A Weighty Issue: Why Are Some Snowfalls Heavier Than Others? — Westword #snowpack #runoff

Photo via Snowflakes Bentley (Wilson A. Bentley)

From Westword (Andy Stein):

Snowflakes are not created equal (though all snowflakes are arguably very pretty). Many factors determine how big snowflakes can get…and how much energy you’ll exert when you finally go outside to shovel that snow.

Snowflakes start as little ice particles that form on dust or pollen in the air, creating an ice crystal. Then gravity takes over and the ice crystal begins to fall; as it does, water vapor starts to freeze on it, ultimately forming the six-armed flake that lands on the ground. The different shapes of snowflakes result from atmospheric conditions present at the time. A crystal might begin to grow arms in one way, and then minutes or even just seconds later, slight changes in temperature or humidity can cause the crystal to grow in another way.

Certain atmospheric conditions exist at different times of the year, and those conditions are what ultimately decide whether those flakes comprise a snowfall that will be light and fluffy or heavy and wet. Springtime snowstorms have a higher water content than wintertime snowstorms. Meteorologists track that by snow ratio, or snow-to-water ratio: how much water you get from melting down a column of snow.

The snow ratio is not a constant number; it changes storm by storm. Some storms might have a 5:1 ratio, which means that if five inches of snow accumulation were to melt, one inch of water would result. But some storms have a 20:1 ratio, and it would take twenty inches of snow to create an inch of water. Most snows in Colorado fall somewhere within the 8:1 to 20:1 range.

A 20:1 snow ratio results from limited atmospheric moisture and very cold temperatures. When there is not a lot of moisture in the atmosphere and the air is very cold, the ice crystals freeze into snowflakes that are really very tiny ice balls. When they bump into each other, they essentially just bounce around rather than sticking together.

In order for a storm to have a low ratio, such as 5:1, temperatures need to hover at or above freezing (32 degrees Fahrenheit). When the air temperature is warmer, the snowflakes don’t freeze as firmly as they do at colder temperatures. As snowflakes fall through the sky, they bump into each other and stick to one another, forming large-looking snowflakes.

Snow with a high ratio usually sticks to roads quicker (because it’s colder out), doesn’t stick to power lines and is easy to shovel; such storms usually occur in December, January or February. Snow with a low ratio typically lands in October, November, March and April. Such snow makes everything super-wet before it starts to stick, and then it will stick to trees and power lines as well as roads and sidewalks.

Depending on moisture content, snow can weigh from 1 pound per cubic foot to over 20 pounds per cubic foot, like the snow that landed March 19, resulting in some very heavy lifting for anyone who shoveled.

Colorado snowpack basin-filled map March 27, 2020 via the NRCS.

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