiation-free "escape cones" over the poles. The
most intense radiation in the inner belt is about 2,000 miles up, and
in the outer belt, about 10,000 miles.

The aneroid barometer has a cell that expands or contracts with the
-"*i--VACUUM decrease or increase of air pressure.

psi, the pressure at sea level; a bar equals about 0.98 at-
mosphere, and a millibar equals 1/1,000 of a bar. The av-
erage atmospheric pressure at the Earth's surface is 29.92
inches, or 1,013.2 millibars. You will often hear the baro-
meh'ic pressure readings given in inches of merclUY on
TV weather forecasts. Ivlillibars, however, are normally
used on ·weather charts.
An airmass is a large body of air with the same tem-
AIR AIR perature and humidity An air mass takes on the charac-
PRESSURE PRESSURE teristics of the surface over which it forms. Thus, cold-air
masses originate in the cold polar regions, and ·warm-
j j air masses originate in the tropics. The tropical or polar
ail' masses can develop over either continental or mar-
itime surfaces. These two surfaces give their names to the
different kinds of air masses. Since land and sea reflect
the Sun's radiation differently, the two kinds of air
masses have different characteristics.
It takes more heat to change water temperature than
soil temperature, and in seawatel; that heat is absorbed
Principle of the mercurial barometer.
to depths in excess of eighty feet. Howevel; only a few
top inches of soil will absorb radiation. This means that
oceans are slmNer to ·warm up, and slovver to cool down,
to a needle, which points to a graduated scale around the than are land or continental stU"faces. Maritime ah~ there-
face of the barometer. As the cell expands or contracts, fore, ''\Till tend to bring moderate temperahU'es, neither
the needle indicates the atmospheric pressure on that too hot nor too cold l as it moves over land areas.
scale. In the wintel; the United States is swept by continen-
Barometers may be graduated in either inches of tal air masses from the cold Arctic. In the SlUnmer, it is
mercury or millibars. Both inches and millibars are mea- swept by warm, moist maritime air masses from the Gulf
surements of the height of the mercury column sup- of Mexico, the Caribbean Sea, and the Pacific Ocean off
ported at a given time. One "atmosphere" equals 14.696 the Mexican coast.