WEATHER; WINDS AND STORMS; CLIMATE 115 



Bureau, for instance, may show that the atmospheric dis- 

 turbance occurs first, perhaps, in the lower Mississippi 

 Valley, then in the Ohio Valley, then in New York State, 

 and finally in New England. It is not so easy to trace the 

 course of storms in the western part of the country. The 

 great variations in altitude in that region cause irregulari- 

 ties in pressure which modify the course of the storms. 

 Their general direction is easterly. 



The study of storms requires a thorough understanding 

 of barometric conditions and humidity, as well as the rela- 

 tive position of mountains, plains, and oceans. It is, there- 

 fore, beyond the range of an elementary course in science. 

 But careful observation of changes in the weather leads, in 

 time, to a good judgment in regard to local probabilities, 

 so that one may learn to forecast the weather with consid- 

 erable accuracy, without understanding the laws that govern 

 the changes. 



124. Tropical Storms. Violent storms called hurricanes 

 sometimes start near the Gulf of Mexico and travel north- 

 ward along the Atlantic coast. These storms are greatly 

 feared because of damage to shipping. When the Weather 

 Bureau receives information of such storms, it directs that 

 storm signals be displayed, warning shipmasters to remain in 

 port. Much life and property are saved by these warnings. 



125. Local Storms. The term local storm usually refers 

 to precipitation which occurs in a small area and for a short 

 time. Such storms generally follow a change in temperature. 

 After a hot sultry day, as the air rises from the heated sur- 

 face of the earth, the vapor which it contains may condense 

 rapidly in meeting the higher, cool air. Rain may then fall 

 for a few minutes or perhaps hours. 



If the condensation is very rapid, electricity may be de- 

 veloped upon the clouds and flashes of lightning will show its 

 passage through the air. The thunder which follows has 

 the same cause as the sound which results from tearing a 



