AIR 



If a line is drawn through all points on the surface of the 

 earth having the same temperature at the same instant, such 

 a line is called an isotherm (Fig. 254). 



Weather maps. Scattered over the United States are 

 about 125 Government Weather Stations, at each of which 

 three times a day, at the same instant, accurate observations 

 of the weather are made. These observations, which con- 

 sist of the reading of barometer and thermometer, the deter- 



tM 160 WO OO 100 80 60 40 20 20 40 W 



FIG. 254. Isotherms. 



mination of the velocity and direction of the wind, the de- 

 termination of the humidity and of the rainfall, are tele- 

 graphed to the chief weather official at Washington. From 

 the reports of wind storms, excessive rainfall, hot waves, 

 clearing weather, etc., and their rate of travel, the chief offi- 

 cials predict what the weather conditions will be at a defi- 

 nite future time. In the United States, the general move- 

 ment of weather conditions, as indicated by the barometer, is 

 from west to east, and if a certain weather condition prevails 

 in the west, it is probable that it will advance eastward, 

 although with decided modifications. So many influences 



