1 84 RELATIONS OF SOIL TO HEAT 



Many irregularities must be expected in observations on 

 subsoil temperature. The melting of snow in spring may 

 at once determine an abnormal cooling of the subsoil, while 

 heavy rain in early summer will at once convey the heat of 

 the surface soil to the lower layers. If any part of the sub- 

 soil holds more water than the soil above and below it, this 

 portion of the subsoil will display an irregular temperature l . 



Prevention of Summer Frost. Immense damage is occa- 

 sionally done to growing crops in certain districts by early 

 morning frosts. These injurious frosts are most common in the 

 spring or early summer, or again in the early autumn before 

 harvest. In the spring they are dreaded by the fruit grower, 

 even in climates as mild as those of Italy or Florida. In late 

 summer they are disastrous to the yet unripened cereal grain, 

 maize or wheat, in Canada, and in the more northern of the 

 United States. In countries such as Finland, which lie near 

 the extreme limit of possible cultivation, summer frosts will 

 not unfrequently destroy all the crops and produce a famine. 



The conditions which produce these unseasonable frosts are 

 well understood. The frost arises from the unchecked radia- 

 tion of heat from the surface of the earth, or more particularly 

 from the surface of the growing plant. The rapid loss of heat 

 during the hours succeeding sunset culminates shortly before 

 sunrise, and the surface temperature of all bodies exposed to 

 the sky is then found to have sunk below the freezing point. 

 For this rapid radiation of heat to occur it is necessary that 

 the sky should be free from cloud, the air very dry, and 

 perfectly still and calm. The clearness of the sky, and dry- 

 ness of the air, prevent the retention of radiant heat by the 



1 The Keport by H. Mellish on the subsoil temperatures at stations of the 

 Roy. Meteor. Society was issued after the above section was in type. 



