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 ; it will show the areas of clear, cloudy and windy conditions. The man in Chicago may learn at a glance the weather conditions at Los Angeles, New York City, Winnipeg, Key West, Bermuda, or Havana. A merchant who has shipped perishable goods may learn whether or not his consignments are threatened by washouts, snow blockades, or cold waves. In other words, the daily weather map is very much more than a mere bird’s-eye view of the air and its conditions; it is a survey—a topographic map—with measured values.

The Movement of Weather Conditions.—The ripples, whirl-pools, and waves of a river are carried along in its flow; so also the waves and whirlpools of the air are carried along with the great streams of the air. Throughout the greater part of the United States, this movement is from a westerly to an easterly quadrant—that is, the greater part of the main body is in the belt of Prevailing Westerlies. The Gulf Coast, together with the Florida Peninsula are in the Trade Wind belt in summer, but not in winter. Therefore, such movements as cyclonic areas or lows will move from a westerly to an easterly quadrant with the velocity of the general movement of the air.

The paths of the principal types of cyclonic storms which have already been described are shown on the accompanying map. They are discovered by means of the isobars. That is, somewhere in the middle western part of the United States an isobar of 30.00 inches will be found to inclose an elliptical area. Within this area isobars are drawn for every tenth of an inch of decreasing pressure. This area is a low, and probably an area of updraught; if the pressure is below 29.50 inches it is pretty certain to be a strong updraught, and the arrows which indicate wind direction are pointing toward the center of the low.

If the pressure at the central part of the low is only two- or three-tenths below 30.00 inches, the updraught is not very strong and the winds blowing into the low are light. Rain or snow may or may not be falling. On the other hand, if the pressure within the low is 29.50 inches or less, rain or snow, followed by heavy winds, is pretty certain to occur, mainly on the east and