284 PHYSIOGRAPHY 



Weather maps are made by the Weather Bureau, a branch of 

 the National Department of Agriculture. They are prepared in 

 various offices of the country, where telegrams are received daily 

 from numerous stations in different parts of the country. Each 

 telegram tells the pressure, the temperature, the direction and 

 velocity of the wind, the cloudiness, the rainfall and the snowfall, 

 etc., at the station whence the telegram is sent. These stations 

 are established and maintained by the Government. 



Explanation of the Map 



1. Isobars. The isobars of the map show a range of pressure 

 from 30.6 inches in the area centering about the Hudson River 

 Valley, to 29.5 in North Dakota. The pressure is more than 30 

 inches in the eastern half of the country, less than 30 inches in the 

 western interior, and more than 30 inches in an area near the Pa- 

 cific coast. 



The isobar of 30.6, in the eastern part of the United States, 

 is a closed line. Outside of it is the isobar of 30.5. Since the 

 pressure rises as the isobar 30.6 is approached from outside, it is 

 inferred that it continues to rise after this isobar is passed. The 

 area inside it, therefore, has a pressure of more than 30.6 inches, 

 but not so much as 30.7 inches, else another isobar would have 

 been represented. Similarly, all points between the isobars of 

 30.6 and 30.5 have pressures between those indicated by these 

 figures. 



The center of this high-pressure area is marked " high." " High " 

 on the weather map means an area where the pressure is distinctly 

 higher than that of its surroundings, and generally exceeds 30 

 inches, and the word is placed in the center of such an area. 



To the west of the high over the Hudson River Valley the 

 pressure decreases steadily to North Dakota, where there is a center 

 of low pressure, marked "low." "Low" means an area in which 

 the pressure is less than that of its surroundings, and generally 

 less than 30 inches. On the weather map the word is placed in that 

 part of such an area where the pressure is lowest. 



The isobar of 29.5 about the low in North Dakota is a closed 

 line. Since the pressure is becoming less as this line is approached, 



