Page:Popular Science Monthly Volume 53.djvu/328

312 of Montana, and they follow either of two well-defined paths in their eastern course. One path leads southeasterly to the Georgia coast, from which locality the high follows the warm, moist air over the Gulf Stream to the Gulf of St. Lawrence, there to pass beyond our observation (Fig. 3). The other path extends across the Great



Lakes and New England to the same destination. In summer the larger number of highs enter our country from the Pacific Ocean about the latitude of Oregon, and either pass northward along the Pacific slope, whence a crossing is made over the Rockies to the northern circuit from Alberta, or through the Salt Lake region to the southern circuit from Alberta. Having determined the circuit which a high will follow, and having found its rate of travel from observation, it becomes largely a matter of calculation to foretell its progress across the continent.

While the highs have only two points of entrance to the United States, the lows or storms originate in nine different districts throughout the country. In Fig. 3 the heavy full lines represent the paths of the highs. The lighter full lines indicate the origins and paths of the lows, and the heavy dotted line is the axis or path of the middle of the cold waves. It will be seen that the lows follow the two circuits, northern and southern, of the highs, and that they occasionally cross over from the southern to the northern circuit. Some of them originate in the West Indies and travel up the Atlantic coast.

Fig. 4 shows the nine districts into which the United States has