284 



Common Science 



way. From several such 

 reports the weather men 

 to the east can tell how 

 fast the storm is travel- 

 ing and exactly which 

 way it is going. Then 

 they can tell when it 

 will reach their station 

 and can make the correct 

 prediction. 



Weather men do not 

 have to wait for an ac- 

 tual storm to be re- 

 ported. If the reports 

 from the west show that 

 the air is rising as it 

 swirls along that is, if 

 the barometer readings 

 in the west are low - 

 they know that this low- 

 pressure air is approach- 

 ing them. And they 



know that low pressure usually means air that is rising 

 and cooling and therefore likely to drop its moisture. 

 In the same way, if the barometers to the west show 

 high pressure, the eastern weather men know that the 

 air that is blowing toward them is being compressed and 

 warmed, and is therefore not at all likely to drop its 

 moisture ; so they predict fair weather. 



The weather man is not ever certain of his forecasts, 

 however. Sometimes the air will begin to rise just 



FIG. 155. 



The kind of mercury barometer 

 that you buy. 



