Our Heralds of Storm and Flood 



595 



A Typical Weather Map 



The solid lines are isobars ; the broken lines are isotherms. The shaded portion of the 

 map indicates the area over which precipitation has occurred during the twelve hours preced- 

 ing 8 A. M., 75th-meridian time. The arrows point in the direction the wind is blowing. 



it grew to immense proportion. Thence 

 it dashed upon our western coast, ahnost 

 simultaneously striking California, Ore- 

 gon, and Washington. It swept over 

 the Rocky Mountains as if they were a 

 five-foot fence, dashed over Wyoming, 

 South Dakota, Nebraska, Kansas, Okla- 

 homa, Missouri, Illinois, and Wisconsin, 

 and finally disappeared in the Great 

 Lakes four days after its entrance. A 

 storm like this revolves all the time it is 

 advancing. It moves like a spinning 

 plate flung across the room,* or like the 

 top which the small boy shoots spinning 

 across the sidewalk ; in fact, the storm is 

 a gigantic top about a thousand miles in 

 diameter and several miles high. 



This map illustrates perfectly the dif- 

 ferent kinds of weather that such a great 



* The cyclone revolves in a direction opposite 

 to the hands of a watch. 



cyclone will bring. As the storm ad- 

 vances, it brings a deluge of rain or 

 snow, but it restores the sunshine before 

 it disappears. The reason is as follows : 

 The wind in the front half of the cyclone 

 is from the south, and as this warm wind 

 comes into colder latitudes, it cools, and 

 the moisture in it is condensed, so that 

 we have rain and snow-storms. The 

 wind in the rear half of the cyclone is 

 from the north, and is thus cold; as it 

 comes into warmer latitudes, it grows 

 warmer and is able to absorb the moist- 

 ure in the air, so that we have clearing 

 weather. 



Such a cyclone may be generated by 

 the clashing of two antagonistic currents 

 of air, one current coming perhaps from 

 the south and the other from the north. 

 As the two currents wrestle, they are 

 caught by the never-ending stream of at- 



