Oct. =s. 1915 Temperature and Capillary Moisture in Soils 1 59 



is attributed by many writers almost entirely to this thermal movement 

 of vapor. Thus, in discussing the subject Hilgard (7, p. 307) states 

 that "dew is formed from vapor rising from the warmer soil into a colder 

 atmosphere, and condensed on the most strongly heat-radiating surfaces 

 near the ground, such as grass; leaves, both green and dry; wood; and 

 other objects first encountering the rising vapor." Farther on he says: 

 "The fact that dew is most commonly derived from the soil could have 

 been foreseen from the other fact, long ascertained and known, that 

 during the night the soil is as a rule warmer than the air above it." Other 

 writers, such as Ramann (9), etc., claim in substance the identical belief. 

 But really, is there a rising of vapor or warm moist air from the warm 

 soil below to the cold soil above ? And is the source of water of the dew 

 ascribable to this soil vapor? During the day the soil receives its heat 

 at the upper surface, and its temperature rises. The heat is conducted 

 downward, and the temperature of the various depths of the soil increases 

 correspondingly. The temperature at the surface continues to increase 

 until a maximum is reached and then begins to decrease. As the tem- 

 perature increases and moves downward, the soil air expands, and as the 

 volume of the pore space remains constant, it is expelled into the atmos- 

 phere. The pressure of the soil air at the different depths tends to be 

 the same at any one time and equal to the atmospheric pressure, pro- 

 vided the communications are ideal. When the temperature at the 

 surface soil is at the maximum, it is generally many degrees higher 

 than that of the air above, amounting sometimes to 30° C. In fact, 

 the air temperature decreases in calm and clear weather with an increase 

 in height at the adiabatic rate of approximately 0.9° per 300 feet. When 

 the temperature of the surface soil and of the air is highest, the atmos- 

 pheric pressure also tends to be at its minimum, so that the air escapes 

 from the soil with greater facility. After the surface soil attains its 

 maximum temperature and then begins to cool, its air contracts, tends 

 to produce a partial vacuum, and consequently draws air from the 

 atmosphere, so that its pressure will be in equilibrium with that of the 

 latter. The fall of temperature is also conducted downward and pro- 

 ceeds as a wave, and as it descends it causes a dimunition in volume at 

 the corresponding depths and therefore produces an inward flow of air. 

 This cold wave, however, is preceded by the m.aximum temperature 

 wave, which as it proceeds downward causes a further expansion of air, 

 which goes to make up for the decreased volume of air caused by the 

 cold wave following immediately after. The difference in temperature, 

 however, of the soil at any depth immediately before and after the 

 tnaximum temperature wave is reached is very small, as experiments 

 at this Station show; consequently the expansion and expulsion of air 

 caused by the downward march of the minimum temperature wave is 

 not very appreciable. Hence, as the cold wave proceeds downward and 

 produces a Oecrease in volume of the soil air, the air that comes to make 



