158 



GENERAL SCIENCE 



the atmosphere at which the water vapor begins to collect 

 into droplets of water, becomes visible, and forms fog or 

 clouds. If cooling is continued after the dew point is 

 reached, heavy clouds will be formed, the drops will be 

 too heavy for the air to carry, and they will fall as rain. 

 If the air is below freezing temperature, the condensed 

 vapor will be frozen and snow or hail will fall. 



The hygrometer is an instrument used to determine the 

 per cent of moisture in the air, that is, to ascertain how 

 near the air is to the dew point. The hy- 

 grometer determines the humidity of the 

 air. Humidity is indicated on the instru- 

 ment in per cent. Humidity is the per cent 

 of water vapor in the air, or it is the amount of 

 vapor present compared to the amount of vapor 

 that the air can hold at a given temperature. 

 If the humidity is 75 per cent at a tem- 

 perature of 85 F., that means that the air 

 has three-fourths as much water vapor as it 

 can hold at that temperature. If the tem- 

 perature rises, the humidity will decrease unless more 

 water vapor is added by evaporation. If the temperature 

 falls, the humidity may increase until it reaches 100 per 

 cent, when condensation will occur; 100 per cent humidity 

 is dew point. 



106. Isobars and Isotherms. Isobars are the lines 

 drawn on weather maps to connect places having the 

 same barometric reading at a given time. Since the 128 

 Weather Bureau stations are at various altitudes, the 

 barometers in the different offices have varied readings 

 and would not mean anything if they were put on the 

 maps as they are read; so the readings are reduced to 

 what they would be if the stations were at sea, level. ISO- 



HYGROMETER 



