SOIL HEAT 231 



ward slopes varied witli the time of year. For example, the 

 southeasterly inclination was warmest in the early season, the 

 southerly slope during mid-season and the southwesterly slope 

 in the fall. Such a relationship is of course governed entirely 

 by local climatic conditions, especially cloudiness, and might 

 not be true of any other place. A southeasterly slope is gen- 

 erally preferred by gardeners. Orchardists also pay strict 

 attention to the aspect as it is often a factor in sun-scald and 

 certain plant diseases. 



120. Rise of temperature and the factors involved. — 

 The rise of temperature of a layer of soil following a given 

 absorption, depends (1) on the specific heat of the soil, (2) on 

 the rate at which the heat moves to other parts of the soil 

 mass, and (3) on the losses of heat to the atmosphere. It is 

 evident that in a study of the influence of insolation on soil 

 temperature, specific heat should receive the first attention. 



121. Specific heat and soil temperature. — The specific 

 heat of any material may be defined as its thermal capacity 

 compared with that of water. It is expressed as a ratio to the 

 quantity of heat required to raise the temperature of a given 

 amount of a certain substance 1° C. to the quantity needed 

 to change an equal amount of water from 15° to 16° C. 



The specific heat figure for soil generally refers to the heat 

 capacity of the dry substance. Under normal conditions, soils 

 contain variable amounts of pore spaces and consequently 

 have different weights to the cubic foot. A specific heat figure 

 based on weight, therefore, does not give a true idea of the 

 relative heat capacities of two soils. The expression of spe- 

 cific heat by volume seems a more rational basis of compari- 

 son.^ The specific heat of the soil is important because of the 

 relation it has to the warming up of soil in the spring, the 



Erwarmung desi Bodens; Forseh. a. d. Gebiete d. Agr.-Physxk, Band I, 

 Seite 263 294, 1878. This publication contains a number ot other papers 

 on this subject by Wollny. 



* Weight specific heat of a substance may be expressed by the number 

 of calories required to raise the temperature of one giam, 1° 0. Volume 



