288 The National Geographic Magazine 



of the centers of the cyclones for August 

 during a ten-year period, the anti-cy- 

 clones 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 west- 

 ern 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 sys- 

 tem that runs through Colorado, Wy- 

 oming, 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 ulti- 

 mately 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 Au- 

 gust, they finally pass over New En- 

 gland, which fact explains the varia- 

 bility of the weather of the latter region. 



As regards storm conditions, the year 

 may be divided into three parts in the 

 Northern Hemisphere. December, Jan- 

 uary, February, and March are domi- 

 nated by swiftly moving storms, swing- 

 ing far to the south and carrying wide 

 oscillations of temperature clear to the 

 northern boundaries of the tropics, with 

 general precipitation ; June, July, Au- 

 gust, 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 be- 

 tween the winter and the summer con- 

 ditions. 



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 in- 

 tervening oceans that throws weather 

 forecasts temporarily into confusion. 

 It is difficult to assign a reason to such 

 sudden departures from usual condi- 

 tions. It may be due to the accumula- 

 tion of large bodies of air over conti- 

 nents or oceans from which no daily 

 reports can be received. When mo- 

 mentum 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 



