Snow mm to cm Ratio Calculator

Snow-to-liquid ratio: converting rainfall equivalent into snow depth

What ties snow depth to its liquid-water equivalent is the snow-to-liquid ratio (SLR). Most people start with 1:10 as a rough guide, meaning 10 mm of rainfall equivalent ends up as roughly 10 cm of snow. In reality the number swings a lot depending on crystal shape and temperature. Dry powder near −15 °C can hit 1:30, whereas wet snow that falls close to freezing might only reach 1:5. So 20 mm of liquid equivalent at SLR 10 gives you 20 cm of snow, but those same 20 mm at SLR 20 double it to 40 cm. The US NWS leans on the Cobb/Maxwell algorithms to pin down SLR for each storm.

Applications

It shows up in winter weather forecasting and snowfall accumulation maps, in ski-area operations where snowmaking has to be planned ahead, and in winter road management, where it informs chains, plowing schedules and salt dosing. The same figure feeds avalanche risk assessment, the structural load people calculate for roofs in cold climates, and the briefings prepared for aviation winter operations.

FAQ

Why is the 1:10 ratio just an average? Crystal habit (dendrites, needles, plates), temperature, wind and compaction all push the density around. Once you account for that, real SLR can land anywhere from 1:3 to 1:40.

What is liquid-water equivalent? Melt all the snow in a column and measure the depth of water left behind; that is the liquid-water equivalent. It is the same amount a pluviometer would catch if the snow had fallen as rain instead.

Does Brazil get measurable snowfall? Hardly ever. A few sporadic events turn up in the highlands of southern Brazil (São Joaquim, Urupema, Bom Jardim da Serra), and they tend to be a few centimetres of wet, high-water-content snow with a low SLR.