30 



BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



changes are in one direction, still there is no basis for assuming that 

 for whole years there can be any essential difference. Therefore, if, 

 for example, the mean annual temperature at 1 foot, as shown by 

 8 a. m. observations, is 49°, and the corresponding temperature at 

 4 feet is 50°, there is every reason to believe that the 8 a. m. readings 

 at 1 foot give values, on the average, 1° below the corresponding 

 daily means. When the oscillations are greatest at midsummer, this 

 correction would be too small, and in winter it would be too great; 

 but its use should, at least, bring us nearer to the true mean tem- 

 perature for any given period. 



Table 1 indicates the correction factors thus obtained for a num- 

 ber of stations and sites, with sufficient description of each to show 

 why the morning temperature is much or little below the mean for 

 the day. Practically all of these records were obtained from ther- 

 mometers in iron pipes, which, by conduction, tend to create a 

 greater dairy range of temperatures in their vicinity than occurs in 

 the soil naturally. From these data it will be seen that a small 

 variation from the mean is likely to be secured if (1) the aspect is 

 easterly so that the site receives early insolation, or (2) if the 

 observation hour is relatively late, or (3) if the natural daily range 

 is small, as is usually the case with heavy cover and to some extent 

 on slopes which do not receive vertical rays. Finally, insolation late 

 in the day, though probably causing a large daily range, may bring 

 a morning observation relatively high on the descending curve. 

 These data will be principally valuable in indicating that every site 

 must be studied independently. 



Table 1. — Probable error in mean 1-foot soil temperatures obtained through 



single daily observations. 



[Determined .by comparing annual means for 1 foot and 4 feet.] 



Site. 



Moderate southerly slope open 



Southwesterly slope, some trees 



Northeast slope, steep, heavy cover. 



East slope, some cover 



Canyon bottom, heavy cover 



Northwest slope 



North slope, no cover 



North slope, full cover 



Flat, little cover 



Do 



North slope, one-third cover 



Do 



North slope, high altitude, no cover. 



Gentle easterly slope, little cover 



North slope, steep, cover 



South slope, steep, some cover 



North slope, heavy cover 



8 a. m., strong radiation. , 

 Insolation late in day. 

 Insolation early, range small. 

 Insolation- early. 

 Insolation latej if any. 



Do. 



9 a. m., some insolation early. 

 9 a. m., little insolation. 



7-8 a. m. ; heavy snow blanket. 

 8-9 a. m., insolated all day. 

 9 a. m., little insolation. 



Do. 

 10-12 a. m., radiation intense. 



8 a. m., early insolation. 



9 a. m., small daily range. 

 Great daily range." 



11-12 a.m. 



1 This station equipped with telethermoscope, so that depression is due solely to normal depression of 

 soil temperature at 9 a. m. 



