Low barometric pressure, or the storm centers, are indicated on the map
by the word “low.” High barometric pressure centers are indicated by the
word “high.” Note how they move in an easterly direction; how they are
progressive. They can be compared to a series of waves, which we will
call atmospheric waves. The crest of the wave may be likened to the
“highs” and the troughs to the “lows.”
Usually the winds are southerly or easterly and therefore warmer in
advance of a “low.” When the “lows” progress east of a place, the wind
generally shifts to westerly and the temperature lowers. The westward
advance of the “lows” is preceded by precipitation, and almost always in
the form of rain or snow, following which the weather is generally
clear. Note how a “low” is followed by a “high,” and so on as they move
along eastwardly.