(Weather Bureau, U. S. Dept. Agric.)
The first impression of a student of the weather maps, as they present their seemingly endless forms and combinations of the temperature and pressure lines, is often one of confusion. This feeling is likely to be attended by one of discouragement, and the impulse to abandon the task of seeking an underlying plan is more powerful with many persons than the incentive, which depends upon curiosity, to know what it all really means.
The storms of the United States follow, however, year after year a series of tracks, not capricious, but related to each other by very well-defined laws.
The positions of these tracks have been determined carefully for the United States by studies made in the Forecast Division of the Weather Bureau, on the long series of maps that have been made during the past twenty years. The track that the central point of a high area or that the center of a storm follows in passing over the country from west to east is laid down on individual charts, these are collected on a group chart, and from this the average track pursued can be readily described. The chart herewith (see fig. 1) shows the general result of
Fig. 1. —Weather map, showing mean tracks and average daily movement of storms.
a study of tracks of storms in the United States. It indicates that, in general, there are two sets of tracks running westerly and easterly, one set over the northwestern boundary, the Lake region, and the St. Lawrence Valley; the other set over the middle Rocky Mountain districts and the Gulf States. Each of these is double, with one for the "highs" and one for the "lows." Furthermore, there are lines crossing from one main track to another, showing how storms pass from one to the other. The transverse broken lines show the average daily movement. On the chart the heavy lines all belong to the tracks of the "highs," and the lighter lines to the "lows." Let us trace them somewhat in detail. A "high" appearing on the California coast may cross the mountains near Salt Lake, and then pass directly over the belt of the Gulf States to the Florida coast; or it may move farther northward, cross the Rocky Mountains in the State of Washington, up the Columbia River Valley, then turn east, and finally reach the Gulf of St. Lawrence. The paths are determined by the laws of the general circulation of the atmosphere and the configuration of the North American continent. This movement of the "highs" from the middle Pacific coast to Florida or to the Gulf of St. Lawrence is confined to the summer half of the year — April to September, inclusive. In the winter months, on the other hand, the source of the "highs" is different, though they reach the same terminals. In the months October to March, inclusive, many "highs" enter the United States near the one hundred fiftieth meridian and move south along the Rocky Mountain slope into the southern circuit, and thus reach the South Carolina coast; or else they turn more abruptly eastward and move in the northern circuit over the Lakes to Newfoundland. The chief difficulty in the art of forecasting is to decide which of these paths will be pursued and the probable rate at which the movement will take place.
The weather map.
The daily maps of the Weather Bureau show stations in the United States and Canada that make telegraphic reports of the weather each day at 8 a. m. and 8 p. m., seventy-fifth meridian time. The reports consist of observations of the barometer and thermometer, the velocity and direction of the wind, amount, kind, and direction of movement of clouds, and amount of rain or snow, and the 8 a. m. reports are furnished to nearly one hundred stations of the Weather Bureau for use in the preparation of maps and bulletins.
On the weather maps solid lines, called isobars, are drawn through points that have the same atmospheric pressure, a line being drawn for each one-tenth of an inch in the height of the barometer. Dotted lines, called isotherms, are drawn through points that have the same atmospheric temperature, a line being drawn for each ten degrees of temperature. Heavy dotted lines are sometimes used to inclose areas where decided changes in temperature have occurred during the preceding twenty-four hours. The direction of the wind at each station is indicated by an arrow which flies with the wind. The state of the weather - clear, partly cloudy, cloudy, rain, or snow - is indicated by symbols. Shaded areas are used on the maps issued at Washington, and at several stations, to show areas within which precipitation in the form of rain or snow has occurred during the preceding twelve hours. The tabular data give details of maximum and minimum temperatures, and 24-hour temperature changes, wind velocities, and amount of precipitation during the preceding twenty-four hours. The text printed on the maps presents forecasts for the state and the station, and summarizes general and special meteorological features that are shown by the lines, symbols, and tabulated data.