244 



THE WEATHER 



been found. Pellets of solid ice about the size of rain drops are 

 frozen rain, caused by falling through freezing air below the 

 cloud. Such pellets are sometimes called sleet. Damaging 

 hail is rare in most localities. Falls of hail 8 in. or a foot in 

 depth have been known. Stones J^Sjn.Jn circumference have 

 been measured. 



271. The Tornado. The tornado is the most violent dis- 

 turbance of earth's atmosphere. It, too, is a local storm 

 usually not more than 50 to 500 yd. in width. A few tornadoes 

 have destroyed everything in a path a mile or more wide. The 



FIG. 173. Tornado. "" Rather small funnel cloud extending to ground. 



tornado appears to be caused by a special and violent devel- 

 opment of the ascending air movements that are always pres- 

 ent in a cumulo-nimbus cloud. The uprush of warm air is 

 stronger in some cumuli than in others (see Figs. 159, B, and 

 160, B). Under favorable conditions it may become strong 

 enough to form a whirl, or eddy, within the cloud as the air goes 

 upward, much like the whirl that water forms in a tub or pan 

 as it runs down through an opening in the flat bottom. This 

 air whirl when well established within the cloud readily extends 



