Page:Popular Science Monthly Volume 86.djvu/136

132 toward its center, or point of lowest pressure. Marked "Low" on the weather map, the cyclone is variously called storm, depression, or disturbance. 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 precipitation, and as they pass alternately with the "Highs" in endless procession across the northern and central portions of the country they produce 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 particular kind of whirl. However, as used by the Weather Bureau their application is that described above.

The frequent expression in winter that "another storm is brewing 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 office of the Weather Bureau at Washington, show that the cyclones and the anticyclones which determine our weather move from west to east in endless procession. Some of the individual areas may be followed throughout the entire circuit around the earth, while others can be traced for only short distances. Neither Medicine Hat nor any other single station 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 known as the equinoctial storm is supposed to