Apr. 3.I9I6 Climatic Conditions and Cercospora beticola 31 



taken at the Weather Bureau station ; hence, the place where the records 

 were taken for use in the present correlations with the development of 

 the disease is an important consideration. 



Air temperature. — At Rocky Ford the maximum temperatures taken 

 among the plants near the surface of the ground from June 13 to 30 

 ranged from 2 to 19 degrees higher and the minimum temperatures 

 generally from i to 14 degrees lower than those taken at 5 feet above 

 the ground (fig. 4). This was due to the fact that, the plants were small 

 during this period and covered only a portion of the ground; conse- 

 quently during the daytime the temperature of the soil became higher 

 than that of the air, and in turn the temperature of the air near the 

 ground became higher than that of the air a few feet above. During the 

 night the reverse occurred, the surface soil losing its heat by radiation 

 and conduction faster and finally reaching a lower temperature than 

 that of the air in contact with it, after which the heat of the latter 

 gradually passed into the soil and as a result the temperature of the air 

 immediately above the ground eventually became lower than that a few 

 feet higher up. It is possible that convection currents also tended to 

 lower the temperature of the air immediately above the ground; for, as is 

 well known, when it is not disturbed by other factors, the coolest air 

 settles to the lowest levels. 



The maximum temperatures of the air near the ground, as shown by 

 the records, were higher for a longer period during June than at any 

 time during the season, varying from 100° to 106° F. on nine different 

 days between the 14th and 26th of that month and rising above 100° 

 only once thereafter, on August 16. The maximum temperature of the 

 air 5 feet above the ground, on the other hand, was lower during June 

 than during the middle of the season, ranging from 90° to 93° on six 

 different days during the month, while it was above 90° and some- 

 times as high as 100° on 12 different days during July. 



As shown by the records, the temperature of the air near the ground 

 among the plants was lower during the middle than during the early 

 part of the season. This was probably due to the difference in the size 

 of the plants, the larger plants practically covering the ground in mid- 

 season and preventing the heating of the surface soil, while early in the 

 season the smaller plants covered the ground but sparsely and con- 

 sequently afforded less protection against heating. Comparison of the 

 records also shows that during the middle of the season the temperature 

 of the air among the plants near the ground was practically the same as 

 that of the air 5 feet above the field and that throughout the entire period 

 the latter was quite comparable \vith the temperatures taken at the 

 Weather Bureau station (fig. 4). 



A similar marked variation was shown at Madison, the maximum tem- 

 perature there being almost constantly higher and the minimum tempera- 

 27469°— 16 3 



