NINETEENTH ANNUAL YEAR BOOK— PART X 547 



Weather Conditions Favorable for Tornadoes. 



Atfention is invited to a copy of the daily weather map of the United 

 States Weatlier Bureau on page 34 showing the general weather condi- 

 tions prevailing at 7 A. M. May 9, 1918. An area of low barometer or 

 general storm center was located in central Nebraska, the lowest bar- 

 ometer reading being 29.18 inches at North Platte. The barometric grad- 

 ient or slope was steep from the Mississippi River across Iowa to eastern 

 Nebraska, amounting to .40 inch in 500 miles. As usual in such cases, 

 warm, southern, moisture laden air was being transported northward over 

 Iowa in tremendous volumes. Intervals of sunshine warmed the air 

 rapidly at the surface. From around 60 degrees in the morning the tem- 

 perature rose about 30 degrees by early afternoon, which set up strong 

 by the proximity of the general storm center. Towering cumulo-nimbus 

 convectional (up and down) currents, the up-currents being greatly aided 

 clouds here and there over the State raised their heads and in some cases 

 a large part of their bodies into the rapidly moving, wintry, westerly winds 

 aloft that had not had time to be warmed by the springtime sunshine 

 caught and radiated by the ground. Where these conditions were most 

 pronounced tornado vortices formed. By 7 p. m. of the 9th the general 

 storm center was crossing the Mississippi River at almost the identical 

 time and place that the tornadoes occurred in Clayton county. By the 

 next morning the general storm center had moved to a point in Canada 

 northeast of Lake Huron and the danger of further tornadoes was over 

 till a similar set of conditions could be pieced together in the meteor- 

 ological kaleidoscope. 



In this case there' was not long to wait, for May 21 was one of the 

 worst tornado days in the history of Iowa. The weather map of 7 a. m. 

 that date on page 35 shows a general storm center with a barometer 

 reading of 29.62 inch at Valentine, Neb. The barometric gradient or 

 slope across Iowa from the Mississippi River to extreme eastern Nebraska 

 was .50 inch in 400 miles, showing considerable more energy than the map 

 of May 9. Much the same temperature, moisture, wind and cloud condi- 

 tions prevailed, only that, if anything, the summits of the cumulo-nimbus 

 clouds rose higher. The great cloud mountains in which the Boone and 

 Newton tornadoes were generated were plainly visible at Des Moines. 

 In this case the general storm center took a wide detour. At 7 p. m. it 

 was near the northwest corner of Minnesota and by the next morning 

 It was north of Lake Superior. 



