u 



WINDS AND WEATHER 



The arrows point the direction of the winds in 

 each case. 



In your notebook show by an arrow the wind 

 direction about a "low" and the wind direction 

 about a "high." Complete the following state- 

 ment. 



Compared with the motion of the hands of 

 a clock, the winds about a "low" blow in the 

 _ direction and about a "high" in the - 

 direction. 



Experiment 28. In what direction do 

 weather changes move? 



Study the first of the weather maps shown 

 in the three-day series (Fig. 66) and locate a 

 low-pressure area over some western state and a 

 "high" over some other portion of the country. 

 Study the second map of the series and deter- 

 mine what movement of each has taken place. 

 Study the third map and see if you can reach a 

 conclusion regarding the question of this ex- 

 periment. 



In your notebook complete the following 

 statement. 



From this experiment I am able to conclude 

 that low- and high-pressure areas travel across 

 the country from - to -- 



READINGS WHICH WILL HELP ANSWER 

 THE PROBLEM QUESTIONS 



What happens to air when it is heated or 

 cooled? Nearly everyone is familiar with 

 the fact that the air at the top of a room 

 is warmer than near the floor ; that air over 

 a radiator or register rises and that fresh 

 air from a window comes in at the bottom 

 while the warmer, used air goes out near 

 the top. These conditions all result from 

 a difference in weight of cold and warm air. 



When any gas, like air, is heated, it ex- 

 pands or grows larger and as a result gets 

 lighter. This causes other parts of the gas 

 which are cooler and heavier to rush in and 

 push the lighter, heated portions upwards. 

 This sets up currents in the air called con- 

 vection currents, for as the warmer air rises 

 and the cooler air sinks a circulation is 

 brought about. We make use of these con- 

 vection currents in warm-air heating sys- 

 tems. They may also be caused in a liquid 

 such as water when parts of it are unequal- 

 ly heated. Convection currents are also 

 used to circulate the water in hot-water 

 heating systems. 



What causes winds? As the sun shines 

 on the earth it warms some parts more 

 than others. Water warms up more slowly 

 than land and holds its heat longer. Some 

 places on the earth's surface absorb more 

 of the sun's heat than others. Air which is 

 in contact with these unequally heated 

 portions of the earth's surface becomes 



FIG. 66. WEATHER MAPS FOR THREE CONSECUTIVE DAYS 



