Accurate monitoring of the large-scale dimensions of global snow cover is essential for understanding details of climate dynamics and climate change. Presently, such information is gathered individually from ground station networks and satellite platforms. Efforts are in progress to consolidate and analyze long-term station records from a number of countries. To gain truly global coverage, however, satellite-based monitoring techniques must be employed. A 27-year record of Northern Hemisphere continental snow cover produced by the National Oceanic and Atmospheric Administration (NOAA) is the longest such environmental record available. Records of Southern Hemisphere continental cover and snow on top of Arctic sea ice have been produced by similar means for a portion of this interval. The visible imagery charting technique used to generate these data provides information on snow extent but not on snow volume. Satellite microwave analyses over Northern Hemisphere lands show some promise in this regard, however, large-scale monitoring of snow extent with microwave data remains less accurate than visible charting. This paper updates the status of global snow cover monitoring, concentrating on the weekly snow charts prepared by NOAA and discussing a new and consistent record of monthly snow cover generated from these weekly charts. The NOAA charts show a reduction of hemispheric snow cover over the past five years, particularly in spring. Snow areas from the NOAA product are then compared with values derived using passive microwave data. The latter consistently reports less snow cover than the more accurate visible product. Finally, future snow monitoring initiatives are recommended. These include continuing the consistent NOAA product until an all-weather all-surface product is developed. The latter would use multiple data sources and geographic information systems techniques. Such an integrative product would need extensive comparisons with the NOAA product to ensure the continued utility of the lengthy NOAA observations in studies of climate change. In a retrospective sense, satellite charts from the middle 1960s to early 1970s need reevaluation and techniques to merge satellite products with historic station time series must be developed.

Northern hemisphere snow cover during !988 and 1989 was at its lowest extent since the advent of reliable satellite snow-cover monitoring in 1972; running some 8-! 0% below the eighteen-year annual mean of 25.7 million km 2. Monthly minima for the period of record occurred six times during these two years. In general, the last nine years of the satellite record had less extensive cover than the 1972-80 interval. Negative anomalies during the 1980s were largest over Eurasia in all seasons, and in the Spring over North America. Hemispheric seasonal means for the most recent nine years were 3.7% to 8.4% lower than those between 1972 and 1980. Resttits are based on analyses of National Oceanic and Atmospheric Administration weekly snow charts, which are produced from visible satellite imagery. Inu'oducdon Snow is a key variable in the global climate system, influencing the global heat budget chiefly through its effect of increasing surface albedo [Kung et al., 1964, Robinson and Kukla, 1985]. Accurate information on snow cover is

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Many researchers are presently interested in detecting long-term trends in annual or seasonal daily temperature range (DTR), and attributing these changes to anthropogenic origins. However, very little work has been done to confirm the mechanisms that are important to determining the long-term average annual cycle of the DTR. Therefore, the focus of this work is to examine the spatial and temporal difference in the DTR average annual cycle across the United States, and to associate the patterns of these cycles with potential causal variables. Three major types of DTR annual cycle exist in the United States: high sun season maximum (northern and western U.S.), low sun season maximum (south central and southeast U.S.), and transitional season maxima (middle latitude in the U.S.). The annual cycles of the DTR in the northern and western U.S. are well related to average annual cycles of cloud cover and dew point temperature; only areas to the west of the Rocky Mountains have a strong linkage between DTR and precipitation frequency annual cycles. Across the northern tier of the U.S., the loss of snow cover is important to DTR transitions during the spring season. However, the onset of snow cover in the fall does not appear to be the major factor in DTR variations, which are instead more strongly associated with cloud cover effects. As expected from their sinusoidal annual cycle, maximum and minimum temperature cycles are linearly related to the DTR in regions with a warm season or cold season DTR maximum, while non-linear relationships exist where the DTR annual cycle has maxima in the transition seasons.

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