44 S. L. Dingman et al. 



seek out natural deep snow areas to take advantage of the more moderate 

 interface climate. Deep-drifted snow in narrow gullies, between high- 

 centered polygons, and near ponds is a favorable winter habitat. The 

 other two structural types, hard snow and soft snow alone, are relatively 

 unfavorable sites. When the wind slab is very near the surface it effec- 

 tively cuts off the area; no burrowing was found in hard wind slabs. If 

 only a depth hoar layer remains after removal of the wind slab, low tem- 

 peratures occur at the soil/snow interface because of the high air perme- 

 ability of the depth hoar. 



MICROCLIMATE 



Definition of Microclimate Seasons 



A characteristic succession of physical processes is observed each year 

 on the tundra: establishment of the winter regime through large radiant 

 energy losses, modification of the snowpack just prior to melting, gener- 

 ally rapid melting and consequent large runoff, high evaporation from 

 the water-covered tundra after snowmelt, and the relatively dry summer 

 regime followed by freeze-up. This progression is accompanied by a 

 characteristic pattern in the relative magnitudes of the components of the 

 radiation, energy, and water balances that provides a convenient basis 

 for identifying six seasons: winter, pre-melt, melt, post-melt, summer, 

 and freeze-up (Weller and Holmgren 1974a). Although the starting dates 

 and durations of these seasons vary from year to year, it is possible to 

 specify typical values (Figure 2-8). 



Earlier studies at Barrow have suggested somewhat different bases 

 for dividing the year into seasons: Kelley and Weaver (1969) defined six 

 seasons based on soil-temperature regime, and Maykut and Church 

 (1973) described four seasons based on variations of surface albedo. 



Figure 2-8 shows the magnitudes and signs of the energy-balance 

 components typical of each season. The radiation balance is negative 

 during the winter, and positive during the rest of the year. The hydro- 

 logic regime is dominated by increasing storage during the freeze-up, 

 winter and pre-melt seasons, by runoff during the melt season, and by 

 evapotranspiration during the post-melt and summer seasons. 



Radiation Balance 



The radiation balance describes the partitioning of radiant energy 

 into incoming and absorbed solar radiation, longwave incoming radia- 

 tion, and longwave outgoing radiation at the earth's surface as follows: 



