If our weather were uniform, it would furnish little matter for conversation; in fact, would hardly be weather at all. Changeableness is the salient feature of weather, and to understand weather changes one must know something about barometric pressure.
Like all other forms of matter, the invisible air has weight. At sea-level it exerts a downward pressure averaging 14.7 pounds to the square inch. Atmospheric pressure is measured by means of an instrument called the barometer, in which the weight of the air is balanced against a column of mercury. As the height of the mercurial column varies with the pressure of the air, and is taken as the measure of the latter, we follow the practice of expressing pressure (a force) in linear units (inches or millimeters). This practice is retained even in the use of the aneroid barometer, which contains no mercurial column. Hence, when we say that the average barometric pressure at sea-level is 29.92 "inches," we are really expressing in a roundabout way the weight of the air at that level.
Courtesy of U. S. Bureau of Standards and Popular Science Monthly.
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HOW THE CAMERA ANALYZES LIGHTNING
The same flashes photographed with (a) a stationary camera, and (b) a camera revolving on a vertical axis. One of the flashes is seen to have consisted of several successive discharges along an identical path
Barometric pressure not only varies somewhat regularly with altitude—diminishing as we ascend—but also less regularly from place to place in a horizontal direction, and from time to time at a given place. In studying the weather meteorologists frequently wish to compare the barometric pressures prevailing at a certain time at a number of places lying in the same horizontal plane. Given a system of meteorological stations scattered over a certain territory, the first step is to secure simultaneous readings of the barometers at these stations. Then, if the stations are at various altitudes, as they commonly are, corrections must be applied to the readings to reduce all to a common plane; the plane adopted for this purpose is sea-level. Since most stations are above sea-level, and since atmospheric pressure diminishes with altitude, reduction to sea-level generally involves applying an additive correction.