TORNADOES. 147 



year. The most violent, however, take place during the spring and autumn. 

 The width of the path of the storm is very irregular and may vary from many 

 rods to many miles. In either case the velocity at all points within the storm's 

 path is not necessarily the same ; in fact such a conditon never occurs. The dura- 

 tion of the storm is also extremely variable, it may continue for only a few min- 

 utes or for several hours, although in the latter case the maximum velocity is not 

 maintained throughout the entire period. On the contrary there are periods of 

 recurrence alternating with decided diminutions of the highest activity. There 

 are perhaps but few portions of the country altogether free from the possibility of 

 their occurrence. In the low table lands of mountainous regions where most of 

 the country is extremely broken, the habitable portions are shielded from the 

 power of violent wind storms. No surface currents can attain any great velocity 

 in such regions although on the mountain peaks and elevated plateaus dangerous 

 hurricanes at times prevail. 



Whirlwinds. — In defining these disturbances it will be best perhaps that you 

 should be asked to recall the occurrence, on a warm, dry day, of the formation of a 

 dust-whirl as it suddenly bursts upon you in the open street, fairly enveloping your 

 body with fine particles of dirt, straw, leaves and the like. Whirlwinds suddenly 

 start up from some barren, sandy spot unduly exposed to the direct rays of the 

 sun. Over a small surface thus exposed the air rapidly rarifies and ascensional 

 currents form which move spirally inward and upward, carrying dust, leaves, 

 straws and sometimes objects of considerable weight. The air within the whirl 

 moves either from left to right or m the contrary direction. The whirlwind's path 

 has a diameter of several feet (sometimes rods) and the direction of its course of 

 movement is decidedly irregular, possibly moving toward any point in the com- 

 pass. On the sandy plains of Arizona, Southern California and Nevada these 

 phenomena occur with great frequency during the summer months. Columns of 

 whirling sand, sometimes several in a group, move rapidly over the surface. 

 Whirlwinds are harmless and generally of but a few moments duration. In 

 comparison with the tornado let it be born in mind that the former starts from 

 the earth's surface, extends upward and moves onward, not leaving the earth, 

 being solely confined to the region of surface currents. The tornado forms near 

 the superior limit of the lower regions of the atmosphere and between the upper 

 and lower sets of currents or the currents prevailing in the upper and lower re- 

 gions of the atmosphere. The former currents are indicated by the appearance 

 of the fine cirrus clouds and the latter by the heavy cumulous formations. 

 From this lofty seat of origin the tornado cloud gradually descends to the earth's 

 1' surface increasing rapidly in size and augmenting in power. 



Waterspouts. — These disturbances generally form at a considerable height 

 in the air, although at times they seem to ascend from the water's surface; that is 

 to say, there is no visible agent influencing the ascension of the water, but of 

 course in every instance the causative power is from above and in the latter case 

 near the water's surface. When I speak of the formation of the waterspout at a 

 considerable height in the air, I mean that the embodiment of the whirl or the re- 



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