DAVID A. ROBINSON1
In situ and satellite observations of snow-cover duration show a considerable amount of year-to-year variability on local to hemispheric scales. This variability is often embedded in longer-term fluctuations. In situ data from the central United States indicate multi-decadal fluctuations in the duration of snow over the past century, and hemispheric satellite observations suggest variations lasting for several years or more.
Such analyses have become possible in recent years as a result of the recovery, digitization, and validation of historic in situ observations of snow cover and the availability of two decades of satellite observations of continental snow cover. Efforts are needed to recover additional historic data, to improve the recognition of snow using microwave satellite data, and to create global snow products using all available data sources. With these data available for analysis, knowledge of the spatial and temporal kinematics of snow cover will continue to improve. This will contribute to a better understanding of the role of snow in the climate system and to the utility of snow as an indicator of climate change.
Snow cover is a critical influence on the earth's climatic energy and hydrologic budgets. In many regions it may play an influential role in determining the magnitude of any human-induced climate change, and might be a useful indicator and monitor of such change. To understand better the importance of snow in the climate system, it is essential that accurate information on the temporal and spatial dimensions of snow cover be available. Such data have been obtained through in situ and satellite observations, and recent efforts have begun to locate, assimilate, and validate these data. Once available, they are being employed in empirical and modeling investigations of snow-cover kinematics and the dynamical aspects of snow within the climate system. An excellent survey of the latter is provided by Walsh (1995) earlier in this section. This paper will concentrate on the distribution of snow cover in space and time over the past two decades and the past century. Much of the discussion will focus on the newly available in situ and satellite data sets. Given the lack of attention paid to secular snow cover data until recently, only limited analyses of these data have taken place. A few examples of these efforts