The hidden factor that determines snowfall totals

CHICAGO - One of the keys to snowfall forecasting is something called the Snow to Liquid Ratio, or "SLR" for short.

The SLR refers to how much snow fluffs up relative to how much actual "water" is involved. A common misconception is that SLR is always 10:1, meaning that for every inch of liquid, you’d end up with 10 inches of snow. 

Let’s say it’s a cold rainy day with a temperature of 36 degrees. People love to generalize when they hear that a half-inch of rain fell that "if that was snow we would have had 5 inches!" That would be true if the SLR were indeed 10:1. That frequently isn’t the case, however.

Dig deeper:

Many factors go into the SLR. First and foremost is the temperature, both at the surface and aloft. Colder temperatures generally result in a higher SLR, often 20:1 or higher. Conversely, a warmer snow (great for making snowballs and snowmen) would have a SLR of perhaps 7:1. Other variables that impact the SLR include the humidity and the shape of snowflakes. The bigger flakes tend to have more water content than smaller flakes which are free to fluff up more. 

Complicating the SLR’s role in snowfall forecasting is the fact that the ratio changes during the event. The snow might start as a heavy wet snow then transition to a drier, higher-ratio snow. 

By the numbers:

Here’s a useful example of how SLR’s can have a significant impact on snow totals: Let’s say we expect about .30 inches of liquid to fall at a ratio of 10:1. That would be 3 inches of snow. What if the SLR ends up being 20:1? That would end up as 6 inches of snow.

The point is, getting a good handle on the SLR helps determine a good snowfall forecast.