82.  Isobaric and Isothermal Lines.  If a line were drawn through all points on the surface of the earth having an equal barometric pressure at the same time, such a line would be called an isobar.  For example, if the height of barometers in different localities is observed at exactly the same time, and if all the cities and towns which have the same pressure are connected by a line, the curved lines will be called isobars.  By the aid of these lines the barometric conditions over a large area can be studied.  The Weather Bureau at Washington relies greatly on these isobars for statements concerning local and distant weather forecasts, any shift in isobaric lines showing change in atmospheric pressure.

If a line is drawn through all points on the surface of the earth having the same temperature at the same instant, such a line is called an isotherm (Fig. 49).

83.  Weather Maps.  Scattered over the United States are about 125 Government Weather Stations, at each of which three times a day, at the same instant, accurate observations of the weather are made.  These observations, which consist of the reading of barometer and thermometer, the determination of the velocity and direction of the wind, the determination of the humidity and of the amount of rain or snow, are telegraphed to the chief weather official at Washington.  From the reports of wind storms, excessive rainfall, hot waves, clearing weather, etc., and their rate of travel, the chief officials predict where the storms, etc., will be at a definite future time.  In the United States, the general movement of weather conditions, as indicated by the barometer, is from west to east, and if a certain weather condition prevails in the west, it is probable that it will advance eastward, although with decided modifications.  So many influences modify atmospheric conditions that unfailing predictions are impossible, but the Weather Bureau predictions prove true in about eight cases out of ten.

The reports made out at Washington are telegraphed on request to cities in this country, and are frequently published in the daily papers, along with the forecast of the local office.  A careful study of these reports enables one to forecast to some extent the probable weather conditions of the day.

The first impression of a weather map (Fig. 50) with its various lines and signals is apt to be one of confusion, and the temptation comes to abandon the task of finding an underlying plan of the weather.  If one will bear in mind a few simple rules, the complexity of the weather map will disappear and a glance at the map will give one information concerning general weather conditions just as a glance at the thermometer in the morning will give some indication of the probable temperature of the day. (See Laboratory Manual.)

[Illustration:  FIG. 50. weather Map]

On the weather map solid lines represent isobars and dotted lines represent isotherms.  The direction of the wind at any point is indicated by an arrow which flies with the wind; and the state of the weather—­clear, partly cloudy, cloudy, rain, snow, etc.—­is indicated by symbols.