TJdeatber predictions. 113 



tending it, taken in connection with the 

 weather conditions in the region for which the 

 prediction is made, an expert can ordinarily 

 forecast with some degree of accuracy. After 

 all that can be said, however, weather predic- 

 tions based upon maps are and have been far 

 from satisfactory. One who has been a close 

 student of local conditions for a number of 

 years will often predict with as great accuracy 

 as the weather bureau. Areas of low pressure 

 are followed sooner or later by a fall of tem- 

 perature; this is especially true in the winter 

 months. Sometimes this fall is very marked, 

 and then it is called a cold wave. These sud- 

 den changes of temperature are not thor- 

 oughly understood, but are supposed to be due 

 partly at least to rapid radiation of heat into 

 the upper regions, as the clear atmosphere 

 which usually attends areas of high pressure 

 is favorable to such a condition. Undoubt- 

 edly, too, there are dynamic causes, forcing 

 the colder air from the upper regions to the 

 earth, when it immediately flows off toward an 

 area of low barometer. 



Long-time predictions are purely guesses. 

 They sometimes guess on the right side, and 

 this gives them courage to make another. It 

 is an old saying that " all signs fail in dry 

 weather." In time of a drought it is true that 

 the indications which at ordinary times would 

 be surely followed by a rain are of no value. 



