Tuesday, February 8, 2011

Weather 102: Snow ratios and the liquid water equivalent

A couple of days ago I posted a "Weather 102" entry on the basics of the wet-bulb effect and how it plays into the onset of a winter precipitation event. Tonight, I'll continue the "Weather 102" series by providing some basic information on snow ratios and the liquid water that goes into producing snow.

Most of the time when it snows in the Mid-South, we receive a wet, heavy, "sticky" snow that makes for good snowballs, snowmen, and other snow concoctions and slushy streets if it comes down hard enough. Rarely do we see snow that blows freely once it reaches the ground and produces small drifts in the wind.  This is much more common in northern locations. Have you ever thought about why?

In basic terms, it comes down to the water content of the snow and the temperature of the air in which those water molecules freeze (which happens to be much below the expected 0 C/32 F) and subsequently fall through.  The term typically used to describe the water content of snow is the snow ratio, or more accurately snow to liquid water ratio.  The widely-accepted average snow ratio is 10:1.  In other words, for every 10" of snow that falls, when melted, an inch of liquid water results.  Thus, going the other direction, if a snowfall is expected to be produced from .25" of liquid water, 2.5" of snow would result.  In wet snows, the snow ratio may be lower - say 8:1 or 9:1.  Thus, the same .25" of liquid would produce 2.0-2.25" of snow (lower accumulation, but wetter snow).  With dry snow, the ratio goes up, say to 15:1 or 20:1.  Again, given the same .25" of liquid water, the snow accumulation with a dry snow may be 4-5" (higher accumulation, drier snow).

What determines the snow ratio?  Without getting too detailed (I'll save that for Weather 103 or 104!), the temperature of the column of air the snow encounters has a great deal to do with it. In the Mid-South, we typically see snow with surface temperatures in the 28-34 degree range. and temperatures aloft (in the levels below where the snow forms) below freezing but not exceptionally cold. These "warmer" temperatures, especially with a surface temperature near freezing or above, promote high water content in the snow and lower snow ratios (8:1 to 11:1). In addition, wetter snowflakes tend to stick together as they fall and bump into one another, creating what appears to be very large flakes, which are actually multiple flakes aggregated together. Where temperatures are colder at the surface (mid 20s or lower) and aloft, the snow is drier, with lower water content and higher snow ratios. This is more typical up north where it is usually colder when it snows. In addition, drier flakes do not stick together as easily when they fall, so we observe many more smaller flakes at the ground.

So, for tomorrow's snow event, temperatures will be colder than the past few snow events and conditions in the lowest few thousand feet of the atmosphere will be more conducive to a drier snow. The models are pointing to snow ratios in the 15:1 range. Thus, when calculating the amount of snow we will receive tomorrow, I am using a ratio of about 15:1.  With Monday's snow, it was closer to 9:1 as temperatures were 32-33 degrees. Wednesday's snow won't be nearly as good for packing snowballs, but it might mean that the roads are better able to handle it, since it won't stick so much as just blow around.  We'll see if I am right tomorrow!

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