46 



WEATHER AND CLIMATE 



being cooled by expansion so that much of the mois- 

 ture in a low-pressure area may fall as rain or snow. 

 On the other hand, the falling dry air over a high- 

 pressure area tends to absorb moisture, thus clearing 

 the air of clouds and making it invigorating. High- 

 pressure areas bring fair weather generally. 



Figure 76 show an anemometer used at the U. S. 

 Weather Bureau for measuring the velocity of winds. 



Rainfall is measured by rain gauges. Every weather 

 bureau station has a rain gauge for measuring the 



Tycos 



FIG. 77. RAIN GAUGE 



amount of rainfall. There are many types, the one 

 shown in Figure 77 being generally used. Here the 

 top is about eight inches in diameter and is funnel- 

 shaped. The rain is collected in the funnel and then 

 drips through the small curved pipe into the cup be- 

 low. To determine the number of inches of rainfall 

 the contents of the cup are poured into the measuring 

 graduate which is marked off in hundredth parts of 

 an inch. 



How is the weather map made and for what is it 

 used? The making of the weather map requires the 

 cooperation of nearly three hundred branch stations 

 and several thousand observation points scattered 

 over the United States and Canada. Each day at 8 

 a.m. and 8 p.m., Eastern Standard Time, measure- 

 ments are made of the temperature, pressure, relative 

 humidity, wind direction, wind velocity, and general 

 weather conditions at these stations. These are tele- 

 graphed in code to Washington, D.C., and other fore- 

 cast centers in the country, where they are recorded 

 and studied. From these data the weather predictions 

 for the next twenty-four hours are made. At some key 

 stations the data are printed in a weather map and 

 sent out to cities, towns, and important industries in 

 that area. 



Few of us realize the great importance of the 

 Weather Bureau, but when we stop to consider the 

 value of advance knowledge of a cold wave, a frost, 

 or a destructive hurricane, we can better see how dif- 

 ficult it would be to dispense with it. 



The Weather Bureau also publishes a "Monthly 

 Weather Review," which contains a summary of the 

 various weather factors for the month. 



What are some of the recent advances in weather 

 forecasting? A recent study of the reliability of the 

 forecasts of the Weather Bureau reveals that it is 

 correct about eighty per cent of the time. However, 

 with the increasing importance of weather forecasting 

 to aviation, the authorities in charge desire to make 

 the reports even more reliable. The Science Advisory 

 Board of the government has completed plans to join 

 the aerological services of the army and navy with 

 those of the Weather Bureau for the adoption of the 

 air-mass-analysis method of weather study. This new 

 method, worked out over several years of careful 

 study by scientists, would not displace the present 

 method, but would serve to make it more effective and 

 probably more reliable. 



Heretofore most of the weather forecasting has 

 been carried on from the ground. The new attack 



International News Photos, Inc. 



FIG. 78. WEATHER INSTRUMENT ON AN AIRPLANE 



would get the weather data not only at the ground 

 level but at altitudes up to several thousand feet 

 above the earth's surface. Airplanes are now in use 

 daily at many points in the United States, securing 

 weather data above the earth. When these data are 

 put together and studied, they reveal a much more 

 complete picture of the conditions and movements of 

 the air masses which surround the earth. 



Our atmosphere is more than twenty miles in depth, 

 and the forces which control its movements can best 

 be understood by seeing it not only near its bottom 

 but also at all depths. Thus when a great mass of cold 

 air from the arctic or antarctic regions moves in 

 toward the warmer tropic regions, its movement can 

 be plotted more accurately if it has been explored at 

 all depths rather than observed only from the ground 

 level. 



