Tuesday, January 13, 2009

What Is In A Number?

Have you ever wondered how snow totals equate to rain in terms of true precipitation rates? The rule of thumb has always been 10” of snow will equal 1” of rain, but is it true?

Like anything there are numerous variables in place such as humidity and temperature that will affect the specific density of a snow flake. A higher density of a flake leads to a higher moisture content found. One of the main factors that dictate the moisture content found in snow is the air temperature at the time of snowfall.

The U.S. National Weather Service (NWS) uses a conversion table for estimation, with snow/water ratios increasing from 10:1 to 100:1 as surface temperature decreases:

Surface Snow/waterTemperature Ratio

28-34 °F 10:1
20-27 °F 15:1
15-19 °F 20:1
10-14 °F 30:1
0-9 °F 40:1
-20 to -1 °F 50:1
-40 to -21 °F 100:1