THE WEATHER 



41 



from the west If the difference of pressure between Iowa 

 and Indiana were only 0.1 inch, the wind would be light; 

 if the difference were as much as 0.5 inch, the wind would 

 be much stronger. 



In general it may be said that there are always differences 

 of pressure between various parts of the country and that 

 the air is always flowing from regions of high pressure to 

 regions of low pressure. 



FIG. 26. Weather map for March 15, 1913 



40. Pressures as shown by weather maps. A good idea of 

 the distribution of high and low pressures (figs. 24, 25, and 

 26) throughout the country may be obtained by looking at 

 a weather map. The air pressures are represented on the 

 maps by heavy black lines. These lines are drawn in such 

 a way that all places which have the same air pressure are 

 shown on one line, and the figure which represents this pres- 

 sure is marked upon the map close to the line. These lines 

 connecting regions of the same air pressure are called isobars. 



