IV CONTENTS. 



PAGE. 



The Climatic Conditions of the United States: 



1. Introductory j4g 



II. Temperature conditions. 



1. Duration of temperature conditions. 



A. Preliminary considerations 154 



B. The length of the period of the average frostless season (table 2, 



plate 34) 157 



Table 2. — Frost data and length of average frostless sea- 

 son for 1803 stations in the United States 161 



C. Length of period of average frost season 197 



D. Length of period of high normal daily mean temperatures 



(table 3, plate 35) 197 



Table 3. — Length of period with normal daily mean tem- 

 peratures of 68° F. or above, and of period with similar 

 means of 32° F. or below, within the year 200 



E. Length of period of low normal daily mean temperatures 



(table 3, plate 36) 204 



2. Intensity of temperature conditions. 



A. Preliminary considerations 205 



(1) Direct indices of temperature efficiency for plant growth 208 



(2) Remainder indices of temperature efficiency for plant 



growth, Merriam's chart (plate 37) 209 



(3) Exponential indices of temperature efficiency for plant 



growth 211 



Table 4. — Exponential indices of temperature 

 efficiency for plant growth, based on a coefficient 

 of 2.0 for each rise in temperature of 18° above 

 40° F., for each temperature from 41° to 100° F. 212 



(4) Physiological indices of temperature efficiency for 



plant growth 213 



Table 5. — Physiological indices of temperature 

 efficiency for plant growth, based on Lehen- 

 bauer's 12-hour exposures with maize seedlings . 214 



B. Summations of direct indices of temperature efficiency for 



period of average frostless season 216 



C. Summations of remainder indices of temperature efficiency 



for period of average frostless season (table 6, plate 38) 216 

 Table 6. — Summations of normal daily mean remainder 

 indices of temperature efficiency for plant growth, for 

 period of average frostless season, the daily indices 

 being derived by subtracting 0, 32, 39, or 50, from the 

 values of the normal daily mean temperature on the 

 Fahrenheit scale 217 



D. Summations of exponential indices of temperature efficiency 



for period of average frostless season (table 7, plate 39) . . . 225 

 Table 7. — Summation of normal daily indices of tem- 

 perature efficiency for plant growth, for period of aver- 

 age fro.stless season, the mean daily efficiency indices 

 being derived from the corresponding temperatui'e 

 indices, (1) by the exponential equation of chemical 

 reaction velocities and (2) by the emph-ical growth- 

 rate coefficients for maize seedlings as found by Lehen- 

 bauer for a 12-hour exposure to maintained tempera- 

 ture. The temperature efficiency for 40° F. is taken as 

 unity in both cases 226 



E. Summations of physiological indices of temperature efficiency 



for period of average frostless season (table 7, plate 40, and 



fig. 1) 232 



F. Absolute temperature maxima 233 



