Behind the Forecast: Flake News: How snow-to-liquid ratios impact snow type, totals

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Science Behind the Forecast: How snow-to-liquid ratios impact snow type, totals

LOUISVILLE, KY (WAVE) - When it snows 10 inches, does that actually equal one inch of rain? We’ve all heard this old rule of thumb but it’s not always true.

Just like each snowflake is unique, each snow storm is one of a kind.

Snowflakes form when super-cooled water vapor freezes to a microscopic particle, also called a nucleus, and eventually grows into an ice crystal.

The amount of moisture within a snowflake is mainly dependent on air temperatures but there are quite a few other things that factor in. The humidity and temperature greatly impacts the shape of the ice crystal. Needle-like crystals are seen around 23°F and flat plate-like ice crystals form near 5°F.

As temperatures change, so does the snow-to-liquid ratio. For example, when a storm is all snow and temperatures are near 32°, one inch of liquid precipitation would typically equal to 10 inches of snow or in other terms have a 10:1 ratio. The warmer it is, the lower the snow-to-rain ratio.

The amount of warm air between the snow-producing cloud and the surface also has a big impact on the snow-to-rain ratio. Warmer temperatures mean lower ratios. Colder temperatures lead to smaller ice crystals which compact together as they accumulate and produce snow that may have a ratio of 10:1. With temperatures in the teens and single digits snow-to-liquid ratios can easily reach 20:1 or 30:1!

The amount of super-cooled water droplets in relation to the number of ice crystals is important. If there is a large amount of super-cooled water droplets in a cloud then snow ratios will be lower. If there are more ice crystals then snow ratios will be higher.

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In relation to the number of water droplets, the track of the storm also is a factor. Storms that travel closer to oceans usually have more liquid water in their clouds which produces lower snow-to-liquid ratios.

Wind also plays a factor; stronger winds can cause snowflakes to fracture leading to lower accumulations and snow ratios.

Intense cold encourages higher snow ratios. With colder temperatures, ice crystals are smaller so they compact together as they accumulate and produce snow that may have a ratio of 10:1.

Wet snow correlates to lower snow ratios. Wet snow has a higher liquid content which it gets by partly melting before reaching the ground. Wet snow flakes stick together easily while they fall so there will be a lower overall number of snowflakes but flakes will be larger in size. If the ground temperature is sufficiently above freezing then quite a bit of snow can fall but all melt leading to little or no accumulation.

Higher ratios are common with drier snow because of the lower liquid content which keeps snowflakes from sticking together. Drier snow is made up of a larger number of much smaller snowflakes.

The table below from the National Oceanic and Atmospheric Administration (NOAA) shows approximate snow-to-liquid ratios based on temperatures.

Approximate snowfall amounts at specified temperature ranges
Approximate snowfall amounts at specified temperature ranges

Taking all of this into account, a snow storm in the Midwest is likely to producer lighter and fluffier snow than one in the Northeast. The moisture from the Atlantic oceans coupled with temperatures closer to 32°F will lead to slushier conditions. A Midwest snow storm gets the fluffier snow from the higher snow-to-liquid ratios caused by colder temperatures in that part of the country. Melted, the Northeast storm would have more liquid water than the Midwest storm.

The fluffiest snows (with high snow-to-liquid ratios) occur with light winds and surface temperatures near 15°F. Keep that in mind if you’re itching to make a snowball when our next batch of snow comes into the forecast.

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