868 Handbook of Nature-Study 



resistance than elsewhere it moves in that direction. So when heat causes 

 air to expand and become lighter than the surrounding cool air, it moves, 

 and air in motion is wind. 



This diagram represents a section of the atmosphere over a broad, level 

 plain with the air at rest and pressing down equally on every part of the sur- 

 face. The dotted line H represents the top of the quiet atmosphere. Such 

 a condition occurs frequently at night after the heat from the sun is with- 

 drawn and gravity has settled the atmosphere. When the rays of the sun 

 fall upon the earth upon which this quiet air rests they warm the earth first, 

 then the layer of air immediately in contact with the surface, so the atmos- 

 phere is heated from the bottom upward. We will assume that the layer 

 of air between the earth and the dotted line, G, is thus heated to a higher 

 temperature than the air above it. It will, therefore, expand. It cannot 

 expand downward because of the earth. It cannot expand much laterally 

 because it is pressed upon by air that is also seeking more space. It, there- 

 fore, expands upward as represented by the line ABC. Now in expanding 



/^K^//f^f^KK^/////////////'f//Wf''' r/ f"' 



Fig. i. Diagram showing air currents set up by sun's heat. 



upward it lifts all the air above it and the line H , representing the top of the 

 atmosphere, will become bowed upward also as indicated by the line A'B'C' . 

 As a result, the air at the top of the atmosphere over the warm center 

 slides down the slopes on either side toward the cool margins. As soon as 

 the flow of air away from the warm center begins, just that instant the 

 pressure upon the heated layer at the surface is relieved and the warm air 

 rushes upward (is pushed upward) and the whole circulation, as indicated 

 by the arrows, begins. It must be remembered that gravity is the really 

 active force in maintaining this movement, because it pulls down the denser, 

 heavier air at the cool margins with greater force than the warm, expanded, 

 light air at the warm center. The descent of the cool air actually lifts the 

 warm air. 



The normal pressure, or weight, of the atmosphere at sea level is about 

 14.7 pounds on each square inch of surface. It is customary, however, to 

 express the weight of the atmosphere in terms of inches of mercury instead 

 of in pounds and ounces. A column of air one inch square from sea level to 

 the top of the atmosphere will just counterbalance a column of mercury 

 30.00 inches high in a barometer tube of the same size. We, therefore, say 

 that the normal pressure of the atmosphere at sea level is about 30.00 

 inches. If, for any reason, the atmosphere becomes heavier than normal, 

 it will raise the column of mercury above the 30 inch mark, and we say that 

 the pressure is "high." If the atmosphere becomes lighter than normal, we 



