THE WEATHER 



39 



all things connected with the weather, it will no longer be 

 mysterious, and probably we shall be able to foretell it with 

 certainty. Men have not yet been able to discover all the 

 causes which are at work in the production of our weather, 

 but enough have been discovered to make the science of 

 meteorology one of greatest importance. 



FIG. 24. Weather map for March 13, 1913 



The three maps for successive days (figs. 24, 25, and 26) should be compared, to 

 show the progress of the storm and the changes in weather conditions. The 

 unbroken lines are the isobars ; the arrows fly with the wind ; the blackened 

 circles indicate cloudy weather at the time of observation ; the figures near the 

 arrows show temperature; the broken lines are isotherms of zero and freezing 

 temperatures. These figures are from the United States Weather Bureau 



39. Air pressure and winds. The first thing we must study 

 is the winds, for most other features of the weather depend 

 upon winds. Winds are merely air in motion, and of course 

 the faster the motion the harder the wind is said to blow. 



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 



