8 Farmers' Bulletin 1096. 



parent that if the dew point is very low, the temperature may fall 

 low enough to cause considerable damage without the formation of 

 any frost. For example, if the dew point is 18 F. and the lowest 

 temperature reached during the night is 24 F., there will be con- 

 siderable damage to growing crops without any frost whatever. 

 This phenomenon, often called a " black frost," is of rare occur- 

 rence in most localities. 



Another factor that has great influence on the amount of fall in 

 temperature during the night is the liberation of latent heat in the 

 formation of dew and frost. A definite amount of heat is required 

 to raise the temperature of a given amount of water to the boiling 

 point without vaporizing any of it. After the boiling point is 

 reached, another definite amount of heat is required to convert this 

 water into water vapor, the temperature of the water remaining 

 unchanged during the vaporizing process. This latent heat, stored 

 up in the water vapor, is released whenever the water vapor is again 

 changed to a liquid state. A portion of the sun's heat during the 

 day is expended in evaporating moisture from the ground and 

 from the leaves of plants. When the dew point is reached at night 

 and vapor is condensed, its latent heat is given up. The amount of 

 heat given up on a particular night depends on the amount of 

 moisture precipitated. Obviously, the greater the amount of moisture 

 in the atmosphere, the more will be condensed, provided the tem- 

 perature falls below the dew point. When the dew point is high, 

 the latent heat given off in the formation of dew is often sufficient 

 to check the fall in temperature almost entirely. Generally speaking, 

 therefore, other conditions being equal, the higher the dew point in 

 the evening the less danger there is of the occurrence of a damag- 

 ing frost. 



WHEN TO EXPECT FROST 



The weather in the latitude of the United States is controlled by 

 atmospheric disturbances of great size and varying intensity, which 

 follow one another across the country, .moving from west to east. 

 These disturbances are of two types, one of which is marked by 

 low barometer, overcast skies, and rain or snow; the other by high 

 barometer and clear skies. 



The main factor in the occurrence of frost is radiation of heat 

 from the earth. When heavy lower clouds cover the sky, they act 

 as a blanket and prevent radiation to the upper limits of the at- 

 mosphere, making impossible the occurrence of a true frost. A 

 moderate wind is also generally effective in preventing frost. The 

 formation of a thin layer of cold air near the ground is prevented 

 on a wind} 7 night by the mixing of the surface air with the warmer 

 air above. 



