Article — Rain to Snow Calculator
Rain to Snow Calculator: Convert Rainfall to Snow Depth
A rain to snow calculator converts liquid rainfall into the equivalent snow depth by applying a temperature-dependent ratio. NOAA bands run from 5:1 wet snow near 32 degrees F to 30:1 powder below -20 degrees F, with 10:1 to 15:1 for typical winter storms. The Roebber et al. study in Weather and Forecasting (2003, 2005) replaced the old 10:1 rule of thumb with this banded model after analysing 28,000 station records.
Enter your rainfall in inches or centimetres, set the surface air temperature in Fahrenheit or Celsius, and the calculator returns snow depth, the applicable NOAA ratio band, and the snow type label (wet, average, dendritic, powder, ultra-fluff). Results are estimates; mid-storm temperature shifts and wind compaction routinely push real accumulation 10 to 30 percent in either direction.
How the rain to snow calculator works
This rain to snow calculator looks up the temperature you enter against the NOAA / National Weather Service ratio table, multiplies that ratio by the rainfall, and reports the snow depth in your selected length unit. The temperature input drives the entire result; rainfall is just a multiplier. Switching the rainfall unit converts the existing value automatically so you do not have to recompute.
The ratio bands are coarse on purpose. Snow:water ratio depends on more than surface temperature, but air temperature dominates the variance enough that a banded lookup matches the published climatology within a few inches. For research-grade forecasting, NOAA WPC uses additional inputs like vertical temperature profiles, relative humidity, and supercooled liquid content from numerical models.
Mount Baker Ski Area in Washington holds the world record for seasonal snowfall: 1,140 inches (28.96 m) in 1998-1999. The accumulated snow water equivalent at peak was about 95 inches, suggesting a season-average snow:water ratio near 12:1. The next-deepest reliably measured year is the 1971-1972 season at Rainier Paradise (1,122 inches).
The snow to water ratio explained
The snow to water ratio (SR) is the ratio of fresh snow depth to the liquid water that produced it. A 10:1 ratio means 10 inches of snow contain 1 inch of liquid water. The ratio is unitless and varies with snow density: light, dry crystals pile up loosely (high ratio); heavy, partially-melted flakes compact tightly (low ratio). NOAA reports ratios in the 5:1 to 30:1 range for normal precipitation, with extremes at either end.
Snow Water Equivalent (SWE) is the inverse view: how much liquid water is locked in the snowpack. SWE matters for water-supply forecasting because spring runoff depends on the liquid content, not the visible snow depth. The NRCS SNOTEL network measures SWE at hundreds of mountain stations across the western US to forecast irrigation supply for the warm season.
Temperature bands for rain to snow
At temperatures above 32 degrees F, falling snow partially melts before reaching the ground, producing a wet, sticky snow with a low 5:1 ratio. Between 25 and 32 degrees, the snow holds together better but still contains liquid water (8:1). The 15 to 25 degree band is the most common winter-storm regime in the contiguous US, producing classical dendritic crystals at 12:1 to 15:1.
32+ F 5:1 wet28-32 F 8:1 sticky20-28 F 12:1 average10-20 F 15:1 dendritic0-10 F 20:1 powder-10 to 0 F 25:1 very drybelow -10 F 30:1 ultra-fluffWet snow vs dry powder conversion
Wet snow forms when air temperature is close to or above freezing. The crystals are partially melted and refrozen, with high liquid content and density around 300 to 500 kg/m^3. One inch of rain at this regime makes only 5 inches of snow. Wet snow is the kind that breaks tree limbs and downs power lines because it weighs 6 to 10 times more per inch than powder.
Dry powder forms below 15 degrees F in the dendritic growth zone where six-pointed crystals develop with maximum air space between them. Density falls to 50 to 100 kg/m^3, and the ratio climbs to 15:1 or higher. Ski resorts in the Wasatch (Utah) and the Colorado Rockies are famous for this snow; the same inch of rain that would make 5 inches of wet snow at 30 F makes 15 inches of powder at 10 F.
Snow water equivalent and hydrology
SWE is the metric hydrologists care about. A deep but airy snowpack holds little water; a shallow, dense snowpack holds a lot. NOAA NOHRSC produces daily SWE maps for the contiguous US based on remote sensing and ground stations. The 1 April SWE in the Sierra Nevada has been the most-watched water-supply number in California for decades because it determines summer irrigation deliveries.
Two storms can deposit the same SWE but very different visible snow depths. A 1-inch SWE event at 10 F deposits 15 inches of powder; the same SWE at 30 F deposits 5 inches of slush. For driving conditions and road clearance, snow depth matters; for water supply, drought forecasting, and avalanche analysis, only SWE does.
Regional rain to snow ratio differences
Average snow:water ratios vary by region. The Southeast US runs 8:1 to 9:1 because winter air stays close to freezing. The Northeast averages 11:1 to 13:1. The Upper Midwest runs 13:1 to 15:1 in colder winters. Colorado, Utah, and Wyoming average 15:1 to 17:1, with frequent 20:1 events at altitude. Interior Alaska can sustain 25:1 or higher for weeks at a time in January.
Lake-effect snow over the Great Lakes is its own regime. Warm lake water (40 to 50 degrees F in early winter) evaporates into very cold incoming air, creating intense localised bands that drop 1 to 3 inches of snow per hour. Ratios in lake-effect bands run 8:1 to 15:1 depending on how cold the upper-level flow is.
Why the 10 to 1 rule is outdated
The 10:1 rule (1 inch of liquid water makes 10 inches of snow) dates to early 20th-century US Weather Bureau practice (often attributed to George Hellman). Forecasters used it because it was easy. Roebber, Bruening, Schultz, and Cortinas analysed 28,000 station-day records in their 2003 Weather and Forecasting paper and found that the actual US-wide mean is closer to 13:1, with the regional variance described above. NOAA now uses the banded temperature model for operational forecasting.
- 10:1 rule = old rule of thumb, no longer used operationally
- 13:1 = modern US-wide mean snow:water ratio
- 5:1 = wet snow near 32 F (breaks tree limbs)
- 30:1 = ultra-fluff in interior Alaska or northern Rockies
- Mount Baker holds the seasonal record: 1,140 in (28.96 m) in 1998-99
- Density range = 50 to 500 kg/m^3 (10x variation)
- Roebber 2003 paper established the temperature-banded model
- Wind compaction reduces measured depth 10 to 30 percent
Common rain to snow conversion mistakes
The most frequent error is applying the 10:1 rule everywhere. The 10:1 ratio is reasonable for the eastern US in moderate cold but underestimates Rocky Mountain snowfall by 30 to 50 percent and overestimates Gulf Coast snowfall by similar amounts. Use the temperature band instead.
The second mistake is ignoring storm-internal temperature shifts. A storm that begins at 25 degrees F and warms to 33 produces a layered snowpack: dry powder on the bottom, wet snow on top, and the overall ratio falls somewhere between the band predictions. The calculator above uses surface temperature as a single proxy; for research applications you would integrate the ratio along the storm timeline.
The third mistake is forgetting wind. NWS snow gauges and Doppler-radar accumulation estimates assume relatively still air. A 20-mph wind during a storm compacts surface snow by 15 to 25 percent and drifts the rest, so on-the-ground measurements vary widely across small distances. Official observers measure on level boards in sheltered locations to control for wind.