590
POPULAR SCIENCE MONTHLY.
Chart No. 3.—The line of tornado frequency naturally moves north with the sun, the tornadoes of winter and spring occurring in the south or border States, while the maximum of tornado frequency for the northern States is in June. Tornadoes are superinduced by unstable conditions of the atmosphere, which are particularly likely to prevail to the southeast and south of a cyclonic center, and the relation of these violent local storms to the great central disturbances is strikingly shown on the United States weather map of March 27, 1890, the day of the Louisville tornado. The parent cyclone was of enormous, though not abnormal, area. It had caused, and was causing, snow and rains from the Rocky Mountain slope to the Hudson Valley, from Arkansas to Minnesota. Its vortex, with a barometric pressure of 29.10 inches—as low as in some of our most destructive tropical cyclones or hurricanes—covering a large part of Illinois,
Fig. 3.
was drawing to it winds from all over the United States, from the Rocky Mountains to the Atlantic, from the Gulf of Mexico to the Canadian border. In front of the cyclone, pushed up by the warm southerly winds, the temperatures were all above freezing and, in its southeastern quadrant, reached summer temperatures of 70°. Several hundred miles through its center, in the rear, the temperatures were below freezing in its northwestern quadrant and 30° cooler in its southwestern quadrant than in its southeastern quadrant. Compared with this tremendous storm disturbance, the tornadic outbursts it caused in Kentucky were insignificant local eddies which, on this map, can only be indicated by crosses, though their violence caused a loss of 113 lives and property losses of over $3,000,000, 76 being killed, 200 injured, and property damaged to the extent of $2,500,000 in Louisville alone.
