Wednesday, December 18, 2013

Snow to Water Ratios, or How Much Water is in that Snow

If you are a CoCoRaHS observer in a part of the country that receives snow, then you no doubt have reviewed the snow measurement training  that's available on the CoCoRaHS web site, as well as the snow measurement animations on YouTube. You have reviewed these, right?

Taking a snow core
for a water equivalent measurement.
Snow measurement involves several more steps than just measuring what's in your gauge. One of the measurements we ask observers to make is the liquid water equivalent of the new snow measured from a snow core taken from the snow board or other flat surface. Some may ask "Why bother?  Doesn't one inch of water equal ten inches of snow?"   No, not really.

The adage "one inch of rain equals ten inches of snow" is a myth. It happens, of course, but only under certain circumstances. Observers should never "measure" snowfall by using the 10:1 ratio to convert the water in the gauge to snow depth. That's why we ask observers to measure the depth of new snow, and to measure the liquid water equivalent of that snow.

A stellar dendrite snow crystal
So what's with the 10:1 rule? It was first provided to weather observers by the U.S. Weather Bureau in 1875, although it later was qualified to be a rough approximation. It tends to occur or be close to 10:1 when the surface temperature is around 30°F during the snow.  However, the snow to water ratio, or snow density, is dependent on many complex factors. Among these are are the temperature and water vapor within the dendritic growth zone in the clouds (where the temperature is -12°C to -18°C), the depth of the dendritic zone. Dendrites are beautiful, ornate snow crystals that most people are familiar with. Additional factors affecting snow density include the shape of the snow crystals, vertical motion in the clouds, the amount of liquid water in the cloud, and the thermal profile of the layers from cloud level to the surface. Once the snow starts accumulating on the ground, compaction due to the weight of the snow begins to take effect. Wind will also affect the density of the snow. Strong winds break up the snow flakes/crystals into smaller pieces that allow the snow to compact further. Climatologically, the 10:1 ratio tends to occur only over a relatively limited area. A climatology of snow to water ratios was published in 2005 and showed that on average, a ratio of 13:1 was more typical.

30-year climatology of snow to water ratios.
Credit:  A Climatology of Snow-to-Liquid Ratio for the Contiguous United States Martin A. Baxter, Charles E. Graves, and James T. Moore, St. Louis University, October 2005. AMS Journal of Weather and Forecasting

Snow falling on Saturday
Snow to water ratios can change during a storm, and that can be a challenge for forecasters when trying to determine how much snow will result from the amount of liquid water expected. A case in point is the storm that hit my area this past weekend.  The snow started here on Friday afternoon. By Saturday morning at observation time (it was still snowing) there were 6.8 inches of snow that melted down to 0.67 inch of water. The snow to water ratio was 10.1:1. There was little wind during the storm and the snow came straight down, frosting trees, fences, and any horizontal surface, turning everything into a winter wonderland. It continued snowing until about noon Saturday at which point there was another 1.7 inches of snow. However, the lower levels of the atmosphere had warmed some so the snow was denser. That snow melted down to 0.28 inch of water for a snow to water ratio of 6.1:1. That's quite a difference.

Another 0.8 inch of snow on Sunday night melted to 0.06 inch water for a snow to water ratio of 13.3:1.  So over a period of a little over 48 hours I measured 9.3 inches and snow to water ratios ranging from 6:1 to 13:1.

In general, the colder it is, the less dense the snow will be. Snow falling at 20°F will tend to be dry and "fluffy" while snow at 32°F will tend to be wetter and denser, the good-packing snowball-making, snowman-making kind of snow.

1 comment:

  1. How about a precip. map for the year, like the daily one, but showing the yearly totals on a colorful map. That would be nice. Thanks for blogging about the weather