586 Principles of Weather Forecasting. 



September 8, 1900, when more than 5,000 human lives 

 and $20,000,000 of property were lost. 



The severe cold winds which are designated as the 

 "Northers" of Texas owe their origin to storm centers of 

 unusual intensity off the Gulf coast, which set large bodies 

 of air in motion from the northward, drawing it into them- 

 selves as they pass along to the southward and eastward. 



THUNDER STORMS, HAIL STORMS AND TORNADOES. 



Associated with the ordinary storms which have been 

 described in a preceding section there are others much 

 more local in their character, shorter in duration, but often 

 more violent in wind movement and precipitation. These 

 are thunder storms, hail storms and tornadoes. 



745. Relation of Tornadoes and Thunder Showers to Ordi- 

 nary Storms. Careful study of the time of occurrence and 

 distribution of these storms has shown that they are almost 

 always associated in a definite way with some cyclonic 

 wind movement, and that they usually originate to the 

 southeast, south or west of south of a storm center, in the 

 region designated by the cumulus clouds in the diagram, 

 Fig. 269. 



746. Tornadoes. Tornadoes are whirling winds of ex- 

 treme violence which last but a short time, progressing al- 

 most always from the southwest toward the northeast, often 

 at the rate of a mile per minute, sweeping a belt 40 to 80 

 rods wide and several miles long. Sometimes the width 

 of the zone of destructive winds may reach a full mile. 

 At the center of the tornado the moisture is swept together 

 by the revolving winds into a dark funnel-shaped cloud, 

 where the velocity of the whirling air may be so great that 

 few structures can withstand the enormous pressure they 

 develop. 



