THE UNITED STATES WEATHER BUREAU 



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remains at the lowest point reached. Each of these cam 

 then be set again. 



Rain gauge. In order to measure the amount of rain, 

 a rain gauge is used (figure 205). This is so constructed 

 that the area of the top of the funnel, which receives the 

 rain, is ten times the area of the small tube in which it col- 

 lects. This raises the level of the water ten times as high, 

 and makes it easier to read. This is measured by means of 

 a ruler and the result is 

 divided by ten to get the 

 true rainfall. In order 

 to measure snow, a vol- 

 ume equal in area to the 

 top of the funnel and as 

 deep as the fall of snow 

 is collected and melted, 

 and the water is meas- 

 ured the same as the rain. 



When the reports of 

 the weather made by the 

 observers in various 

 parts of the country 

 have been received at Washington and other cities, the fig- 

 ures are put down in their proper places on a blank map 

 of the United States ; from these a weather map like that 

 shown in figure 199 is constructed. 



Weather forecasts. This map when completed gives a 

 general view of the weather conditions for the day through- 

 out the whole country. Having made this map, how are 

 the weather forecasters able to foretell what the weather 

 for a certain place will be on the following day? A study 

 of a great many maps for many years has shown that after 

 a low area has formed in the western part of the country r 

 it moves across the country in an easterly direction at the 

 rate of several hundred miles a day. This speed varies 



FIG. 205. Rain gauge. 



