Ch. 14] 



EXTENT 



249 



Double hatching: Approximate extent of continuous permafrost. Ground temperature 

 at a depth of 30 to 50 feet generally below -5° C. 



Diagonal hatching: Approximate extent of discontinuous permafrost. Ground tem- 

 perature in permafrost at a depth of 30 to 50 feet generally between —5° and — 1° C. 



Dotted diagonal hatching: Approximate extent of sporadic permafrost. Ground tem- 

 perature in perznafrost at a depth of 30 to 50 feet generally above — 1° C. 



Reliability: Eurasia, good; Alaska, fair; all other, poor. (Eurasia after Sumgin and 

 Petrovsky, 1940, courtesy of I. V. Poir6.) 



Fig. 1. Areal distribution of permafrost in the Northern Hemisphere. 



