PERMAFROST — BLACK 



275 



Figure 1. — Areal distribution of permafrost in the Northern Hemisphere. 



Double hatching : Approximate extent of continuous permafrost. Ground tem- 

 perature at a deptli of 30 to 50 feet generally below —5° C Diagonal hatching : 

 Approximate extent of discontinuous permafrost. Ground temperature 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 peruiafrost at a depth of 30 to 50 feet generally above —1° G. Re- 

 liability : P^urasia, good; Alaska, fair; all other, poor. (Eurasia after Sumgln 

 and Petrovsky, 1940, courtesy of I. V. Poire.) 



