WEATHER PREDICTION 



45 



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FIG. 74. MAXIMUM- 

 MINIMUM THER- 

 MOMETER 



by its temperature, warm air 

 being able to hold more mois- 

 ture than cold air. 



At the U. S. Weather Bureau 

 stations temperature readings 

 are taken on maximum-mini- 

 mum thermometers, one type 

 of which is shown in Figure 74. 

 The little steel index riders are 

 carried by the mercury to the 

 highest and lowest tempera- 

 tures for the day. Such a ther- 

 mometer must be set each day 

 by means of a magnet as illus- 

 trated. 



On the weather map dotted 

 lines, called isotherms, are 

 drawn through localities on the 

 map which have the same tem- 

 perature. In general lower tem- 

 peratures prevail near high- 

 pressure areas and higher tem- 

 peratures near low-pressure areas. 



Of what value are barometers in weather forecast- 

 ing? The barometer enables one to tell at any time 

 how the pressure of the air is changing and when a 

 "low" or "high" is passing. Since pressure differences 

 in the atmosphere are the chief determiners of weath- 

 er, the aneroid barometer has become a household 

 device in many places. The one shown in Figure 75 is 

 a common type used in offices and homes. 



Of what importance are winds in weather predic- 

 tion? From the study of a previous topic in this unit, 

 you have learned that the United States lies in the 

 wind belt known as the prevailing westerlies. These 

 warm winds blow over the Pacific Ocean and pick up 

 large quantities of moisture. As they blow over the 

 western slope of the Rocky Mountains, they rise, the 

 air expands and is cooled, and much of the moisture 

 falls as rain or snow. This makes for the ample rain- 

 fall in all Pacific Coast states, buf'is also the cause 

 of the dryness and aridness of the eastern slopes of the 

 Rockies. 



These same winds continue eastward across the 

 continent, bringing us the succession of high- and 

 low-pressure areas which we know as weather 

 changes. "Lows" and "highs" follow one another 

 across the continent with speeds ranging between 250 

 and 1000 miles a day. As a low-pressure area ap- 

 proaches and passes a given locality some interesting 

 changes take place. The following wind observations 

 are given on all United States weather maps : "When 

 the wind sets in from points between south and south- 

 cast and the barometer falls steadily, a storm is ap- 

 proaching from the west or northwest and its center 



will pass near or north of the observer within 12 to 

 24 hours with the wind shifting to northwest by way 

 of southwest and west. When the wind sets in from 

 points between east and northeast and the barometer 

 falls steadily a storm is approaching from the south or 



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FIG. 75. STORMOGUIDE 



southwest and its center will pass near or to the south 

 or east of the observer within 12 to 24 hours with 

 wind shifting to northwest by way of north. The 

 rapidity of the storm's approach will be indicated by 

 the rate and the amount of the fall in the barometer." 



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FIG. 76. ANEMOMETER 



Thus it is clear that by carefully observing the wind 

 direction and its changes one can accurately predict 

 the position of the center of a low-pressure area. 



Rain, snow, or generally unfair weather accom- 

 panies a "low" because the air over it is rising and 



