302 



PHYSIOGRAPHY 



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seasons. Further, they are most common on clays which are 

 unusually warm, and during the warmer parts of 1 these days; but 

 there are occasional thunder-storms in the winter, and there are 

 thunder-storms at night. 



The first indication of a thunder-storm is usually a large cumulus 

 cloud (Fig. 256) which, in the zone of the westerly winds, generally 



appears in the west. It moves 

 eastward, and as it reaches the 

 place of the observer, there is 

 usually a smart breeze, or thun- 

 der-squall, rushing out before it. 

 Shortly after the squall the rain 

 begins to fall. The rainfall may 

 be heavy, and the drops large 

 but the downpour does not us- 

 ually last more than an hour, 

 and, in many cases, much less. 

 A second thunder-storm some- 

 times follows close upon the 

 first, thus prolonging the period 

 of rainfall. When a thunder- 

 storm has moved on to the east, 



the air is notably cooler and fresher, and the barometer distinctly 

 higher. 



When water is condensed rapidly in the air, electricity is pro- 

 duced, and the surface of each water particle becomes charged 

 with electricity. The charge of the individual droplets increases 

 as they increase in size, and the lightning is due to the discharge of 

 the electricity from one part of a cloud to another, or from one 

 cloud to another, or from the cloud to the ground. 



The flash of lightning is followed by thunder, the noise being 

 due to the vibrations in the air caused by the electrical discharge. 

 The thunder has been compared to the noise which follows the 

 explosion of a rocket or the cracking of a whip (Davis). 



In middle latitudes, most thunder-storms occur during the passage 

 of cyclones, though they do not accompany all cyclones. They are 

 more common on the south sides of cyclones than elsewhere , and 



Fig. 256. Ascending currents and 

 cumulus cloud before a thunder- 

 storm. (Ferrel.) 



