with full force at noon. Sailing vessels, yachts, and fishing dories
remain within reach of port if the barometer foretells storms.

[Illustration: FIG. 49.--Isotherms.]

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