4 Farmers' Bulletin 1096. 



HOW FROST IS FORMED 



Whenever two objects or portions of the same object are unequally 

 heated, the colder always gains in temperature at the expense of the 

 warmer, the tendency being to equalize the temperature throughout 

 every portion of the bodies. The interchange of heat is accom- 

 plished in two ways, radiation and conduction, each of which will 

 be discussed separately as it bears on the matter of the occurrence 

 of frost. 



Radiation. The heat and light from the sun come to us through 

 space in a form of wave motion called radiation. The atmosphere 

 offers considerable obstruction to the passage of these waves. Even 

 when the sky is very clear, rarely more than 65 per cent of the radia- 

 tion penetrates to the surface of the earth, the part absorbed being 

 expended in raising the temperature. The region near the upper 

 limits of the atmosphere is one of intense cold. As the sun, having 

 a much higher temperature than the earth, radiates heat to the earth, 

 so from the surface of the earth heat is radiated to the much colder 

 upper limits of the atmosphere. 



The radiation of heat from the earth is continuous both day and 

 night when there are no clouds or other obstructions between the 

 earth and the upper atmosphere. During the day the amount of 

 heat received from the sun is so much greater than the amount lost 

 by radiation from the earth that the temperature rises. After the 

 sun sets, however, no heat is received to counterbalance the loss by 

 outgoing radiation and the temperature falls. 



Conduction. Heat may be interchanged between different por- 

 tions of the same body, or between two separate bodies in actual 

 contact, by conduction. When one end of a bar of iron is held in a 

 fire, the end away from the fire soon becomes too hot to hold in the 

 hand. The heat is transferred from the hot portion of the bar to 

 the cooler portion by conduction. The shortness of the time taken 

 for the heat to reach the cooler end of the bar indicates that iron is 

 a relatively good conductor of heat. On the other hand, one end 

 of a stick of wood can be held in the fire until it is completely con- 

 sumed without the other end becoming very warm; therefore wood 

 is a poor conductor of heat. Of course, if the stick ceased to burn 

 and the heat in the burning end were not lost, the heat in the warm 

 end would eventually be distributed equally throughout the stick. 

 Both the soil and the air are very poor conductors of heat. 



During a clear, calm day the temperature of the ground surface is 

 raised by the heut received by radiation from the sun and the air in 

 immediate contact with the ground becomes warmed by conduction. 

 Since the air is so poor a conductor, the increased temperature of the 

 ground is imparted to only a very thin layer of air at first. How- 



