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