The Weather 43 



gesting circulation of the air in the room? What air moves upward 

 and which settles toward the floor? Compare also the behavior of 

 smoke issuing from a chimney into the open air with the behavior of 

 warm air in a low-pressure area. The heated air from the chimney 

 may sometimes be seen to continue upward and take a slight whirl- 

 ing or rotary motion before it spreads out and disappears. Does this 

 resemble the central draft of a low-pressure area, or of a tornado as 

 described above ? 



Air that contains water vapor is shown by the barometer to 

 be lighter, volume for volume, than dry air. A cubic foot of 

 steam, which is water vapor, is lighter than an equal volume of 

 dry air at the same temperature. Consequently the pressure 

 of air with any mixture of water vapor will be less than that of 

 dry air ; and the greater the amount of water vapor which air 

 contains, the less its weight, as shown by the barometer. 



It appears from this that a cubic mile of cold, dry air encoun- 

 tering near the surface of the earth a cubic mile of warm, moist 

 air will invariably force the latter wherever the resistance is 

 least, which is usually upward. In its upward progress it 

 expands and cools, and its moisture is condensed to form cloud 

 and perhaps precipitation as rain, snow, sleet, or hail. 



26. Humidity of the atmosphere. The atmosphere always 

 contains moisture or water vapor whether visible or invisible, 

 but in varying quantities. Moisture is in fact one of the most 

 important constituents of the atmosphere. It has apparently 

 great influence upon the development of storms (page 35). 

 Its importance in relation to health is generally recognized, 

 though not as yet thoroughly understood. 



Water vapor is supplied to the atmosphere by evaporation, 

 which occurs at any temperature from water, ice, or bodies 

 containing water in any form. The only condition that checks 

 evaporation is complete saturation of the air. Evaporation 

 is facilitated by high temperature and by moving air, as wind, 

 which replaces a mass of air already saturated with a mass of 

 comparatively dry air. Compare the effect of different tem- 

 peratures or moving air upon drying clothes. 



