956 
plants in growth [57]. He divides the world into thermal 
regions defined by reference to what he calls “potential 
evapotranspiration.”® The latter is a function of the 
202i 
Valley. 
temperature, whose value for a month is given by 
EH = 1.6 (10T/1)2 em, (3) 
where T' is the centigrade mean air temperature, J is 
3. This function is defined as the amount of water that 
would evaporate from a plant-covered soil if there were no 
restriction in water supply in the root zone. It is considered 
independent of the floral composition of the vegetation. 
POLAR METEOROLOGY 
a heat mdex dependent on the annual course of mean 
temperature and is equal to the sum of the twelve 
monthly values of (7’/5)!5!4, and a is a constant de- 
208 
pendent on J. The twelve monthly values of # are 
added (after they have been adjusted for variations 
in length of day and month) to give an annual value 
of potential evapotranspiration. 
This value forms the basis of Thornthwaite’s thermal 
zones. Along the tundra/microthermal boundary, H 
should be 28.5 cm (11.2 in.), according to Thorn- 
thwaite’s divisions. In practice, this theoretical line 
