RESULTS OF CHANGING AIR PRESSURE 



59 



RESULTS OF CHANGING AIR PRESSURE 



Effects of Temperature on Air Pressure. Temperature affects 

 the reading of the barometer. The air in a schoolroom 20 feet by 30 

 feet would weigh 903 pounds when the temperature is 60 F., but if 

 the temperature were increased to 80 F. the air would expand, and 

 some of it escape. The air left would weigh 873 pounds. Good 

 barometers must be compensated for temperature, especially for 

 measuring altitude. 



The Air Pressure Varies for Different Kinds of Weather. 

 Torricelli observed, among other things, that the level of the mer- 

 cury in the tube fluctuated as changes in the weather took place. 

 When the air is heavy, the barometer reads high; and usually this 

 occurs during fair weather. For example, 30.2 is a relatively high 

 reading of barometer and 29 is a relatively low reading of barometer. 



Sea Level. An aneroid barometer for reading weather conditions 

 must be corrected for altitude. Sea level means the reading of the 

 barometer at an altitude, corrected in such a manner that it would 



FIG. 51. A diagram showing the relation between altitude and air pressure. 

 Clobourg, 111., Lowell, Neb., Spica, Kan., Kanarado, Kan., and Kanfield, 

 Colo. 



give a reading equal to the reading of the barometer if the place of 

 observation were at sea level instead of at an elevation. The higher 

 we go the less the pressure of air. By referring to the diagram, 

 Fig. 51, air at sea level will be seen to have a pressure of 30 

 inches. At B, Clobourg, 111., 1000 feet above sea level, the barom- 



