Rain to Snow Calculator - Convert Precipitation
Convert rainfall to snow equivalent depth using temperature, humidity, and elevation. Instantly estimate snow accumulation from liquid precipitation.
Enter temperature, precipitation amount, and optional humidity and elevation, then click Calculate to see estimated snow depth.
Rain to Snow Calculator - Convert Precipitation
Convert rainfall to snow equivalent depth using temperature, humidity, and elevation. Instantly estimate snow accumulation from liquid precipitation.
About the Rain to Snow Calculator
Rain-to-snow conversion is a meteorological calculation that estimates how much snow will accumulate on the ground given a known amount of liquid precipitation under specific atmospheric conditions. It is fundamental to winter weather forecasting, ski-resort operations, avalanche risk assessment, and civil infrastructure planning.
The essential insight is that snow has far lower density than liquid water. A millimetre of rain represents 1 mm of liquid water depth. When that same water freezes into snow crystals, the resulting snowpack is much deeper because the interlocked crystals trap a large volume of air between them. The ratio of snow depth to liquid-equivalent depth is called the snow ratio and typically ranges from about 5:1 in wet, near-freezing conditions to 30:1 or more in extremely cold, dry powder conditions.
Temperature is the single most important variable. Near the freezing point (−1 °C to −3 °C), ice crystals grow relatively large and clump into dense, wet flakes that compact easily under their own weight. As temperature drops below −10 °C, crystals become smaller, drier, and more dendritic, trapping more air and producing light, fluffy powder with a higher snow ratio. At −20 °C or below, the relationship asymptotes: very cold, dry powder snow commonly achieves ratios of 20:1 or even 30:1.
Humidity modifies the result. Higher relative humidity causes snow crystals to grow larger and bond more tightly together through a process called sintering, producing denser snow and a lower snow ratio. Lower humidity produces smaller, drier crystals and a higher ratio. The effect is secondary to temperature but meaningful in coastal versus continental climates.
Elevation has a minor influence primarily through its effect on atmospheric pressure and the temperature lapse rate. Higher elevations are typically colder and drier, which tends to increase the snow ratio. At very high elevations the reduced air pressure also affects crystal formation.
The calculator uses a temperature-based formula for the base snow ratio — colder temperatures yield proportionally higher ratios — with a humidity correction factor. The resulting snow depth in centimetres equals the precipitation in millimetres multiplied by the computed ratio. Snow density in g/cm³ is the reciprocal of the ratio divided by ten; typical fresh snow has a density of 0.05–0.10 g/cm³ compared to liquid water at 1.00 g/cm³.
These estimates are approximate. Real-world snowfall depends on wind, crystal habit, solar radiation, ground heat flux, and surface roughness. For official forecasting, meteorological models incorporate far more variables. This calculator provides a rapid, intuitive first estimate suitable for planning, education, and general understanding of winter precipitation physics.
Rain to snow conversion examples
Four weather scenarios showing how temperature and humidity change the snow depth estimate.
| Conditions | Snow Depth | Snow type |
|---|---|---|
| −10 °C, 20 mm precipitation, 70 % RH | ≈ 27 cm | Moderate density snow. Snow ratio ≈ 13.6:1. Typical of cold inland continental conditions at moderate humidity. |
| −2 °C, 15 mm precipitation, 90 % RH | ≈ 13 cm | Wet, heavy snow near the freezing point. Snow ratio ≈ 8.8:1. Dense, clumping flakes ideal for snowballs. |
| −15 °C, 30 mm precipitation, 60 % RH | ≈ 52 cm | Mountain powder conditions. Snow ratio ≈ 17.4:1. Very high ratio produces deep, light accumulation. |
| −5 °C, 25 mm precipitation, 85 % RH | ≈ 26 cm | Coastal winter storm. Moderate-density snow with a ratio near 10.3:1 due to high humidity. |
How to use the rain to snow calculator
- Enter the air temperature in degrees Celsius. The temperature must be below 0 °C for snow to form; the calculator will warn you if it is not.
- Enter the liquid precipitation amount in millimetres. This is the liquid-water equivalent, which weather stations measure using rain gauges with heating elements.
- Optionally adjust relative humidity (default 70 %) and elevation in metres. These refine the snow ratio estimate.
- Click Calculate to see estimated snow depth in centimetres, the snow ratio used, approximate snow density, and snow type classification.
- Click Reset to clear all fields and start a new calculation.
Rain to snow calculator FAQ
What is the snow ratio and how is it calculated?
The snow ratio is the number of millimetres of snow produced per millimetre of liquid precipitation. It depends primarily on temperature: colder air produces drier, less dense snow with a higher ratio. This calculator uses a temperature-based formula with a secondary humidity adjustment. Typical ratios range from 5:1 in wet near-freezing conditions to 20–30:1 in very cold dry conditions.
Is the 10:1 snow-to-rain rule accurate?
The '10:1 rule' (10 mm of snow per 1 mm of rain) is a popular rule of thumb that applies to conditions around −5 °C with moderate humidity. In reality the ratio is highly variable — it can be as low as 5:1 in wet coastal storms near 0 °C and as high as 30:1 in extremely cold interior snowfall events. Using a temperature- and humidity-adjusted calculation gives a much more realistic estimate.
What temperature is needed for snow to form?
Air temperature must generally be at or below 0 °C for precipitation to fall as snow. Even at 0 °C, snow often melts before reaching the ground if lower layers of the atmosphere are above freezing. The best snow-forming conditions are usually between −2 °C and −15 °C, where ice crystals grow efficiently and ground temperatures are cold enough to prevent melting.
How does humidity affect snow density?
Higher relative humidity promotes the growth of larger ice crystals and increases the rate of crystal bonding (sintering), resulting in denser, wetter snow. Lower humidity produces smaller, drier crystals that stay separate, giving lighter, fluffier snow with a higher snow ratio. Maritime climates (high humidity) therefore tend to produce heavier snow than continental climates at the same temperature.
How is snow depth related to snow water equivalent?
Snow water equivalent (SWE) is the depth of liquid water that would result if all the snow in a snowpack were melted. It equals snow depth divided by the snow ratio. A 300 mm snowpack with a 10:1 ratio has an SWE of 30 mm. SWE is the quantity measured by snow pillows and used in hydrological forecasting for river flow and reservoir management.
Why do my results differ from actual observed snowfall?
This calculator provides a simplified estimate based on temperature and humidity. Real snowfall is also influenced by wind redistribution, crystal habit variations, partial melting and refreezing during fall, surface roughness, and topographic effects. Official snowfall measurements use standardised gauges and observation protocols. Treat calculator results as a first-order approximation rather than a precise forecast.