Page:Handbook of Meteorology.djvu/51



Measurement of Atmospheric Pressure.—Practically all the movements of the air are due to differences in its temperature; but, inasmuch as differences in temperature result in differences in the density of the air, it is convenient to express such differences in terms denoting the force with which the air presses upon the earth at sea level. It is also more convenient in weather science to express this pressure in terms of the length of a column of mercury which the air balances—that is, a barometer.

Thus, a column of air 1 square inch in cross-section weighs at sea level about 14.7 lbs., or 1 atmosphere. It balances a column of mercury of the same sectional area, 29.92 inches in length. In metric terms, the weight of a column of air 1 square centimeter in sectional area is 1033.3 grams, and it balances a column of mercury 760 millimeters in length. The length of a column of water which balances a column of air of the same sectional area is about 34 feet.

Distribution of Pressure.—The movements of the air caused by heating, cooling, expansion and contraction include the general or planetary movements, as well as the massing of the air in one locality and the counterbalancing depressions formed in another. The expansion of the air by heating has been determined many times. If 1000 parts of air at 32° F be heated to 33° F, its volume will be increased 2.035 parts; at 50° F the increase will be 36.63 parts; at 130° F, a temperature very common under a summer sun, the 1000 parts will become 1199.43