Page:Handbook of Meteorology.djvu/159

 The arrow points the direction of the wind. If the sky is clear, the circle of the arrow is left clear; if partly cloudy, half the circle is blackened; if cloudy, all the circle is blackened. An R in the circle indicates rain; S means snow; and M means that the report for that particular station is missing. All localities in which rain or snow is falling are shaded. The map thus finished is the daily weather map, and from it the forecasts of the following twenty-four to thirty-six hours are made. These also may include information discovered by the long- distance forecasts.

Distributing Weather Information.—The base maps on which the information is to appear are distributed to such stations as issue daily weather maps. As soon as the matter described in the preceding paragraph has been placed graphically on the map, it is reproduced by a quick process in the form of a printing plate. The matter for the plate is usually ready by half-past nine o’clock, and is printed on the base maps which are usually folded, wrapped, and addressed within a short time. At the New York City Station about 3000 maps are required for the daily issue. They are sent to shippers, railroad offices, merchants, newspapers, educational institutions, post offices, and public places of various sorts. Almost every daily paper publishes a résumé of the weather map; some reproduce the map itself. All told, the daily forecast is so widely published that it is within almost instant reach of everyone within the main body of the country.

Features of the Weather Map.—The chief desire of the public is to learn whether the weather during the succeeding few hours is likely to be pleasant or stormy, warmer or colder, clear or cloudy, quiet or windy. These are features that affect all people; and the daily weather map answers the questions correctly a little more than four times in five. The verification of rain or of snow practically is four times in five; of tempera- ture and wind direction, rather better than four times in five.

A study of the weather map will show the area or areas in which rain or snow was falling at the time of observation; it will show where freezing temperatures may, or may not have