MICHIGAN ACADEMY OF SCIENCE. 



193 



With a view to getting a working formula wliich^ although imperfect 

 and subject to amendment after further research, can be used in connec- 

 tion with records of cloudiness and temperature which have been made 

 in any locality, to evaluate such air temperatures as to their efficiency 

 in promoting plant growth, the 304 simultaneous observations of plant 

 and air temperature made at midday in 1915 and 1916, have been studied 

 in connection with the cloudiness, in order to determine factors which 

 can be used to give proper values to temperature readings made on clear, 

 partly cloudy and cloudy days, respectively. The following table gives 

 the number of clear, partly cloudy and cloudy observations, and the sum 

 of the differences between the plant and air temperature readings at mid- 

 day during tlie months that observations were made: 



Table XVII. 



The Effect of Cloudiness on Difference Between Plant and Air 



Temperatures. 



In round numbers the average difference between the plant and air 

 temperature in full sunshine was 15° F., in partial sunlight, due to thin 

 clouds or intermittent cloudiness, it was 10° F. and less than 1° F. when 

 the sky was thickly overcast, so that the sun's disk was invisible. From 

 these averages we may deduct the following formula for finding 

 the effective temperature from the recorded thermometer readings : 

 T = f -)- 15 C. -|- 10 P; in which T is the sum of the effective tempera- 

 ture for plant growth, t is equal to m — 42X, m being the sum of all 



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