Page:The National Geographic Magazine Vol 16 1905.djvu/324

288 The National Geographic Magazine

of the centers of the cyclones for August during a ten-year period, the anti-cyclones following about the same lines. Adding the numbers at the ends of the lines and at the braces that inclose groups of lines, it is found that 83 storms either had their origin in the states or else came to them from the West Indies or passed up through the ocean near enough to affect the Atlantic coast. The influence of the high western plateau and its mountains in the formation of storms is illustrated by the fact that 57 of these storms had their inception along the mountain system that runs through Colorado, Wyoming, and Montana, and that none came in from the Pacific Ocean. August storms move at the rate of 16 to 26 miles per hour, or about 500 miles a day. Wherever the storms originate they are seen to have a strong tendency ultimately to reach New England.

Now turn to chart XIII, which gives the storm tracks for February for a period of ten years. Against the 83 storms of August there are 98 shown for February for the same period — 1884- 1893. The tracks curve down farther to the south, many of them come in from the Pacific, and a large number form in Texas, but, like those of August, they finally pass over New England, which fact explains the variability of the weather of the latter region.

As regards storm conditions, the year may be divided into three parts in the Northern Hemisphere. December, January, February, and March are dominated by swiftly moving storms, swinging far to the south and carrying wide oscillations of temperature clear to the northern boundaries of the tropics, with general precipitation ; June, July, August, and September, by ill-defined storms and a sluggish movement of them, with many local rains of small area, rather than general storms, while October and November are transition periods between the summer and the winter types, and April and May between the winter and the summer conditions.

At times there is an abnormal change in the rate of drift of the highs and the lows simultaneously over the eastern and the western continents and the intervening oceans that throws weather forecasts temporarily into confusion. It is difficult to assign a reason to such sudden departures from usual conditions. It may be due to the accumulation of large bodies of air over continents or oceans from which no daily reports can be received. When momentum expends itself against gravity there may be a banking up of air in unexplored regions, and its potential may become suddenly available in such a way as to accelerate or retard the gen- eral drift of storms, or it may be due to the complex dynamics of motion of the vast gaseous sphere from which the earth receives light, heat, and various other radiations

When winter has become well estab- lished there often develops a permanent high over the great plain between the Rocky Mountains and the coast ranges, which remains inactive for weeks at a time, lows and other highs passing down from the north along its east front without materially disturbing it. Its principal function is to stop the drift of storms into the continent from the ocean immediately west of it. In mid- summer the high may be replaced by a stagnant low, and hot scorching winds blow steadily for many days over the states lying east and southeast of the low, withering the wheat and corn of the central Mississippi and lower Mis- souri Valleys. Charts XIV and XV show the most frequent routes of storms in the Northern Hemisphere,

The influence of the area of high pressure in deflecting storms from their normal or usual course is set forth by Professor Garriott in his paper on " Tropical Storms in September." In