54 Introduction to the Study of Science 



condensation occurs, white frost is not formed, though plants 

 are frozen. Vegetation thus frozen turns black later, and a 

 black frost is said to have occurred. Frost is one of the adverse 

 climatic conditions with which farmers contend, and because 

 of the importance of protecting growing vegetation from it, the 

 United States Weather Bureau forecasts and issues warnings 

 when frost is likely to occur. Possible damage by frost may 

 be reduced or averted by smudge fires, irrigation to saturate 

 the air with water vapor, and by coverings for more delicate 

 plants. 



Frost is favored in spring and fall by the passage of a high- 

 pressure center with its accompanying clear, calm, and cool 

 nights. If in such a situation, the barometer is above the 

 normal or rising ; if the temperature is falling and the dew point 

 is below 40 F. and falling, frost is quite certain to occur. 



Frost may be predicted for particular localities with much 

 certainty by aid of an hygrometer. If the dew point is near 

 the freezing point, the temperature falling below 50 F., and 

 the sky clear with no winds, then frost is probable. Another 

 method may be used. When the temperature of the dry ther- 

 mometer is below 50 F., multiply the difference between the 

 readings of the two thermometers by 2.5, and subtract the 

 product from the temperature shown by the dry thermometer. 

 This gives the temperature to which the air may cool during 

 the night. 



SUMMARY 



Moist, warm air is less dense than dry, cold air and is forced up- 

 ward by it. 



Evaporation is constantly adding to the invisible water vapor of 

 the air. 



The actual amount of water vapor in the air, which may be expressed 

 as ounces per cubic yard, is the absolute humidity of the atmosphere. 



The ratio of the amount of moisture in the air to the amount it 

 would contain if saturated is the relative humidity. 



Relative humidity may be determined from readings of the wet- 

 and dry-bulb thermometer, by the use of a table. 



