The storms of the United States follow, however, year after year, a
series of tracks, not likely to change suddenly, and not irregular, but
related to each other by very well-defined laws.
The United States Weather Bureau has made a very intensive study of the
positions of the tracks of the storms. Fig. 55 shows the mean tracks and
the movement of storms from day to day. This map indicates that
generally there are two sets of lines running west and east, one set
over the northwestern boundary, the Lake region, and the St. Lawrence
Valley, the other set over the middle Rocky Mountain districts and the
Gulf States. Each of these is double, with one for the “highs” and one
for the “lows.” Furthermore, there are lines crossing from the main
tracks to join them together, showing how storms pass from one to the
other. On the chart, the heavy lines all belong to the tracks of the
“highs,” and the lighter lines to the track of the “lows.”