temperature of the coldest month is less than 9° C. (This defini- 

 tion is the Nordenskj old-Wall formula used by the Soviets for 

 defining the Arctic.) 



Agronomists frequently define the region on the basis of the 

 length of growing season. Thus all areas having less than 50 

 days between frosts are considered Arctic. 



Botanists define it as the region north of the treeline, a boundary 

 which approximates the 50° F. summer isotherm; or as the areas 

 in which trees will not grow because of the cold, excluding other 

 inhibiting factors such as poor soil, lack of water, and excessive 

 winds. 



\0 MILES 



Figure 2-2. — Sharp sounds can be heard at ten miles. 



The astronomical definition, used in navigation and in under- 

 standing the seasonal variations in the length of daylight and 

 darkness, places within the Arctic all areas north of parallel 

 66° 33' N. — the difference between 90° and the maximum northerly 

 declination of the sun. 



There are other definitions based on such factors as extent of 

 permafrost and windchill. 



The subarctic is defined by the temperature factor as the areas 

 where the mean temperature for less than 4 months of the year is 

 higher than 50° F. (10° C.) and the mean temperature of the 

 coldest month is less than 32° F. (0° C.) It should be noted that 

 minimum temperatures are often lower there than in areas farther 

 to the north. This does not mean that all subarctic areas are 

 colder than arctic areas. Some areas included in the subarctic 

 have average winter temperatures fajling so slightly below 32° F. 

 that they are characteristically non-Arctic, such as Iceland where 

 Reykjavik has an average temperature for the coldest month about 

 like that of Philadelphia. 



19 



