Page:Handbook of Meteorology.djvu/205



Two terms may be used to express the gravity of the air—weight and pressure. The weight of air is used more properly to express the gravimetric force of a given volume: thus, 1 cubic foot of air at normal pressure and temperature of 32° F weighs 1.29 troy ounces (1 cu. cm. = 0.0013 gram). In meteorology it is more convenient to consider the weight of a column of air throughout its whole extent, from sea level upward. At sea level such a column of air presses upon the surface with an average force of 14.7 pounds, a pressure empirically termed 1 atmosphere.

The pressure of such a column varies, however, not only from day to day, but from hour to hour. If a mass or a wave of air accumulates over a given locality the pressure increases; conversely, if a depression, or a trough, occurs, the pressure decreases. As the observer goes from sea level to a higher altitude the pressure decreases also. At an altitude of 19,000 feet, a height occasionally reached by airmen, the pressure is about half of that at sea level.

The pressure of the air may be determined by weighing it; it is much more convenient to compare it with a column of mercury which balances it. A balance constructed for this purpose is a barometer. The theoretical construction of the barometer is simple; it consists of a glass tube about 33 inches long, closed at one end, filled with mercury, and inverted with the open end in a cup of mercury. The column of mercury within the tube exactly balances a column of air of equal section. If air accumulates, the increased pressure on the mercury in the cup forces the column higher in the tube; conversely, a decrease in pressure causes the mercury column to shorten;