Physical Characteristics and Basic Climatic Features - The Arctic Climate System - page 47

Figure 2.18. Depth hoar crystals
collected from the base of a tundra
snowpack on the North Slope of
Alaska (courtesy of Kelly Elder,
USDA Forest Service). (See plate
section for color version.)
of field measurements and climatic data for Alaska. Five of the six classes are found
in Alaska.
Tundra and taiga snow dominate most of the Arctic. Tundra snow is a thin, wind-
blown snow, with typical maximum depths of around 750 mm and a bulk density of
typically 380 kg m −3 . It consists of a basal layer of depth hoar, overlain by multiple
wind slabs. Surface sastrugi is common. Depth hoar represents a low cohesion layer
of large, faceted ice crystals at the base of a snowpack. Depth hoar forms when there
is a strong vertical temperature gradient in the snowpack (temperature decreasing
upward). This results in a vertical gradient in water vapor and hence an upward
migration of water vapor within the snowpack. Water sublimates from existing ice
crystals and desublimates (deposits) on others, resulting in large, faceted crystals
( Figure 2.18 ). The combination of fairly thin winter snow covers and low winter
air temperatures makes the Arctic an ideal place for depth hoar formation. Sastrugi
refers to sharp, irregular ridges of snow formed from wind scour and deposition,
aligned along the prevailing wind direction. Ridges typically range from perhaps
0.1 to 1 m in height, and from 0.5 to 10 m in length ( Figure 2.19 ). Travel over a
rough, irregular sastrugi surface can be difficult. Taiga snow is thin to moderately
deep (300-1,200 mm) with a lower bulk density than tundra snow (260 kg m −3 ). It
is found in forest climates in which wind, initial snow density, and air temperature
are all low.
Although there are a number of terrestrial snow depth data sets for Arctic land
areas, they have limited spatial coverage. Figure 2.20 is an attempt to map the mean
annual maximum snow depth for Eurasia, based on Russian data. Depending on
the region, maximum snow depth tends to occur between February and early April.
North of 60°N, maximum snow depths range from 300-800 mm. Assuming an aver-
age density value of about 300 kg m −3 across Eurasia yields maximum snow water
equivalents of about 100 to 270 mm. On a local scale, the snow pack thickness
varies considerably in relation to terrain and wind conditions. Surveys carried out