WEATHER MAPS 303 



they often occur at a considerable distance from the center of the 

 storm. In middle latitudes, thunder-storms move generally from 

 west to east, while in the zone of trade-winds they move from east 

 to west. In both cases they move with the prevailing winds. 



The forward movement of thunder-storms is commonly 20 to 

 50 miles an hour. They often spread, and become weaker as they 

 move forward (Fig. 257). They usually disappear before they 

 have traveled far. The period of a thunder-storm is usually much 

 shorter than that of the cyclone which it accompanies. 



It sometimes happens that lightning at a great distance lights 

 the clouds over a region where the 

 electric discharge itself cannot be 

 seen. This lighting of the clouds 

 is often called heat lightning, be- 

 cause it is more commonly seen 

 in hot weather than at other times. 



Rainbows sometimes aocom- Fig 2 5 7 ._ shapeo f thunder . storm 



pany or follow thunder-storms. in ground-plan, illustrating its 



They are usually seen just after Jj ^WafdoT " * ^ 

 the passage of a thunder-storm, 



while a little rain is still falling, but after the sun has appeared. 

 They are seen opposite the sun, that is, in the west in the morning, 

 and in the east in the evening. There is sometimes a second bow 

 outside the first, but fainter. The rainbow is due to the effects of 

 the drops of water in the atmosphere on the sun's rays. A bow 

 is also seen when water spray, such as that at a great waterfall, is 

 seen in the bright sunlight. 



Whirlwinds. Distinct ascending whirls of air are often seen 

 on hot days. They are most distinct in dusty regions, for there 

 the dust which is swept up makes the whirl conspicuous. From 

 a given point in the Mojave Desert, in California, as many as eight 

 or ten of these whirls, some of them rather large, have been seen at 

 one time on a hot summer day. The whirlwinds are probably 

 caused by the excessive heating of the air at some point, and this 

 excessive heating gives rise to a sharp convection current. It 

 moves on for a time with the prevailing wind, but soon plays out. 



In humid regions, whirlwinds do not usually appear to extend 



