592 Principles of Weather Forecasting. 



lie to the northwest, at right angles to the line of progress. 

 The evidences of a rotary motion of the air in a tornado 

 are abundant and conclusive, and in Fig. 276 are repre- 

 sented some of these. 



754. Formation of Thunder Showers. Thunder showers 

 appear to have an origin similar to that of tornadoes, but 

 evidently occur where there is less air to change places, and 

 probably also where the depth of the overlying stratum is 

 less. Indeed, it appears very often, if not generally, true 

 that a volume of cold heavy air has dropped directly to 

 the ground and is moving bodily against the warmer moist 

 air, which it is forcing upward, as represented in the lower 

 left-hand corner of Fig. 273. The rapidly ascending 

 warm moist air is cooled by expansion and by mixing with 

 the cold air, thus giving rise to the heavy precipitation so 

 often observed. 



The rolling movement shown in the diagram is often 

 violent enough and involves so great a hight in the at- 

 mosphere, that often raindrops are carried round and 

 round until they become very large before they are able to 

 fall. If the vertical circulation reaches above the zone 

 of freezing temperature the raindrops freeze, forming hail. 

 These hailstones, in the most violent storms, are often 

 carried around with such force and so many times that they 

 become very large before they are able to overcome, by 

 their weight, the velocity of the air, and fall to the ground. 



