This project has two objectives. The first is to develop relationships between visibility and surface catch rates of precipitation for a range of snow types, ambient temperatures, and wind conditions, using theoretical concepts. The second objective is to examine the accuracy of the visibility/snow charts in use, and, if necessary, develop a new chart based on the results of the study. Theoretical relationships between snowfall rate and visibility for 29 types of snow crystals were developed with data collected over two winter seasons at the National Center for Atmospheric Research snowfall test site. Observation and theory showed that the relationship between snowfall intensity and visibility varies widely, even during a single storm. The variations depend largely on the type of snow crystal and the degree of crystal riming, aggregation, and wetness. Furthermore, for the same snowfall intensity, visibility at night can be twice as great as it is daylight. On the basis of the study findings, the research team developed a new visibility/snow chart that estimates snowfall intensity using visibility and temperature criteria. The use of visibility is more conservative than the current use of visibility to estimate snowfall rate, but more accurately accounts for the observed scatter in the measured and theoretically estimated relationship between snowfall rate and visibility. (Author/publisher)
Abstract