Climate, Snow Cover, Microclimate, and Hydrology 41 



temperature gradient is accompanied by a vapor pressure gradient, which 

 results in upward transfer of water vapor accompanied by recrystalliza- 

 tion of the lower part of the snowpack into depth-hoar crystals. Most of 

 the mass removed from the lower part of the snow cover is redeposited in 

 the colder upper part, where it aids in the development of wind slabs. 

 The rate of mass transfer within snow in interior Alaska, where stronger 

 temperature gradients persist in the snow, has been calculated by 

 Trabant and Benson (1972) and is on the order of 0.02 to 0.03 g cm"^ 

 day"'. 



The depth hoar layer is so fragile that it often disintegrates with the 

 slightest disturbance, causing the collapsed snow or snowquakes com- 

 monly observed when one walks in an undisturbed area. These snow- 

 quakes are first observed in November and then with increasing fre- 

 quency until April, and are generally restricted to a few thousand square 

 meters in area because of the support afforded by hummocky 

 microrelief. 



The second structural type, a hard layer of snow without underlying 

 depth hoar, develops after wind erosion entirely removes the snow cover 

 from a small area. Subsequently, a new wind slab forms almost directly 

 on the surface. This process has been observed during a smgle storm, i.e. 

 as the winds shift slightly in direction, an area may change from an ero- 

 sional regimen to one of deposition. Rarely does an area remain denuded 

 of snow for long. 



The third structural type is rare and occurs only when wind erosion 

 removes the wind slab layers, leaving the depth hoar. The structure may 

 be somewhat stabilized by a thin wind crust, which is usually removed or 

 covered by the next wind event. 



Almost any irregularity on the surface serves as a drift trap, at least 

 under some wind conditions. The snow depth is generally related to the 

 height of the vegetation. Snow is also caught in the low areas between 

 polygons, which generally become filled by mid-October. Along the 

 coast drifts often exceed 4 m in depth. However, the most extensive drifts 

 accumulate in stream channels incised a meter or more below the tundra 

 surface. For example, on the Meade River at Atkasook drifts are often 

 several kilometers long, up to 20 m wide, and 10 m deep. 



The large drifts that form on the banks of rivers and lakes are sepa- 

 rated into two groups, one formed by storm winds from the west which 

 bring most of the new snow and the other by the prevaiUng northeasterly 

 winds. The general shapes of the drifts are reproduced each year. The 

 sizes and shapes of the prevaihng-wind drifts are virtually independent of 

 the amount of snowfall. However, the sizes of storm-wind drifts vary 

 significantly with the amount of snowfall. As an example of this process 

 cross sections of drifts on the banks of the Meade River were measured 

 between 1962 and 1973 (Figure 2-6). Drifts caused by storm winds were at 



