THE PHYSICAL PROPERTIES OF THE SOIL. 963 



when heavy i^recipitation occurs in late autumn. A low atmospheric 

 temperature is accompanied, in the majority of cases, in autumn by an 

 excess of atmospheric precii)itation, and canses then a regular and often 

 considerable fall in the soil temperature. In the less frequent cases of 

 cool and dry autumn months there is, as a rule, only a very slight varia- 

 tion in the soil temperature. 



THE DISTRIBUTION OP HEAT IN THE SOIL. 



Daily variations. — During the cooler season, when the exposure to 

 the sun's rays is slight, the temperature of the surface of the soil runs 

 parallel to the minimum temperature of the air, but the maximum in 

 the upper layers of the soil exceeds that of the air during the warmer 

 season, and all the more tlie greater the exposure to the sun's rays and 

 the greater the absorptive capacity of the soil. 



The daily variations of temperature are greatest in the top layers, 

 and decrease with the depth, generally disappearing at a depth of 1 

 meter. Considering the fact that the conduction of heat downward is 

 gradual, it is clear that the extremes of temperature are later in the 

 lower strata than at the surface and in proportion to the depth. The 

 extent of this retardation depends on the physical properties of the 

 soil, especially the conductivity for heat. In general the influences 

 causing variations in temperature at the surface of the soil have less 

 effect in proportion to the depth of the layer under consideration. 



Yearly variations. — The yearly temperature variations are of consid- 

 erable magnitude in the upper layers, decrease with the increasing 

 depth, and finally disappear entirely. The depth at which a constant 

 temperature obtains, or at least a temperature unaffected by the sea- 

 sons, is dependent upon climatic conditions. In the tropics this depth 

 is about 6 meters and in the temperate zones 20 to 30 meters. All fac- 

 tors which moderate the extremes of temperature, for instance, nearness 

 to the sea, soil coverings of different i<inds, etc., diminish also the vari- 

 ations in temperature. Observation teaches that in the temperate 

 zone the variations in soil temperature cease at a dej)th of about 20 

 meters near the coast, but at about 30 meters at a distance from the 

 sea. In the warm season the temi)erature of the soil diminishes on 

 descending, but in the cold season increases with the depth.^ 



1 C. S. Cornelius, Meteorologie, 1863, p. 96. J. Soyka, Dcr Boden. Von Pettenko- 

 fer and Zieiusseu, Handbuch der Hygiene, Theil 1, Abtheihmg 2, Heft 3, 1887, p. 

 142. J. R. Lorenz vou Liburuau, Lelirbuch der Klimatologie, 1871, p. 34. E. Ramann, 

 Forstliche Bodenkunde, 1893, p. 95. 



