THE WEATHER 29 



In order to explain the cause of winds we must go back 

 to the subject of air pressure and barometers. Anyone who 

 has watched a barometer for several days knows that even if 

 it remains in one place, its indicator goes up and down, thus 

 showing that the pressure is constantly changing. If we should 

 secure a record of the readings of barometers at a. great many 

 places in the United States at the same time, we should find 

 that they would not be the same. The readings would show 

 that in some parts of the country the pressure is high, while 

 in others it is low. We might find, for instance, that the 

 barometers read 29.8 inches in Iowa but only 29.5 inches in 

 Indiana. If we should do this for several days, we should 

 find that the pressure is never the same all over the country, 

 and that it does not long remain stationary at any place. 



If the pressure of the air is high in Iowa and relatively low 

 in Indiana, we can see that the air next to the ground in Iowa 

 will tend to flow away toward places of less pressure. It will 

 move from Iowa toward Indiana across the intervening state 

 of Illinois, and Illinois will, therefore, have a wind 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. 

 What has been said about these three states is only an example 

 of what is actually happening in the country all the time. 



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. 



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

 the distribution of high and low pressures (figs. 19, 20, and 

 21) throughout the country may be obtained by looking at a 

 weather map. On such a map many things are represented, 

 but we shall give our attention just now to the air pressures 

 only. These are represented on the maps by heavy black 

 lines. These lines are drawn in such a way that all places 



