WEATHER 



163 



Movements of the Atmosphere. Movements of the 

 air are caused by the force of gravity wherever differences 

 in atmospheric pressure exist. These j 



differences are due to the differences in 

 the temperature of the air, at any area 

 of low pressure the heavier air from all 

 sides forcing up the lighter air. A simple 

 experiment may be performed to show 

 the origin of the winds. 



Experiment 48. Cut a piece of paper in 

 the form of a spiral (Figure 138) and hold it 

 over a lighted Bunsen burner. The rapid up- 

 ward current of the air will cause the spiral to 

 turn, while colder air will be moving toward the 

 flame, as shown in the diagram. ; FIG. 138. 



The horizontal movements of the air are called winds, 

 when the movements are strong enough to be perceptible. 

 The vertical movements are called calms. To summarize : 



1 . Winds are caused by the force of gravity ; 



2. Winds blow from re- 

 gions of high barometric 

 pressure to regions of low 

 pressure. 



3. The velocity of the 

 wind depends upon the 

 barometric gradient. The 

 steeper the grade, or the 

 greater the difference in 

 pressure between two 

 areas the greater will be 

 the velocity of the wind. 



FIG. 139. -The Anemometer an In- Figure 139 shoWS an 



strument for Determining the Velocity of 



the Wind. instrument which is used 



