By Bryan Allegretto, Forecaster Updated 2 months ago December 5, 2023
When we talk about snow quality, such as "light and fluffy" or "heavy and wet", we are talking about the snow-to-liquid ratio. To calculate the snow-to-liquid ratio, divide the amount of snow by the depth of water you would measure if you melted the snow into liquid.
The higher the snow-to-liquid ratio, the lighter the snow quality, and vice-versa.
Typical Snow-to-Liquid Ratios
A ratio of 10 inches of snow to 1 inch of liquid (10-to-1) will be fun but feel a little heavy.
A ratio of 15 inches of snow to 1 inch of liquid (15-to-1) will offer some faceshots and feel pretty light.
A ratio of 20 inches of snow to 1 inch of liquid (20-to-1) will be incredibly light, almost like skiing through nothing but air.
When the ratio is higher, the snow feels lighter because the new snow contains a lot of air rather than tightly-packed snowflakes.
While there are average snow-to-liquid ratios for each region of the world, every storm is different so there is no single number that works for each mountain all of the time.
What Controls the Snow-to-Liquid Ratio?
Temperature is the main factor that determines the ratio. The lightest snow is usually created by temperatures that are roughly between 0°F and 10°F. This is the temperature where snowflakes are being created, usually in the cloud near or above the top of the mountain.
This temperature range is called the "Dendritic Growth Zone" and is the favored range for big, beautiful dendrite snowflakes to form.
The amount of moisture in the air also plays a role in determining the snow-to-liquid ratio because it impacts the type of snowflakes that form.
The following chart shows the various shapes of snowflakes created by a range of temperatures (bottom axis) and humidity levels (vertical axis).
Image source: http://www.snowcrystals.com/morphology/morphology.html
The wind speed is the final factor that determines the ratio. A faster wind will knock snowflakes into each other, breaking the flakes and allowing them to pack more tightly together on the ground with less air space in the snow. This decreases the snow ratio and the new powder can feel thick and heavy.
How To Use Snow Ratios to Make a Snowfall Forecast
First, we look at a forecast for the total amount of precipitation. These amounts are given as depth of liquid, which is either rain or melted snow. Below is an example precipitation forecast.
Second, we look at the temperature forecast (or estimate temperatures based on the lapse rate) to determine the snow-to-liquid ratio. For example, if the temperature just above the mountain top where snowflakes are forming is 5°F, we might estimate a ratio of 20-to-1.
Finally, we multiply the precipitation forecast by the ratio. For example, if the precipitation forecast is for 1.0 inches, and the snow-to-liquid ratio is 20-to-1, we multiply 1.0 inches x 20 = 20 inches of snow.
In other words, this would be a day with deep, blower (light and fluffy) powder!
If you are looking for the lightest powder, search for temperatures between 0-10°F near or just above the summit. If temperatures are colder or warmer than this, you'll likely find that the snow quality becomes heavier and thicker.
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