264 REPORT OF STATE BOARD OF HORTICULTURE. 



FROST. 



WHEN TO EXPECT IT AND HOW TO LESSEN THE INJURY 



THEREFROM. 



By Prof. W. H. Hammox. 

 FORMATION OF FROST. 



Before proceeding to a consideration of methods of preventing injury by 

 frost, it is essential that the conditions under which it forms be quite thor- 

 oughly understood. 



The two principal methods by which plants lose their heat are convection 

 and radiation. 



The movement of the air is continuously bringing new particles of it in 

 contact with the plants, and if the air be cooler than the plants it will take 

 from the plants a portion of their heat, until both air and plants are at the 

 same temperature. This is known as convection, and is very effective on 

 windy nights when a cold wave is approaching, and the breeze is continu- 

 ally bringing new portions of the atmosphere about the plants. We can 

 hardly attribute to this process the great loss of heat on the quiet, clear 

 nights when frosts mostly occur : for on such nights the plants are usually 

 colder than the surrounding air, and any mixing of the air tends to raise 

 their temperature. 



The chief method by which plants lose their heat on calm, frosty nights 

 is by radiation. By this term is meant that peculiar process by which heat 

 escapes from an object and passes through the surrounding space in direct 

 lines in the same way that rays are emitted from a source of light. Heat. 

 lost by radiation, does not appreciably warm the air thi^ough which the ray 

 passes, but its effects are manifest at any surface which obstructs the pas- 

 sage of the raj'. 



The surface of the earth is continually losing heat by radiation into 

 space, but during the day it usually receives heat from the sun more rapidly 

 than it loses it by radiation, and consequently it grows warmer. At night, 

 however, heat from this source is cut off and the continued radiation causes 

 the temperature to fall. 



Under favorable conditions this fall continues until condensation of vapor 

 begins. Aqueous vapor, although invisible, is always present in greater or 

 less quantities in our atmosphere, and can always be condensed into water 

 if the temperature be sufficiently lowered. If the condensation takes place 

 at temperatures below the freezing point of water, the moisture is deposited 

 , in the form of frost. 



The heat given off by the condensation of vapor is enormous. The con- 

 densation of enough vajior to make a pint of water will evolve enough heat 



