A STUDY OF WEATHER MAPS 263 



With the passing of an elliptical LOW on November 11, 1911, the 

 temperature at Peoria, 111., dropped from 75 at 4:05 p.m., to 52 at 

 5 o'clock, freezing shortly after 7 p.m., and reaching 17 at midnight. 

 This was an exceptional storm. 



Considering both types of LOW, what portion has warm or 

 rising temperature? Which portion cold or falling tempera- 

 ture? State the general law of temperature changes with the 

 passing of a LOW. 



287. How the Clouds Changed. Figure 188 is based on the 

 map of December 5, when the storm center was near Omaha. 

 It shows a vertical cross-section of the storm taken along a 



1500 MILE ?( 



FIG. 188. Cross-section of a storm on December 5th along a line from 

 Omaha to Northern New York. The illustration shows a common arrange- 

 ment of clouds and air movements in the lower few miles of air in such a storm. 



line from Omaha to the St. Lawrence Valley. The clouds 

 are shown in the order of their heights. Notice that the heavy 

 lower clouds and the rain or snow extend at 7 a.m. from 

 Nebraska to Lower Michigan. The cirrus extend to western 

 New York in the morning. They reached the St. Lawrence 

 Valley shortly before noon. 



288. General Rule ; Cloud Indications. The cirrus clouds 

 of a well-developed storm usually spread farthest ahead and 

 appear first as the storm approaches. Lower and denser 

 clouds follow. Often they come in the order, cirrus, cirro- 

 stratus, alto-stratus, strato-cumulus; but there is great variety. 



