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S. C. Colbeck

Snow precipitation degenerates rapidly once it reaches the ground. A wide variety of particle types develop in seasonal snow covers, thus leading to a wide range of snow properties. The most common varieties of particles are shown here. The physical processes responsible for the growth and development of these particles are described in general terms, although these processes are not understood as well as the processes of crystal growth in the atmosphere. The heat and mass flows associated with the development of these crystals in the snow cover are complicated because of snow's complex geometry.

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David A. Robinson, Kenneth F. Dewey, and Richard R. Heim Jr.

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.

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Kenneth F. Dewey and Richard Heim Jr.

The purpose of this article is to acquaint the research community with a new data base—a digitized archive of Northern Hemisphere snow cover. Historically, those researchers who needed snow cover data for climatic and atmospheric boundary layer studies have had to rely on the irregularly spaced (and in some regions, sparse) grid of point observations. Northern Hemisphere Weekly Snow and Ice Cover Charts, which are created from analyzed satellite imagery at the National Earth Satellite Service (NESS), have been available on an operational basis since late 1966. Each of these weekly charts for the period November 1966 through December 1980 was digitized and stored in a new data archive. Snow cover area and snow cover frequency climatologies were created and examples are presented. The significance of this unique data archive is examined by comparing the 14-year mean annual snow cover frequency climatology with several published snow cover climatologies. The potential uses for this data archive in meteorological and climatological studies also are reviewed.

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Pavel Ya. Groisman, Eugene L. Genikhovich, and Pan-Mao Zhai

This paper is a continuation of empirical studies of cloud and snow cover effects on climate based on a blend of observational meteorological data for the past several decades. It employs the idea that the analysis of climate variability observed during the period of intensive instrumental observations can provide “overall estimates” of these effects.

A climatology of clear skies for northern extratropical lands is presented in the form of deviations from the average climate conditions. Clouds are an internal component of the climate system, and these deviations indicate specific climate conditions associated with clear skies. At the same time, they may be considered as estimates of the overall cloud effect on the regional climate. A similar approach is applied to estimate the potential effect of snow on the ground, and an attempt is made to divide the effects of snow and clouds.

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Ralph R. Ferraro, Fuzhong Weng, Norman C. Grody, and Alan Basist

The Special Sensor Microwave/Imager (SSM/I), first placed into operation in July 1987, has been making measurements of earth-emitted radiation for over eight years. These data are used to estimate both atmospheric and surface hydrological parameters and to generate a time series of global monthly mean products averaged to a 1° lat × 1° long grid. Specifically, this includes monthly estimates of rainfall and its frequency, cloud liquid water and cloud frequency, water vapor, snow cover frequency, and sea ice frequency. This study uses seasonal mean values to demonstrate the spatial and temporal distributions of these hydrological variables. Examples of interannual variability such as the 1993 flooding in the Mississippi Valley and the 1992–93 snow cover changes over the United States are used to demonstrate the utility of these data for regional applications.

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Phil E. Church
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Hans H. Neuberger

Summary

Under symmetrical light conditions the optical modification produced by a snow surface decreases the antisolar distance of Arago's neutral point. Therefore, for investigations of the atmospheric turbidity by means of observations of Arago's point this effect must be taken into consideration.

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Imke Durre and Michael F. Squires

For many, Christmas is directly associated with snow. Christmas cards, carols, stories, and movies tend to depict a “white Christmas.” Often these depictions portray a snow-covered landscape or falling snow. In most parts of the United States, however, a white Christmas is actually far from certain. Where, then, are the best places for experiencing snow on the ground or snowfall on Christmas Day? Where is there likely to be sufficient snow for skiing at that time? In which parts of the country

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Gary McManus, Thomas W. Schmidlin, and Christopher A. Fiebrich

study) found to be a few tenths of a degree cooler than a nonaspirated thermometer of the maximum-minimum temperature system (MMTS) while temperatures were cooling at night over snow-covered ground. Lin and colleagues attributed the small “bias” to the nonaspirated MMTS thermometer. The area within 1,000 m of the station is open, flat terrain with agricultural land use ( Fig. 1 ). The station is not in a valley or depression that might contribute to the development of extreme cold, as exists at some

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