Climate, Snow Cover, Microclimate, and Hydrology 31 



(Kbppen 1936). More recently, temperature data were used to identify air 

 masses, and Bryson (1966) showed that arctic tundra areas in North 

 America have at least a 50% frequency of arctic air in July. The median 

 location of the Arctic Front in July corresponds approximately with the 

 northern limit of boreal forest (see map inside front cover), although in 

 eastern North America the definition of the frontal zone and its relation- 

 ship with vegetation boundaries is less certain (Barry 1967). The exact 

 causal relationship between this frontal position and the biota remains 

 uncertain, but the lack of trees in the arctic tundra must be attributable 

 in part to the poleward decline of available surface energy. Annual net 

 radiation at the treehne is about 670 MJ m"^ in Alaska and 750 MJ m'^ in 

 central Canada, and decreases to 400 to 600 MJ m"^ over the tundra 

 (Hare and Ritchie 1972). 



The general weather conditions in northern Alaska are a result of 

 the patterns of atmospheric circulation. The mean sea level pressure map 

 shows that in winter Alaska is influenced by easterly arctic airstreams as- 

 sociated with a deep low over ihe Aleutians and a ridge of high pressure 

 from the Mackenzie Valley across the Arctic Ocean towards eastern 

 Siberia. A ten-year analysis of daily weather maps by Putnins (1966) il- 

 lustrates the winter maximum of anticyclonic patterns with high pressure 

 cells predominantly to the north and east of Alaska. 



Winter conditions at Barrow are similar to those over most of the 

 Arctic Slope, but during summer its coastal location gives it a modified 

 arctic tundra climate (Watson 1959, Searby and Hunter 1971). The temp- 

 erature regime north of the Brooks Range is continental, with an annual 

 range of 32 °C for monthly mean temperatures (Table 2-1). Extreme max- 

 ima and minima have ranged between 26 °C and -49 °C at Barrow, and 

 between 39°C and -61 °C (unofficial reading) at Umiat in the central Col- 

 ville River Valley (Conover 1960). Mean daily temperatures are below 

 -20 °C at Barrow from December through March. February is the coldest 

 month, with 90% of hourly temperatures below -18 °C (Rayner 1960a, 

 b). The severe temperatures are accompanied by moderate wind speeds, 

 averaging 5 ms"', which often cause blowing snow. In contrast to inland 

 localities, the wind is seldom calm at Barrow; speeds above 12 ms'' are 

 also infrequent (Figure 2-1). Winds along the arctic coast are easterly ex- 

 cept in the vicinity of Barter Island, where the regional topography fre- 

 quently causes strong westerly winds in winter (Schwerdtfeger 1975). 



The winter temperature regime is closely related to the presence of a 

 surface temperature inversion which has an average intensity of about 

 12°C over a vertical distance of 750 m (Bilello 1966). A ground-based in- 

 version is present on 62% of winter days, and for half of these days the 

 inversion layer is more than 1000 m thick (Holzworth 1974). The result- 

 ing stability of the lower troposphere implies that day-to-day tempera- 

 ture changes are largely determined by changes in cloud cover and the 



