etric 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
repres