10 



l-o at least 45 degrees and tliat tlie loiii[)eraLure at a given hour (as "> 

 p. m.) to-morrow will be at least 15 degrees lower than it is at the 

 same hour to-dajr. Cold waves do not go with every anti-cyclone, for 

 in July the temperature rarely falls to 45 degrees, but there never is a 

 cold wave without an anti-cyclone. They originate in Canada just 

 east of the Eocky Mountains and follow three general courses — one 

 from west to east along the Great Lakes, another from Manitoba di- 

 rectly south to Texas, and a third in a southeasterly direction. This 

 last course crosses Indiana. 



The knowledge of tlie general movements of low and liigh areas 

 is the basis of the prediction of weailior conditions. T\vi(-e each day 

 the forecast ofBcial rocei\es telegiams giving the conditions at a large 

 numl)er of stations. These lie enters on a map and after observing 

 the changes since the last observation, he ventures to predict what 

 will take place during the next 36 hours. 



The movements of storm areas are influenced to a considerable ex- 

 tent by local conditions, such as the presence of lakes, river valleys 

 and mountain ranges. The forecast official must know not only the 

 laws governing the general movements of the storm ai'eas, but he 

 inust also keep in mind all these local variations. He is the most suc- 

 cessful forecaster who knows best the etl'ect ot these local conditions. 

 These are learned only by constant observation of the things about 

 him, and here we have another illustration of the fact that the sound- 

 est and most useful knowledge is that which we derive from a careful 

 (iliscrvntion of the things with which we are daily brought in contact. 



LITEIt.VTUKK. 



The iinly tiiil;ible wiirk for general readiiij; for American students is "American 

 Weather," by ( u-u. ,\. W. Greely. New York : Dodd, Mead * Co., $-J.50. 



