130 ILLINOIS ACADEMY OF SCIENCE. 



May. Doubtless this may be explained by the unusual condi- 

 tions that obtained during this month. It had the highest tem- 

 perature record experienced since the establishment of the Chi- 

 cago Weather Bureau in 1871. The average temperature for 

 the month was 10 degrees above the normal; for six days it 

 reached or exceeded 90° F., and it reached 94° on the 25th, 26th 

 and 27th of the month. The percentage of sunshine, 79 per cent, 

 was also greater than that observed during any previous May. 

 In contrast, during September, the period with least evaporating 

 power, the temperature was about normal, while the relative 

 humidity was somewhat above and the percentage of sunshine 

 considerably below the normal mean. All these meteorological 

 factors influence the evaporating power of the air profoundly, 

 and their great variation gives emphasis to the necessity of rec- 

 ords extending over more than one season before definite con- 

 clusions may be reached with safety. 



The unusual conditions during May, combined with the absence 

 of foliage during a portion of the month, gives such an excess- 

 ively high evaporation record for this portion of the season that 

 this portion of the record will be disregarded in making the com- 

 parisons which follow, although they are considered in arriving at 

 the average rates for the season. This course has further justi- 

 fication in the fact that the high rate reached at midsummer more 

 nearly represents the extreme of aridity for most of the vegeta- 

 tion of the associations concerned. 



At station one (Fig. i), in the depression, the evaporation 

 rate was at all times the lowest. It reaches a maximum of 16.21 

 cc. daily in July, a minimum of 1.74 cc. daily for the third week 

 of September, and has an average for the season of 189 days of 

 8.3 cc. per day. This is very slightly higher than the average of 

 8.1 cc. reported last year by Fuller^ for the beech-maple associa- 

 tion and indicates a very high degree of mesophytism. 



The most important record for the forest is that expressing the 

 mean of the two stations in the normal upland oak-hickory asso- 

 ciation with the average undergrowth (Fig. 2b). The readings 

 from stations two and three, which are combined in order to 

 obtain this record, differed very little at any time, and both gave 

 the same average for the season — namely, 9.89 cc. daily. The 

 mean of the two stations shows a midsummer maximum of 17 

 cc. per day, an autumnal minimum of 2.87 cc. daily, and an aver- 

 age for 189 days of 9.89 cc. The effect of the absence of under- 

 growth is plainly shown by the record (Fig. 2a), exhibiting the 



