64 S. L. Dingman et al. 



outlet streams, changes in their volume during the summer are caused 

 solely by rainfall and evaporation. 



The freeze-up season marks the transition from summer to winter. 

 By the time of freeze-up, towards the end of September, the net radiation 

 has decreased substantially (Figure 2-11) as a result of much lower solar 

 elevation and greatly reduced duration of daylight — 13.5 hours in mid- 

 September compared with 24 hours two months earlier (Table 1-1). Light 

 snowfalls, which generally melt, may temporarily reduce the net radia- 

 tion further by increasing the albedo of the tundra; the albedo fluctuates 

 between 18 and 60*^0 before the establishment of the "permanent" 

 winter snowpack. The bulk of the available radiation energy is used in 

 melting these snowfalls, but typically little or no runoff results. 



The other major physical process of this season is the freezing of the 

 thawed soil layer. As freezing progresses downward, and occasionally 

 upward, a steadily increasing slab or sandwich of soil remains isothermal 

 as the latent heat of fusion is being extracted (Brewer 1958, Nakano and 

 Brown 1972). The result is the zero curtain or the period during which 

 temperatures at a given depth remain at the freezing point. Once the soil 

 is totally frozen the cold wave can penetrate into the permafrost. Diurnal 

 variations of the surface soil temperature decrease because variations in 

 air temperature and insolation are smaller and snow depth is increasing. 



SUMMARY 



Data collected prior to and during the Tundra Biome program pro- 

 vide a reasonably complete and consistent picture of the climate, micro- 

 climate and hydrology of the coastal tundra of northern Alaska. The 

 average net radiation at the surface is between 420 and 450 MJ m"' yr"'. 

 Of this, 55% is sensible heat transferred to the air, 36*^0 is used in evapo- 

 transpiration, 1% is used to melt snow, and 2% is sensible heat trans- 

 ferred downward to snow and soil. 



Two-thirds of the year is characterized by a negative net radiation 

 balance, very low surface temperatures, and a gradually increasing snow- 

 pack subject to substantial drifting. The snow reduces extremes of temp- 

 erature and wind at the ground surface, providing a more moderate 

 microclimate for surface- and near-surface-dwelling organisms. 



When the net radiation balance becomes positive in late May, the 

 snowpack, upper soil, and air temperatures approach the freezing point. 

 Surface melting of the snow redistributes water and heat downward, 

 causing the first in a series of rapid changes in the immediate surface en- 

 vironment. Profound changes occur over the few days when the snow- 

 pack melts and the upper layers of the soil thaw. During this time there is 

 a rapid increase in net radiation, which is accelerated by decreasing snow- 



