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A Climatology of Snow-to-Liquid Ratio for the Contiguous United States

TitleA Climatology of Snow-to-Liquid Ratio for the Contiguous United States
Publication TypeManual Entry
Year of Publication2005
AuthorsBaxter, Martin A., Charles E. Graves, and James T. Moore
Weather and Forecasting
Pagination729 - 744
Date Published2005
PublisherAmerican Meteorological Society


A 30-yr climatology of the snow-to-liquid-equivalent ratio (SLR) using the National Weather Service
(NWS) Cooperative Summary of the Day (COOP) data is presented. Descriptive statistics are presented for
96 NWS county warning areas (CWAs), along with a discussion of selected histograms of interest. The
results of the climatology indicate that a mean SLR value of 13 appears more appropriate for much of the
country rather than the often-assumed value of 10, although considerable spatial variation in the mean
exists. The distribution for the entire dataset exhibits positive skewness. Histograms for individual CWAs
are both positively and negatively skewed, depending upon the variability of the in-cloud, subcloud, and
ground conditions.
Short TitleWea. Forecasting
Citation Key1201
Community Notes

In forecasting snow depths scientists first forecast the amount of liquid that is expected and then using a Snow Liquid Equivalent Ration (SLR) the amount of snow is calculated.  A standard ratio of 10:1 used to be used but it has been found insufficient.

Variables affecting SLR:

"The primary factor that determines SLR is the amount of air space trapped in the interstices between ice crystals within the newly fallen snow"

"After the crystal forms, the surrounding environment will determine the type of growth. The process of crystal growth is very complex; as the crystal falls through the atmosphere it may encounter many different temperatures and degrees of saturation."

"lower-level temperatures (and to a lesser extent, relative humidities) play a strong role in determining SLR"

"Snowflake or ice crystal melting is a function of air temperature near the hydrometeor surface, relative humidity, the size of the crystal or snowflake, and the amount of liquid water present."

Snow data have four main sources of uncertainty:

"There are four primary concerns with regard to snow measurement for COOP observers. Two are related to the effects of wind: the “undercatch” of precipitation in the gauge due to high wind speeds, and the settling of the snow due to wind and destructive metamorphism.  High winds can cause precipitation to be underestimated in the gauge, resulting in an overassessment of SLR. Settling of the snow would lead to an underestimation of the amount of snow that fell, and thus an underassessment of SLR. So the effects of wind are twofold and in opposite directions with respect to SLR.  Third is the possibility of mixed precipitation or rain being included in the liquid-equivalent measurements.  If mixed precipitation or rain occurs in the same day as the snow, the amount of precipitation in the gauge is increased, thus inappropriately reducing SLR values.  The final concern is the possible tendency for observers to erroneously record SLR values of 10.0."




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