74 STORMS AND WEA THER FORECASTS 



tilting about the low in a direction contrary to tlie movement of 

 the hands of a watch and you have a very fair conception of an 

 immense atmospheric eddy. 



Have you ever watched the placid water of a deep running 

 brook and observed that where it encountered a projecting crag 

 little eddies formed and went spinning down the stream ? Well, 

 our storms are simply great eddies in the air which are carried 

 along by the general easterl}^ movement of the atmosphere in 

 the middle latitudes of the northern hemisphere. But they are 

 not dee]) eddies, as was once supposed. The low marks the center 

 of an atmospheric eddy of vast horizontal extent as compared 

 with its thickness or extension in a vertical direction; tlius a 

 storm condition extends from Washington to Denver in a liori- 

 zontal direction and yet extends upward but four or five miles. 

 The whole disk of whirling air four or five miles thick and 1,500 

 miles in diameter is called a cyclone or c}' clonic system. It is 

 important that a proper conception of this fundamental idea be 

 had, since the weather sequences experienced from da}'^ to day 

 depend almost wholly on the movement of these traveling eddies, 

 cyclones, or areas of low pressure. 



The large figures in the four quarters of the cyclone show the 

 average temperature of each quadrant. The greatest diflference 

 is between the southeast and northwest sections. This is due in 

 part to the fact that in the southeast quadrant the air is drawn 

 northward from warmer latitudes, and in the northwest quadrant 

 the air is drawn southward from colder latitudes. The shaded 

 area shows the region of rain or snow fall during the preceding 

 12 hours. Unfortunately for the science of forecasting, precipi- 

 tation does not show that relation to the configuration of the 

 isobars that temperature, wind velocity, and wind direction do. 



Chart III, constructed from observations taken 12 hours later, 

 shows that the storm or cyclonic center, as indicated by the word 

 " low," has moved from central Iowa since 8 a. m. and is now. at 

 8 p-. m., central over the southern point of Lake Michigan. The 

 shaded areas show that precipitation has occurred during the 

 l^ast 12 hours in nearly the entire region covered by the cyclone. 



Chart IV, 12 hours later, shows that the precipitation has been 

 general tbroughout the entire area swept b}^ the cyclonic whirl. 



Chart V is quite dissimilar, in the information it conveys, to 

 any other of the charts accompanying this paper. From July 

 28 to August 10, inclusive, 1896, there was a remarkable hot wave 

 in the United States, extending from the Rock}"" mountains to tlie 



