National Geographic : 1897 Mar
STORMS AND WEATHER FORECASTS ture can occur until the high pressure of the northwest is replaced by a low pressure, and convectional currents are drawn toward the northwest instead of being forced southward from that region. To summarize in regard to cold waves, it may be said that when the charts indicate the formation of a body of dense, cold air in the northwest, as shown by the barometer readings, the skilled forecaster is on the alert. He calls for special observa tions every four hours from the stations within and directly in advance of the cold area, and as soon as he becomes convinced that the cold wave will sweep across the country with its attend ant damage to property, destruction to animal life, and discom fort to humanity, the well-arranged system of disseminating warnings is brought into action, and by telegraph, telephone, flags, bulletins, maps, and other agencies the people in every city, town, and hamlet, and even in farming settlements, are usually notified of the advancing cold twelve, twenty-four, or perhaps even thirty-six hours before it reaches them. Charts XIV and XV show the cyclonic systems prevailing at 8 p. m. on the days of the Louisville and St Louis tornadoes. Several tornadoes occurred on each day; their tracks are shown by rows of crosses in the southeast quadrants of each cyclone. Especially do I wish to emphasize the distinction between the cyclonic storm and the tornado. The press and nine out of ten people who should know better use these terms as synonymous. The cyclone shown on Chart XIV, which is fairly typical of all cyclones, is a horizontally revolving disk of air, covering the whole United States from the Atlantic ocean westward to and including the Mississippi valley, with the air currents from all points flowing spirally inward toward the center, while the tor nado is a revolving mass of air of only 500 to 1,000 yards in diameter, and is simply an incident of the cyclone, nearly always occurring in its southeast quadrant. The cyclone may cause moderate or high winds through a vast expanse of territory, while the tornado, with a rotary motion almost unmeasurable, always leaves a trail of death and destruction in an area infini tesimal in comparison to the area covered by the cyclone. The tornado is the most violent of all storms, and is more fre quent in the central valleys of the United States than elsewhere. It has characteristics which distinguish it from the thunder storm, viz., a pendent, funnel-shaped cloud and a violent, rotary motion in a direction contrary to the movements of the hands of a watch, together with a violent updraught in the center.