How this calculator works
The ratio compares the depth of freshly fallen snow with the depth of water it contains after melting. A 10:1 ratio, the old rule of thumb, means 10 cm of snow holds 1 cm of water. Real ratios range from about 5:1 for wet, slushy snow to 50:1 or more for very cold, fluffy snow, and they change within a storm. This calculator uses a surface-temperature table from the NWS Observing Handbook: warm snow near freezing gets 10:1, and each colder band raises the ratio, up to 100:1 below about -29 degrees Celsius. You can override the estimate with your own measured ratio.
Formula
snow depth = rain depth x ratio. rain depth = snow depth / ratio. The temperature estimate uses bands at -2.22, -6.67, -9.44, -12.22 and -17.78 degrees Celsius, with ratios of 10, 15, 20, 30, 40, 50 and 100.
Worked example
At -5 degrees Celsius the table gives a ratio of 15. A 10 mm rainfall therefore becomes 10 x 15 = 150 mm, or 15 cm, of snow. Going the other way, 15 cm of snow at the same temperature melts to 15 / 15 = 1 cm, or 10 mm, of water. If the snow is wet and heavy, a measured ratio of 8 would turn 10 mm of rain into 8 cm of snow instead.
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Frequently asked questions
Why is the 10:1 rule sometimes wrong?
The rule assumes a fixed snow density. In reality the crystal type, the depth of the warm layer the flakes fall through, the amount of supercooled water and the wind all change density. The NWS notes that a ratio near 12:1 may be more representative on average, and individual storms can range far wider.
Which temperature should I enter?
Use the surface air temperature at the time of the snowfall, in the shade. The NWS table is based on surface temperature. If you know the temperature higher in the cloud, or a measured snow density, the custom ratio option gives a better answer.
Does this work for sleet or freezing rain?
No. Sleet and ice pellets have a different density and do not follow the snow-ratio table. Freezing rain adds a glaze rather than depth of snow. Use a measured water equivalent for mixed precipitation.
Can snow depth exceed the rain depth?
Yes, that is the point of the ratio. Snow is mostly air, so a small amount of liquid water can produce a much deeper layer of fresh snow. The ratio falls as snow becomes wetter and denser.