588 Principles of Weather Forecasting. 



747. Schools of Tornadoes. When the conditions are ex- 

 tremely favorable for the formation of tornadoes they often 

 appear in schools, originating one after another or simul- 

 taneously, as the main storm center progresses across the 

 country, and Fig. 273 shows how these local but violent 

 storms are related to a storm center and how many may 

 develop in the southeast quadrant as it travels along. In 

 this figure the short, heavy straight lines to the southeast 

 of the center represent the paths of tornadoes which devel- 

 oped during its course. 



748. Distribution of Thunder Showers. Thunder show- 

 ers, like tornadoes, originate in the great majority of cases 

 to the southeast and south of a well developed storm center 

 and often large numbers of them, scattered over consider- 

 able areas, form as the storm progresses, much as is the 

 case with tornadoes, and Fig. 274 is a diagram showing 

 the advance of the front along which thunder showers orig- 

 inated in a storm of early May, 1892, as recorded in the 

 Monthly Weather Review of that month, p. 138. 



On May 3 a long low area had advanced from the south 

 and west and at 8 P. M. its lowest portion was central 

 north of Lake Huron. The front of the thunder shower 

 line had reached the east end of Lake Erie at 2 P. M. of 

 the same date and showers were in progress along the line 

 marked 2 P. M. in Fig. 274. As the storm center ad- 

 vanced the thuiider-shower-front also moved forward and 

 swept across the state, as shown by the curves on the dia- 

 gram, reaching Long Island at 2 A. M. on the morning of 

 May 4th, the front thus progressing from 20 to 30 miles 

 per hour. 



749. Conditions Tinder Which Thunder Showers and Tor- 

 nadoes Originate. In the diagram of Fig. 273 are repre- 

 sented the wind directions and temperature relations which 

 exist when conditions are favorable for the formation of 

 both of these classes of storms. There is a region of warm 

 moist southerly winds to the south and east of the low area 



