Snow Ratios: An Important Role in Snowfall Forecasting
Published: February 8, 2018
You may have heard that you can melt 10 inches of snow to an inch of water. But is that really true?
While this rule of thumb is valid at times, there are factors in the atmosphere which can result in higher or lower snow amounts from that same inch of water. Meteorologists call this the snow ratio, and it plays an important role in snow forecasts for a given storm, in addition to determining a weather system's future path.
(MORE: Winter Storm Central)
In general, lower snow ratios result in wet, heavy snow accumulations commonly found in early and late-season storms. That type of snow is difficult to shovel, and sometimes can down trees and power lines. Conversely, higher snow ratios lead to light and fluffy accumulations, often prone to blowing around in the wind.
Historically speaking, there is variability by region when it comes to snow ratios, as this map illustrates from the National Weather Service (NWS) in Aberdeen, South Dakota.
The contours show the average snow ratio across the United States 1986-2015. Highest ratios are in the nation's northern tier, lower ratios are in the South. (NWS-Aberdeen/NOHRSC, Fall and Sparrow)
Snow ratios in the nation's northern tier are the highest based on 1986-2015 historical data from the National Operational Hydrologic Remote Sensing Center (NOHRSC).
Locations from the Dakotas eastward into the Great Lakes have seen average snow ratios anywhere from 12-to-1 to 15-to-1 during that two-decade span. That means one inch of water has produced, on average, 12 to 15 inches of snow in a given storm. In parts of the northern Rockies, the average ratio is even higher, topping 15-to-1 in some cases.
Locations from New York City southward into the mid-Atlantic have historically seen ratios on either side of 10-to-1.
The lowest ratios are in the South where during many storms the snow ratio is below the 10-to-1 rule of thumb. In cities such as Atlanta and Dallas, the average ratio for the 20-year period examined is between 8-to-1 and 9-to-1, which means an inch of water only produces 8 to 9 inches of snow.
So what causes this variability in how much snow an inch of water can produce?
Factors affecting snow ratios. (NWS-Aberdeen)
For one, temperature plays a critical role, as snow that occurs in an environment where there is deep cold air produces higher amounts of snowfall. As mentioned earlier, this higher-ratio snowfall is typically fluffier and confined to the nation's northern tier as the climatological history map shows above.
It some cases, the snow ratio in this colder situation can be 30-to-1 or greater. Keep in mind, however, that a storm in such a cold environment likely would not produce an inch of water over a given location, and, therefore, allow 30 inches of snow to pile up.
It can be too cold at times for that higher ratio snowfall to occur, since the snow crystal type is sometimes not ideal, according to research from NOAA.
(MORE: All-Time Snow Record in This U.S. City)
Lower-end snow ratios are often found in situations where temperatures are marginal for snow either aloft or near the surface. This is a common situation in the South, where temperatures are often near 32 degrees during winter storms, leading to lower ratios and wet, gloppy snow accumulations.
High winds can also result in lower snow ratios since snowflakes can break apart in that situation, says NOAA.
It's also worth noting that during a particular winter storm, snow ratios may increase or decrease to some degree depending on how temperatures evolve in the large-scale weather situation.
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