34 S. L. Dingman et al. 



Hunter 1971). This cold surface cools the lowest layers of the atmos- 

 phere, thus providing a source of cool air and sea breeze conditions 

 (Moritz 1977, Walsh 1977). Forty-three percent of hourly temperatures 

 in July are at or below 0°C (Rayner 1960a, b), and on the average there 

 are only 91 days with a mean daily temperature above 0°C each year. 



Cloudiness increases during May and June as moisture becomes 

 available from open areas in the sea ice. Fog formed over the Arctic 

 Ocean drifts inland, often as low stratus that forms following warming 

 of the surface air. Convective activity further modifies the cloud type to 

 stratocumulus. At the coast, heavy fog occurs on about one day in three 

 from June through August. In May and June this fog and cloud cover re- 

 duce the proportion of possible solar radiation actually received at the 

 surface and depress temperatures near the coast. Simulations by Lord et 

 al. (1972) show the importance of such cloud cover in suppressing the di- 

 urnal range of temperature at the surface. However, until snowmelt the 

 effect of cloud cover is in part offset by the multiple reflections between 

 the snow surface and the clouds. 



During the summer temperatures increase from the coast inland. 

 The mean daily temperature in July at Umiat for the years 1947-53 was 

 12°C. This inland warmth was demonstrated in July 1966 when the air 

 temperature at the coast averaged 10.6°C when winds were southerly and 

 2.7 °C when winds were off the ocean (Weaver 1970, Barry et al. 1976, 

 Myers and Pitelka 1979). According to Brown et al. (1975) the mean tem- 

 perature gradient inland from the arctic coast is 6°C per 100 km, al- 

 though they suggest that much of this warming occurs close to the coast 

 where the gradient may be two to three times greater (Walsh 1977). The 

 tundra at Barrow is typical of the very cool, moist zone in the immediate 

 vicinity of the coast. In the Prudhoe Bay region, the area south of the 

 bay is transitional to the warmer inland zone which extends into the 

 Foothills, but still is significantly cooler than locations 20 km or more in- 

 land (Walker and Webber 1979). Average thawing indices, the sum of the 

 positive differences between daily mean temperature and 0°C, for 

 1970-73 are 304 at Barrow but 477 at Prudhoe Bay (Brown et al. 1975). 



From July through September the strong temperature gradient from 

 the snow-free land to the cold, largely ice-covered Arctic Ocean sets up a 

 horizontal density gradient that is referred to as the Arctic Front. An 

 average of four weak lows per month travel eastward along this frontal 

 zone from the Siberian arctic coast. About 35% of the annual precipita- 

 tion falls, mainly as rain, during these three months in association with 

 these systems (Table 2-1). The Arctic Front is generally at a height of 300 

 to 700 m over the coast, and reaches the ground about halfway between 

 the coast and the Foothills (Conover 1960). When the front moves in- 

 land, the shallow layer of arctic air that covers the whole Arctic Slope 

 produces cold, foggy weather. This air tends to advance up the river val- 



