EXPERIMENT STATION BULLETINS. 551 



them on the soil temperature. In the first i)lace, some of them run in 

 a direct or parallel course with each other, while others run in an 

 inverse course. This opposing or contrary action makes it very hard 

 to notice or correlate the influence of any element on soil temperature 

 unless it is very predominant. In the second place, all these elements 

 do not obey the same law from day to day and hence this makes it still 

 more difficult to trace their influence on the soil temperature. 



As a rule, however, they tend to act with one another in the fol- 

 lowing manner: (1) the air temperature tends to vary inversely with 

 the pressure; (2) the sunshine varies directly with the pressure; (3) 

 the air temperature tends to vary directly with the sunshine in the 

 summer; (4) the dew point varies directly with the air temperature; 

 (5) the wind velocity may vary directly with either extreme of pres 

 sure; (6) the precipitation may be inversely proportional to the pres- 

 sure. 



Rome of these meteorological factors tend to increase the soil tempera- 

 ture and others to decrease it. nigh air temperature would tend to 

 raise the temperature of the soil while low barometric pressure would 

 favor greater evaporation and consequently the soil temperature would 

 be kept down, all other conditions being equal. Sunshine varying di- 

 rectly with the pressure would have a very positive influence in the 

 warming of the soil. The wind velocity would have a cooling effect both 

 on account of the continual movement of cool air and because of its 

 accelerating effect upon evaporation. Precipitation in early spring 

 may and may not raise the soil temperature, but in the summer it would 

 tend to lower it both because its temperature is lower than that of 

 the soil, and becauise of the greater evaporation that ensues immedi- 

 ately after a rainfall. 



Space does not permit to discuss and to ])oint out in detail the in- 

 fluence of these elements singl}' or in combination on the present soil 

 temperatures study. The reader can get a fair idea of their general 

 run and influence by an examination of the charts. It will suffice to 

 say that the only factor which the soil temperature follows very closely 

 and overwhelmingly is the air temperature and hence, also, the dew 

 point. The relationship of the soil temperature to the other elements 

 is not very regular simply because their influences are secondarv to 

 that of the air temperature, also on account of the fact that they vary 

 so much from day to day, also because their influences are opposed, 

 hence the efl:ect of the one is ofl'set by the efiiect of the other. 



Special attention may be called to the effect of rainfall and snow 

 on the soil temperature. It is the common belief that both these ele- 

 ments exert a large influence on soil temperature. It is claimed that 

 in the spring the precipitation tends to raise the soil temperature very 

 perceptibly and in the Avinter the snow acts as a blanket over the soil 

 and prevents its rapid loss of heat and thereby keeps its temperature 

 high. 



Data collected on both these factors have led to the conclusion that 

 the rainfall has been given more importance than it really deserves but 

 that the snow does perform well the function that is attributed to 

 it. As to the former, it will be seen by referring to the charts for 

 April and May that whenever it rained the temperature of both the 



