370 ENVIRONMENTAL CONDITIONS. 



IV. MOISTURE-TEMPERATURE INDICES. 



A. INTRODUCTORY. 



An attempt on the part of Livingston^ to obtain a single climatic 

 index for moisture and temperature efficiencies combined resulted in a 

 climatic chart of the United States that has certain interesting charac- 

 teristics. These indices are based on the tentative suppositions: (1) 

 that the temperature efficiency of a climate, to produce plant growth, 

 is proportional to the temperature summation-index of that climate 

 (obtained by whatever method may prove most satisfactory), and 

 (2) that the moisture efficiency is proportional to the Transeau ratio 

 of precipitation to evaporation. This last supposition considers that 

 if two stations differ only in rainfall and in the intensity of the 

 evaporating power of the air, then plant growth at these two stations 

 should be directly proportional to the rainfall and inversely propor- 

 tional to the atmospheric evaporating power, as far as chmatic condi- 

 tions are concerned. In short, the moisture-temperature index of a 

 climate (for any given duration factor) is taken as the product of the 

 temperature summation-index and the moisture ratio. To employ 

 Livingston's terminology, if Imt represents the moisture-temperature 

 index, if It represents the temperature summation-index, if Ij, repre- 

 sents the summed precipitation for the period considered, and if /« 

 represents the total evaporation from some standard atmometer for 

 the same period, then 



Inspection of this formula shows that the value of this moisture- 

 temperature index is increased by higher temperature (supposing that 

 the optimum temperature for plant growth is not surpassed) and also 

 by lengthening of the time period taken into account. The higher are 

 the daily temperature-index values and the more of them are summed, 

 the greater must be the resulting sum (/,), and the product index is 

 of course increased by increasing its first factor. Also, this product 

 index is increased by higher values of the Transeau moisture-ratio 

 (Ip/Ie)' This ratio value, in turn, is increased by more rainfall and by 

 lower atmospheric evaporating power. The efficiency, for plant 

 growth, of the moisture-temperature complex is thus greatest with a 

 long growing-season, with high temperatures (not surpassing the 

 optimum), with great rainfall, and with low evaporating power. 



Livingston's product indices were based upon the duration factor 

 of the length of the period of the average frostless season and upon 

 the physiological summation-index of temperature efficiency. We 

 have calculated these values also for temperature indices derived 

 by the remainder method and for those derived by the exponential 

 method. The results obtained for these two forms of moisture- 



'Livingston (1916, 2). 



