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Snow to Liquid Ratios (SLRs): Understanding the Science Behind Snowfall Forecasting

Snowfall can occur in many different areas around the world. This makes accurately forecasting the snowfall amounts, an important part of meteorology. Looking at Snow to Liquid Ratios or SLRs helps determine how much snow one can expect out of a storm. It is not as simple as one would think and can vary depending on factors such as terrain, humidity, and temperature.

The Evolution of Snow Ratios

For many years, the rule of thumb was a 10:1 ratio across the US – 10 inches of snow equated to 1 inch of water. However, studies have shown that this may not be the most accurate ratio for everyone. 10:1 may work for some locations such as the Great Plains but has less accuracy in places near the Rocky Mountains and the like and would fit closer to an 18:1 ratio. For areas such as the Upper Midwest, a more representative average might be a 12:1 ratio as another example. Complicating things further, SLRs can also fluctuate significantly within a single snow event. One snow event can drop 1.0” of snow in one location and 12.0” in another location, simply due to the variation in the SLRs across the country.

Variables Influencing Snow Ratios

Relying on location is just a small portion of the factors that need to be considered for SLRs. Several variables come into play, shaping the relationship between liquid water and snow during precipitation events:

  • Snowflake Type: Different types of snowflakes can contribute to the density of snowfall. For example, dendrites, due to their shape have a much lower density on the ground than that of columns, contributing to higher SLRs.
  • Depth of the Warm Layer: The presence of a warm layer that is closer to 32°F reduces the ratio. The warmer it is, the lower the SLR tends to be. Thus, creating the potential for heavy, tree-limb-breaking snow.
  • Ice Content in Clouds: The composition of snow-producing clouds matters. More super-cooled water droplets result in lower ratios, while a higher concentration of ice crystals leads to higher SLRs. This creates drier, lighter, and fluffy snow.
  • Wind Impact: Windy conditions can alter snowflakes’ structure, diminishing their “lacy” form and resulting in lower accumulations, thereby lowering snow ratios.
  • Temperature: Deep cold promotes higher ratios, but extreme cold may not favor crystal types conducive to high SLRs.
  • Ground and Boundary Layer Temperatures: Warm ground and boundary layer temperatures tend to keep snow-water ratios low.
  • Storm Tracks: The direction and speed of a storm can impact SLRs as different atmospheric conditions vary by location. For example, if a storm tracks closer to an ocean, the SLR will likely be lower due to higher amounts of liquid water in the atmosphere.


The complex task of predicting snow to liquid ratios involves a nuanced understanding of various meteorological factors. Pay attention to the snowflakes that fall during the next snowstorm and remember that each flake tells a unique story dictated by the temperature, moisture, and winds above.