6 THE WEATHER 



and attended by precipitation in the form of rain or snow, and their 

 passage is usually followed by clearing weather. The temperature on 

 a given parallel west of a "low" may be reasonably looked for on 

 the same parallel to the east when the "low" has passed, and when 

 the night is clear and there is but little wind, frost is likely to occur 

 along and north of an isotherm of 40. A "low" is generally 

 followed by a "high," which in turn is followed by another "low." 



By bearing in mind the usual movements of "lows" and "highs" 

 and the general conditions referred to that attend them, coming 

 weather changes may be frequently foreseen. "Lows" often move 

 south of east from the Rocky Mountains to the Mississippi Val- 

 ley, and then change direction to north of east. " Lows " of tropi- 

 cal or subtropical origin often move in a westerly direction to our 

 south Atlantic and Gulf coasts, and then recurve to the northeastward. 

 The centers of " lows " do not as a rule cross isotherms, but generally 

 follow the general trend of the isothermal lines. Cold waves are 

 always accompanied by, and forerun, " highs." 



When isotherms run nearly east and west, no decided changes in 

 temperature are likely to occur. When isotherms directly west of a 

 place incline from northwest to southeast, the temperature will rise; 

 when from northeast to southwest, the temperature will fall. 



Southerly to easterly winds prevail west of a nearly north and south 

 line passing through the middle of a " high," and also east of a like 

 line passing though the middle of a " low." Northerly to westerly 

 winds occur west of a nearly north and south line passing through the 

 middle of a " low," and also east of a similar line passing though the 

 middle of a " high." 



An absence of decided and energetic " lows " and " highs " indicates 

 that existing weather conditions will continue until later maps show 

 a change, which usually appears in the west. 



Weather Bureau Forecasts 



Forecasts of the weather expected during the ensuing thirty-six 

 hours are issued by the United States Weather Bureau daily at about 

 10 A.M. and 10 P.M. and are distributed to all parts of the country 

 by telegraph, telephone, mail, and by means of flag and whistle signals. 



Nearly all telephone companies cooperate or are willing to cooperate 



