8 BULLETIN NO. 161 



trouble for this reason, and it is awkward to get to the very top 

 of the trees to clean them, and if the amount of water applied 

 be reduced sufficiently to prevent condensation on the trees, it is 

 very doubtful whether enough has been applied to form effect- 

 ively a smudge, as the experiments to be reported later on in this 

 bulletin, using carbon dioxide as a smudge, tend to show- 



AMOUNT OF HEAT IN THE SOIL. THAT THE 

 SMUDGE IS TO RETAIN 



The essential idea in smudging is to maintain an atmospheric 

 blanket over the orchard that will keep in the heat that was 

 stored in the ground during the previous day when the sun was 

 shining. Is there much available heat stored in the ground dur- 

 ing the day? Except for four hours in the middle of the day, 

 the temperature of the ground is colder than the air during 

 December, January, and February. In June, July, and August, 

 except for five hours around noon, the ground is only 2 degrees 

 warmer than the air. (e) . The ground, then, during the night 

 is never much warmer than the air. Again, suppose that the 

 difference in temperature between the maximum of the afternoon 

 and the minimum of the air for the early morning be 30 degrees 

 for the day, then at a depth of four inches in the soil the daily 

 range of temperature instead of being 30 degrees will be one- 

 half of thirty, or .fifteen degrees, and, at a depth of eight inches, 

 the range of temperature is one-fourth, or 7 degrees. The daily 

 change in temperature is not felt at depths in the soil below 

 three feet. It is thus seen that this small excess in temperature 

 of the soil over the air is only found in a comparatively thin 

 layer of the soil at the top. Furthermore, in Utah these freezes 

 in the spring nearly always follow a storm when the ground is 

 wet, the sky has been overcast with clouds and very little heat has 

 collected in the ground during the day. (f). "In general the 

 ground will be warmer even in winter than that which is pre- 

 cipitated upon it," hence the rain has cooled the ground and 

 what little sunshine there has been during the day has been 

 used in evaporating the water instead of warming up the ground 

 during the day. Whether the frost follow a storm or not, it is 

 nearly always true that when we have one night with frost, we 

 usually have three to six in succession and that the days between 

 these frosty nights are not nearly so warm as usual ; 



(e). Descriptive Meteorology Moore, p. 87. 

 (f). Descriptive Meteorology Moore, p. 88. 



