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Blowin' in the Wind, Issue #073 Rain To Snow Equivalent, Converting Precipitation - Dec 1, 2009
November 30, 2009
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Getting Rain to Snow Equivalent

Ever wondered about the rain to snow equivalent of last night's precipitation; how much you would have gotten if that rain had turned out to be snow or vice versa? Well, it turns out that there's a simple mathematical formula that will let you find out! We'll find out how much rain that fluffy snow really is, and vice versa, as well as how to measure density of snow, what kind of snow depth you can expect, and some other important information. Let's take a look.

Rain To Snow Equivalent Conversion

The amount of rain you'd receive versus the amount of snow falling depends a lot on the kind of snow you're getting. If you get the very fine, dry powder snow that Alaska often sees, the kind that's great for skiing, fifteen inches of snow equals just one inch of rain. This very fluffy stuff has small snow crystals that don't compress easily, and takes up more space than normal snow.

Extremely wet snow - the kind you get when the temperature is just below freezing, takes up only five inches of space for every inch of melted water, the rain to snow equivalent. The average in the middle is about ten inches of snow for every inch of rain. Using the example above, a one foot Wisconsin blizzard of fine powder snow is equivalent to less than an inch of rain.

How Dense Is Snow?

Snow density is measured in mass per volume. Usually this is specified as kilograms per cubic meter, though some people still do it in pounds per cubic inch. Water has a density of 1000kg/m^3. Snow density is usually measured as a ratio of water density. A snow that had a density of 100 kg/m^3 would be stated as 100/1000, or as ten percent of the density of regular water.

We can often find out snow density be dividing the water content of the snow by the depth. New snow is between seven and twelve percent water, but sitting on the ground for awhile and compressing under its own weight can make it up to twenty or thirty percent water. We usually get lighter snow falling from very cold air, -20°C or less, and heavier snow from warmer air, near the freezing mark. Old packed snow may be up to half water and even more if it has been subject to heavy traffic.

How Much Snow Can We Expect?

Want to measure the amount of snow on the ground? It's remarkably simple, and the experts do it this way, too. Right before you expect to get snow, put a board on the ground. The new snow will accumulate on top of it, and you can measure how much has fallen on the board. Sweep it clean before the next snowfall to measure it again. This is how you can tell whether your local precipitation forecast is accurate. Then, you'll be able to predict approximately how much you'll get next time!

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