132 THE POPULAR SCIENCE MONTHLY 



toward its center, or point of lowest pressure. Marked "Low" on the 

 weather map, the cyclone is variously called storm, depression, or disturb- 

 ance. Cyclones vary greatly in size, some being as large as the whole 

 Mississippi Valley, while others are no larger than New England. In 

 the United States they usually move from west to east at an average 

 rate of 300 miles a day, the rate being faster in winter than in summer. 

 In general, the wind velocity varies directly as the barometric gradient, 

 that is, the rate of change of barometer as measured outward from the 

 center. Cyclones are regions of clouded sky, with more or less precipita- 

 tion, and as they pass alternately with the " Highs " in endless pro- 

 cession across the northern and central portions of the country they pro- 

 duce the frequent weather changes which are characteristic of these 

 regions. In winter, when they are most common and follow the more 

 southerly routes, they bring warm weather at the front and cold weather 

 at the rear. A tornado, on the other hand, is a violent local storm of the 

 thunderstorm type, with whirling and ascending winds of extremely 

 high velocity, causing destructive effects over paths varying in width 

 from a few feet to a few miles. They occur during the summer half- 

 year only, and usually during the hottest part of the day. Not only are 

 they always associated with thunderstorms, but they may be considered 

 overdeveloped storms of that class. While cyclones and tornadoes thus 

 have many common characteristics, custom has identified the use of the 

 terms with certain meanings. Cyclone, as a general term, refers to any 

 whirling mass of air, while tornado, the special term, refers to a par- 

 tirjlar kind of whirl. However, as used by the Weather Bureau their 

 avplication is that described above. 



The frequent expression in winter that "another storm is brew- 

 ing at Medicine Hat " seems to be based upon a false association of that 

 station with the origin of our weather. Charts of the weather of the 

 whole northern hemisphere, now made daily at the central oflBce of the 

 Weather Bureau at Washington, show that the cyclones and the anti- 

 cyclones which determine our weather move from west to east in end- 

 less procession. Some of the individual areas may be followed through- 

 out the entire circuit around the earth, while others can be traced for 

 only short distances. Neither Medicine Hat nor any other single sta- 

 tion serves as a starting point. However, well-defined storm tracks are 

 now recognized. Certain stations in the Canadian northwest are closely 

 watched for indications of an oncoming storm, which, if it follows the 

 usual route, will enter the northwestern states one to three days later, 

 subsequently passing eastward and finally passing off the Atlantic coast. 

 Because of their positions on the storm tracks, and not because of any 

 center of storm formation, should stations like Medicine Hat be of 

 meteorological interest. 



What is popularly Imown as the equinoctial storm is supposed to 



