Wind and Weather 143 



sent to all sections of the country, usually at regular periods and 

 in time to give full warning of storm or cold. 



In many leading cities, daily weather maps are prepared and 

 are widely distributed, thus giving the public information in 

 regard to the probable course of storms and the prospects for 

 good or bad weather. Condensed reports also are given by news- 

 papers and by radio. The saving to the business interests of 

 the country every year brought about by the work of the Weather 

 Bureau is large and justifies the expense of its maintenance. 

 Among the kinds of bulletins sent out are warnings to ships and 

 shipping interests of approaching storms, to railroads and shippers 

 of perishable goods of coming cold waves, and to growers of 

 fruits and garden products of probable frosts. 



Construction of Weather Graphs and Maps. Anyone can 

 construct a weather graph if he has the data. It is merely a 

 graph showing the variation of a weather condition, such as 

 atmospheric pressure, temperature, or inches of rainfall, for a 

 period of time. To obtain this data, make daily observations of 

 weather conditions for a given length of time and keep a record 

 of them. Then make a graph, based on the records kept. 



To construct weather maps, secure blank weather maps from 

 the Weather Bureau at Washington, D. C, and fill in the maps 

 from the table on page 142, or similar data. This will help you 

 to understand weather maps and many things about the weather. 

 By reference to the table, draw on the map a connecting line 

 through all places with a temperature marked 50. Draw other 

 lines through places marked 40, 30, and so forth. These lines 

 are called isotherms. They pass through places having the same 

 temperature at a fixed time on a certain date. 



On the same map, wind directions may be indicated by 

 arrows pointing with the wind ; that is, for a wind coming from 

 the southwest make the arrow point toward the northeast. Cer- 

 tain symbols may be used to show the weather conditions, as a 

 white circle for fair, a 'black circle for cloudy, or a letter "R" 

 for rain. 



The pressure data may be plotted in the same manner for 

 the different places on the map. You know that air has weight 



