February August
Figure 4.1: Mean snow-cover extent (dark blue) and sea-ice extent (light blue) in the Northern Hemisphere between 1966 and 2006, for
February and August. The difference in snow cover between seasons causes significant differences in the surface reflectivity (albedo).
Source: Based on Armstrong and Brodzik 2005
1
cant influence on climate and hydrology. Snow cover major factor in transportation, winter sports, agriculture
affects large-scale atmospheric circulation. Early season and animal husbandry such as reindeer herding. It in-
snow-cover anomalies in the Northern Hemisphere, for fluences ecosystems and is important for conservation
example, are known to lead to changes in atmospheric of biodiversity.
circulation. Autumn snow cover can also affect climate
on a seasonal scale, with impacts extending into the sub-
sequent winter. Snow cover is also a sensitive indicator Trends and outlook for snow
of regional climate variability and change. Realistic sim-
ulation of snow cover in models and forecast schemes Snow accumulation and melt are governed primarily
is essential for simulating surface energy balance and by air and soil surface temperature, precipitation, wind
predicting winter water storage and year-round runoff. and surface relief. Precipitation determines the overall
amount of snow but air temperature determines wheth-
Snow cover influences human activities directly and er the precipitation falls as rain or snow and governs the
indirectly. Seasonal snow cover is the main source of rate of snow melt. The recent rise in global tempera-
runoff in many mountain regions, and over one billion tures, and the warming trends predicted for the future
people depend on it for their water supplies. Snow is a (see Chapter 3) thus affect global snow cover.
CHAPTER 4 SNOW 41