# Snow-to-liquid ratio: why some storms ski deeper than others

> Snow-to-liquid ratio explained: why the US average is nearer 13:1 than 10:1, how temperature and wind change it, and how SkiBuds will estimate it.

Source: https://skibudsapp.com/guides/snow-to-liquid-ratio
Updated: 2026-10-24

Snow-to-liquid ratio (SLR) is how many inches of snow fall for each inch of liquid water. At 10:1, an inch of water makes ten inches of snow; at 20:1 the same inch makes twenty inches of much lighter snow. Ten to one is the old rule of thumb, but a 30-year study of US weather stations found the average closer to 13:1, and 15:1 to 18:1 through Colorado, Wyoming and Montana. The ratio is the main reason two storms with the same amount of water ski so differently, and the temperature inside the cloud, where the flakes grow, has the biggest say in it.

## A 30-year study puts the US average nearer 13:1

The best look at real ratios comes from three atmospheric scientists at Saint Louis University. Martin Baxter, Charles Graves and James Moore took 30 years (1971 to 2000) of daily snowfall and melted-water readings from 7,760 National Weather Service cooperative stations, kept only snowfalls over 2 inches, and mapped the ratio across the country. They published it in 2005 as [A Climatology of Snow-to-Liquid Ratio for the Contiguous United States](https://www.eas.slu.edu/CIPS/PUBLICATIONS/Baxter2005.pdf).

Their headline finding is that "a mean SLR value of 13 appears more appropriate for much of the country rather than the often-assumed value of 10." The spread across the West is the part a skier should remember. Average ratios run 15 to 18 through Colorado, Wyoming and Montana, then drop abruptly to between 9 and 11 along the West Coast. Within the mountain West, the middle half of all observations fell between 10 and 16. On the West Coast it ran from 4 to 12.

The paper opens with an example of why this matters to a forecaster. A quarter inch of water makes 2 inches of snow at a ratio of 8, or 5.5 inches at a ratio of 22. Same storm, same water, nearly three times the depth.

## The study can't tell how many of its 10s were measured

The most common single ratio in the whole dataset, across 668,832 reports, was exactly 10.0. That could be real snow. It could also be habit, and the authors say plainly that they can't tell the difference: no method can separate correctly measured 10s from ones written down by an observer who measured the snow and then "assumed an SLR value of 10 to compute a liquid equivalent in lieu of an actual measurement." The rule of thumb may be propping up part of its own evidence.

Other errors pull in both directions. In strong wind, gauges catch less of the falling snow; the paper cites work showing unshielded gauges missing 70% or more of the precipitation in winds of 9 meters per second (about 20 mph) or higher. A gauge that misses 70% of the water reports a ratio more than three times too high. Rain mixed into the same day's gauge pushes the ratio down, and so does snow that settles before it's measured. The maps also blend nearby stations so that features smaller than a few hundred kilometers are smoothed away, and the stations are general weather stations rather than ski-area plots. Treat the regional figures as a sense of scale; your own mountain's average could sit well above or below them.

## The same water can make 3 inches or 10

Take a **hypothetical** forecast of 0.8 inches of water and run it through the study's regional numbers. In the mountain West, where half of all observations fell between 10:1 and 16:1, that water makes somewhere between 8 and 13 inches of snow. Along the West Coast, where the middle half ran from 4:1 to 12:1, the same water makes anywhere from about 3 inches to 10. Part of that coastal low end may be rain caught in the same gauge, one of the errors above, but the spread is real enough: a coastal storm with a respectable amount of water can show up as a thin, heavy layer or a solid 10-inch morning, and the water forecast alone won't tell you which.

The difference is how much air is trapped between the flakes, and that's the difference you feel on the first turn. Light snow floats you and fills tracks quickly but gives less cover over rocks for the same water. Dense snow builds a base and holds up under traffic, and it's heavy to ride when it's deep. What decides it, more than anything, is the temperature where the flakes grow.

## The temperature inside the cloud sets the ratio

Snowflakes take different shapes at different temperatures. The large, branching crystals called dendrites grow best between about -12 and -18 °C (roughly 0 to 10 °F), the range the National Weather Service office in Chicago calls the [dendritic growth zone](https://www.weather.gov/lot/2009dec2526). Dendrites stack loosely with a lot of air between their arms, so they make the highest ratios. Outside that zone, crystals grow smaller and pack together better, and [avalanche.org](https://avalanche.org/avalanche-encyclopedia/weather/snow-ratio/) notes that snow forming there, or falling through a warmer layer, comes out denser. Wind makes snow denser too, battering flakes into smaller pieces as they fall, and Baxter's paper cites research finding that winds above about 20 mph can fracture and move crystals at the surface.

Chicago's Christmas 2009 storm shows why the cloud matters more than the thermometer on the ground. The office reported ratios between 30 and 50 to 1, which it called abnormal, "particularly with temperatures in the 20's as they were for much of the snowfall." The explanation was overhead: the dendritic growth zone that day was nearly 11,000 feet deep. Baxter's team makes the same point about the NWS's table for converting water to snow by surface temperature, which "is only marginally effective" because it ignores location and what happens inside the cloud.

## SkiBuds will estimate the ratio from the air at the mountain, and that's its weak spot

The rebuilt SkiBuds app, coming soon, will start each snowing hour's ratio from a temperature curve with the dendrite peak built in. The curve is a rule of thumb, not something calibrated against measured snow at resorts:

| Temperature | Starting ratio |
| --- | --- |
| 32 °F or warmer | 10:1 |
| 20 °F | 16:1 |
| 10 °F | 21:1 |
| 5 °F | 23.5:1 (the peak) |
| -20 °F or colder | 9:1 |

Between those points the ratio rises half a point for every degree colder down to 5 °F, then falls back toward 9:1 in arctic air. Each hour is then adjusted: humidity of 70% leaves it alone, 50% adds 1.5 points and 90% takes 1.5 away; wind takes off up to two points; and in the rare hour when more than an inch of water falls, it drops a point because fast snow compacts under its own weight. Every hour is held between 6:1 and 30:1.

The storm's ratio is total snowfall divided by estimated total water over the snowing hours only, so one fluffy hour that drops two inches outweighs six damp hours that drop a trace. Each hour also gets a plain label, from heavy, wet snow at 6:1 up to champagne powder above 20:1 in air colder than 10 °F.

The weak spot is the starting temperature. The app will use the forecast air temperature at the mountain, which is close to the approach Baxter's team calls marginally effective. On Chicago's Christmas storm, the starting curve would have read air in the 20s as about 11:1 to 16:1, nowhere near the 30 to 50 the office reported. The 30:1 ceiling sits at the top of avalanche.org's natural range, but the 6:1 floor sits above the 5:1 it gives for wet "Sierra cement", so the very wettest snow will read slightly lighter than it is. Use the app's number to compare one storm with another, and trust measured snow over it.

## Divide the snow by the water before you drive

If a forecast shows both the water amount (often labeled QPF, for quantitative precipitation forecast) and the snow amount, divide the snow by the water. **Above 15:1 means light snow that rides deep and fills in tracks; under 10:1 means dense snow that covers rocks well and tires legs.** Use matching time periods and elevations for that comparison; [how to read a ski resort snow forecast](https://skibudsapp.com/guides/how-to-read-snow-forecasts) explains where to find them.

Then look at when the cold arrives. Storms often start warm and finish cold, and light snow falling on a denser layer can make a good first morning. Every ratio is an estimate until the snow is on the ground, so check the resort's report before you commit; [snow depth vs snowfall vs SWE](https://skibudsapp.com/guides/snow-depth-vs-snowfall) explains how each number is measured.

Try the division on the next forecast you open. Eight inches from 0.4 inches of water is 20:1, light snow that rides deep and fills in behind you. Eight inches from a full inch of water is 8:1, dense snow that will cover the rocks and tire your legs. The headline says 8 both times, and only the water beside it tells you which day you're driving to.