TEMPERATURE OF AUSTRAL AND TROPICAL REGIONS. 



55 



The Austral Region. 



Transition zone species require a total quantity of heat of at least 

 5,500° C. (10,000° F.), but can not endure a summer temperature the 

 mean of which for the six hottest consecutive weeks exceeds 22° C. 

 (71. G° F.). The northern boundary of the Transition zone, therefore, 

 is marked by the isotherm showing a sum of normal positive tempera- 

 tares of 5,500° C. (10,000° F.), while its southern boundary is coinci- 

 dent with the isotherm of 22° C. (71.6° F.) for the six hottest con- 

 secutive weeks. 



Upper Austral species require a total quantity of heat of at least 

 6,400° C. (11,500° F.), but apparentlj^ can not endure a summer tem- 

 perature the mean of which for the six hottest consecutive weeks 

 exceeds 26° C. (78.8° F.). The northern boundary of the Upper 

 Austral zone, therefore, is marked by the isotherm showing a sum of 

 normal positive temperatures of 6,400° C. (11,500° F,), while its 

 southern boundary agrees very closely with the isotherm of 26° C. 

 (78.8° F.) for the six hottest weeks. 



Loiver Austral species require a total quantitj^ of heat of at least 

 10,000° C. (18,000° F. ). The northern boundary of the Lower Austral 

 zone, therefore, is marked by the isotherm showing a sum of normal 

 positive temperatures of 10,000° C. (18,000° F.). 



The Tropical Region. 



Tropical species require a total quantity of heat of at least 14,400° C. 

 (26,000° F.); and since the tropical life region is a broad equatorial 

 belt, it is probable that both its northern and southern boundaries 

 are marked by the isotherm showing a sum of normal positive tem- 

 peratures of 14,000° C. (26,000° F.). 



Governing temperatures of the zones. 





Zones. 



Governing temperatures. 





Northern limit. 



Southern limit. 



Regions. 



Sum of normal 

 mean daily tem- 

 peratures above 

 6° C. (43'^ F.). 



Normal mean 

 temperature of 

 six hottest con- 

 secutive weeks. 



Boreal 



Arctic 



°C. 



op 



°C. 



UO 



114 



18 



23 



26 



°F. 

 "50 











157.3 





Canadian _ 



Transition 







64.4 



Austral 



5,500 

 6,400 

 10,000 

 14,500 



= 10,000 

 11,500 

 18,000 

 26,0(j0 



71.6 



78.8 





Lower Austral 





Tropical 

















1 Estimated from insufficient data. 



= The Fahrenheit equivalents of centigrade sum temperatures are stated in round numbers to 

 avoid small figures of equivocal value. 



