The snow-to-water ratio does all the work
One inch of liquid does not become a fixed amount of snow. The snow-to-water ratio, how many inches of snow come from one inch of water, depends mostly on temperature. The familiar 10 to 1 is only an average; real values run from about 5 to 1 for wet snow to 30 to 1 for dry powder.
Ratio by temperature
| Air temp | Snow type | Ratio | 1 in of rain makes |
|---|---|---|---|
| 34 F / 1 C | Wet, sticky | 5 : 1 | ~5 in |
| 28 F / -2 C | Wet to normal | 8 : 1 | ~8 in |
| 20 F / -7 C | Normal, dendritic | 12 : 1 | ~12 in |
| 10 F / -12 C | Dry | 18 : 1 | ~18 in |
| -10 F / -23 C | Dry powder | 25 : 1 | ~25 in |
The 0 F to 10 F band grows the large, feathery dendrite flakes that trap the most air, which is why the coldest snow piles up deepest for the same amount of water.
Same water, very different depth
A half inch of rain is a half inch of rain. What changes is the snow it stacks into. Near freezing you get a shallow, heavy layer; in bitter cold the same water becomes a deep, light pile.
Both bars hold the exact same amount of water. Cold powder at 25 to 1 stacks about five times deeper than wet snow at 5 to 1.
What the number can and can't tell you
- Depth is not water content. Deep powder can hold little water; shallow wet snow can hold a lot. To go back the other way, divide snow depth by the ratio.
- The ratio shifts mid-storm. As temperature changes during a snowfall, so does the ratio, so a single figure is always an estimate.
- Local factors matter. Wind, humidity and elevation all move the result. Colorado and Utah powder runs much drier than Great Lakes lake-effect snow.
Common questions
How much snow does 1 inch of rain make?
On a typical winter day, about 10 inches. That is the old 10 to 1 rule. In practice it ranges from around 5 inches for wet snow near freezing to 20 inches or more for dry powder in very cold air.
What temperature gives the fluffiest snow?
Very cold air, roughly 0 F down to minus 20 F, produces the driest, lowest-density snow, with ratios of 15 to 1 up past 30 to 1. Near 32 F the snow is wet and heavy and the ratio drops to about 5 to 1.
Is the 10 to 1 rule accurate?
It is a rough average, not a law. Studies across the United States put the real average closer to 12 or 13 to 1, and the ratio can shift several times within a single storm as the temperature changes.
