Now please attend carefully to what follows; because I am going to attempt to put into a minimum number of words the essential facts concerning the weather map, the best clue to weather mysteries yet devised by man.
At about 200 stations of the Weather Bureau, distributed over the United States, the barometer and other meteorological instruments are read twice a day; viz., at 8 A. M. and 8 P. M., eastern standard time. The readings are promptly telegraphed in cipher to Washington, where they are entered on a map.
The barometer readings at the different stations, reduced to sea-level as just explained, will vary, say, from 29 to 31 inches. Lines, called isobars, are now drawn through places having the same pressure; the intervals between the lines corresponding to differences in pressure of one-tenth of an inch. Lines (isotherms) are also drawn to connect places having the same temperature, a little arrow at each station shows the direction of the wind at that point, and various other symbols are used to facilitate the interpretation of the map; but the isobars are more important than anything else.
THE KITE HOUSE AT AN AEROLOGICAL OBSERVATORY
Some of the kites are much the worse for wear after flying in a storm
Here is the weather map for the morning of January 9, 1886. The solid curved lines are isobars, representing barometric pressures ranging all the way from 28.7 to 30.8 inches. It will be seen at a glance that these lines tend to assume roughly circular forms, inclosing regions where the pressure is lower or higher than the average. Moreover, the little arrows (which "fly with the wind") show that the winds round a center of low pressure tend to blow in a direction contrary to that followed by the hands of a clock (in the southern hemisphere the reverse is true), but instead of blowing in circles are inclined somewhat inward toward the center. Round a center of high pressure (in the northern hemisphere) the typical circulation of the winds is exactly opposite ("clockwise," and inclined outward), though the accompanying map does not show this particularly well.
An area of low pressure, with its system of winds, is called a cyclone, or low. An area of high pressure, with its system of winds, is called an anticyclone, or high. Note that a cyclone is not necessarily a storm, though the one shown on this map, with its center not far from New York City, was a very violent storm, which, when this map was drawn, was sweeping up the Atlantic coast. (Popular usage applies the term "cyclone" to the tornado.) The strength of the winds in a cyclone depends upon the contrast in barometric pressure between its center and its outer border. A cyclone with crowded isobars always has strong winds; when the isobars are widely spaced the winds are gentle.
These areas of low and high pressure, in addition to their movements about their centers, move bodily across the country, in a general west-to-east direction, at an average speed of over 500 miles a day. This double movement may be compared to that of a carriage-wheel, rotating and advancing at the same time. Most of our cyclones enter the country from the Canadian North-west—though many come from other regions—and nearly all of them pass off to sea in the neighborhood of the Gulf of St. Lawrence. Their route across the country varies greatly, depending in part upon the season.
