784 



STATE BOARD OF AGRICULTURE. 



It is readily seen from the above table, therefore, that the calculated 

 and observed diurnal amplitudes are remarkably close in all the diverse 

 types of soil and consequently the law is valid. 



Since the actual diurnal-nocturnal fluctuations follow the geometric 

 progression law they are expressable then by the formula A=FR°'i, 

 where A is the amplitude, F the first fluctuation, R the ratio of any 

 amplitude (except the first) to the preceding one, and n the number of 

 the amplitude. According to this equation, therefore, if the amplitude 

 is known at two different depths it can be calculated at any other depth 



THE RELATION BETWEEN THE MINIMUM TEMPERATURE OF THE SOIL AT THE 



SURFACE AND THAT OF THE AIR ABOVE. 



On account of the absorption of the solar energy the temperature of 

 the soil immedately at the surface at the time that it reaches its maxi- 

 mum is many degrees higher than that of the air above, amounting 

 sometimes to about 30 °F. In certain sections of the world this dif- 

 ference must be far greater. 



During the night, however, i. e., when the minimum temperature is 

 attained at the surface soil, no such great difference exists. Indeed, both 

 the air and surface soil reach practically equal minimum temperature, as 

 the data in Table 76 show. These data were obtained by placing on the 

 surface soil and about four feet in the air, minimum thermometers of 

 the standard type, and taking records every morning for six months. 

 During the months of June, July, August, and September the ther- 

 mometers were exposed to the sky, but during September, October and 

 November they were covered with wooden boxes with perforations ' all 

 around them to allow free circulation of air and consequently the rapid 

 attainment of the actual air temperature. For lack of space only the 

 averages for the various months are presented here. 



TABLE 76.— MONTHLY AVERAGE MINIMUM TEMPERATURE OF THE SOIL AT THE SURFACE AND THAT 



OF THE AIR ABOVE. 



It is to be noted, therefore, that even during the night the surface 

 soil had a higher temperature than the air above, but the difference is 

 comparatively small, amounting to less than 2°F. In many individual 

 nights the air temperature would be that many degrees warmer. 



The above results on the soil surface temperature were obtained on 

 a sandy loam. Now it has already been seen that the mineral soils 

 maintain a slightly higher temperature during the night than peat. 

 Hence peat would always have equal or slightly lower temperature at the 

 surface than the air above. 



It is an interesting fact that during the night, say at the time that 

 the minimum temperature is attained at the surface soil, the tempera- 

 ture of the air increases with the elevation up to a certain height and 



