CHAPTER XXVI. 

 WEATHER CHANGES. 



The forecasting of weather changes from 24 to 36 hours 

 in advance is based upon several well established facts: 

 (1) Rainy or cloudy weather is usually associated with 

 areas of low pressure, about which the winds move as rep- 

 resented in Fig. 269. (2) Fair or clear weather is usu- 

 ally associated^ with regions of high pressure. (3) Both 

 low and high areas have prevailing dimensions and move 

 in the United States from the west toward the east. 



If areas of low pressure always had the same diameter, 

 and if they traveled at the same rate and in the same di- 

 rection, it would be possible for anyone to forecast the 

 weather changes with much certainty 12 to 36 hours in ad- 

 vance. But with all the irregularity of form, dimension, 

 intensity, rate and direction of motion, it is possible for 

 even a local observer to form a rational judgment of the 

 approach, time of arrival and passage of an ordinary 

 storm. Indeed, it will seldom happen that a strongly de- 

 veloped storm can approach a locality without giving sure 

 signs of its coming 12 to 24 hours in advance. 



734. Prevailing Winds. In the forecasting of weather 

 changes it is important to have clearly in mind the direc- 

 tion of the prevailing winds of the locality, or those which 

 are not due to the storm whose approach is to be forecast. 



In most parts of the United States east of the Rocky 

 Mountains the prevailing fair weather winds are from 

 some westerly quarter and they should be the southwest 

 winds of the general world and continental system unless 

 modified by local conditions, such as give rise to "land and 

 sea breezes" or "mountain and valley winds." 



