IV RATHER MAPS 89 



any shift in isobaric lines showing change in atmospheric 

 pressure. 



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. 49). 



83. 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- 

 mination of the velocity and direction of the wind, the 

 determination of the humidity and of the amount of rain or 

 snow, are telegraphed to the chief weather official at Wash- 

 ington. From the reports of wind storms, excessive rainfall, 

 hot waves, clearing weather, etc., and. their rate of travel, the 

 chief officials predict where the storms, etc., 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 certajn weather condition prevails 

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

 though with decided modifications. So many influences 

 modify atmospheric conditions that unfailing predictions are 

 impossible, but the Weather Bureau predictions prove true in 

 about eight cases out of ten. 



The reports made out at Washington are telegraphed on 

 request to cities in this country, and are frequently published 

 in the daily papers, along with the forecast of the local office. 

 A careful study of these reports enables one to forecast to 

 some extent the probable weather conditions of the day. 



The first impression of a weather map (Fig. 50) with its 

 various lines and signals is apt to be one of confusion, and the 

 temptation comes to abandon the task of finding an underly- 

 ing plan of the weather. If one will bear in mind a few sim- 



