Snow-to-liquid ratio compares the depth of new snow with the depth of water it produces when melted. A ratio of 15:1 means 15 inches of fresh snow melts down to 1 inch of water. Forecasters often shorten it to SLR and use it to turn a forecast of liquid precipitation into a snowfall forecast.
Why 10:1 is only a starting point
The 10:1 figure is old. The U.S. Weather Bureau gave observers a typical 10:1 value in 1875, and by 1894 it was already telling them the number was only a rough approximation.
A national climatology by Baxter and colleagues (2005) found that a mean near 13:1 fits much of the country better than 10:1, with large differences from place to place. A National Weather Service (NWS) technical attachment summarizes climatological values from near 17:1 in the Northern Rockies to near 9:1 along the East Coast, with extremes from about 3:1 to 100:1.
What controls the ratio
The ratio starts in the cloud. Temperature and moisture where ice crystals grow set their shape. Dendrites, the branching six-armed crystals that form mostly between about -12 and -18 C (about 10 to 0 F), stack loosely with air pockets between them and often produce ratios of 20:1 or more.
Other processes pack snow tighter. Riming, where supercooled cloud droplets freeze onto falling crystals, fills the gaps and lowers the ratio. Strong wind can fracture crystals and compact the surface, and sublimation or partial melting on the way down changes flakes before they land.
How forecasters use it
A snowfall forecast usually begins with a quantitative precipitation forecast (QPF) of liquid water, and a ratio converts it to snow depth. The 10:1 method assumes 10 inches of snow for every inch of liquid, while other operational options include climatological ratios and algorithms such as the Kuchera and Cobb methods.
A 2023 study of five winter storms found that snowfall errors from the choice of ratio were similar in size to errors from the choice of ensemble member, and that no single method was clearly best.
What it does not tell you
A ratio cannot rescue a poor liquid forecast. If the QPF is half of what falls, the snowfall forecast will likely come in too low even with the right ratio.
Depth is not the whole story either. Low-ratio snow holds more water per inch, so the same depth weighs more on roofs, trees and lines. Precipitation type and the rain-snow line decide whether a ratio applies at all.