Snow Facts

How does 1 inch of snow compare to 1 inch of rain, and how will the melting affect area rivers?


There is no absolute rule for a "snow to liquid" ratio ... the rule-of-thumb is a 1-to-10 ratio: 1" of water makes 10" of snow.  But this  is a very general rule and depends on a variety of factors.

The complete explanation is complicated, but a simplified summary goes like this:

"Wetter" snow (snow falling through warmer air) tends to produce bigger flakes because smaller flakes tend to stick together and make larger flakes as they fall.  The bigger the flakes the more space they take up ... because there is more air space between them when they lay on the ground.  "Drier" snow (falling through colder air) tends to deliver smaller flakes and smaller flakes pack together more tightly.

Hence, "wetter" snow tends to make greater depths for the same amount of water content.

At Metro AP, the "official" snow accumulation reported was 3.0" for Dec 11 . . . the "liquid total" for the day was 0.28".  Now that would come to about a 1-to-10 ratio, except that the preliminary reports suggest that less than half of that liquid was in snow form -- which would mean something closer to a 1-to-20 ratio!

Of course, the whole issue is complicated by the fact that raingauges are lousy snow collectors, and we often under-catch the water equivalent number in snow events because of the raingauge design.

As for the impact on area rivers?  Likely to be less of an impact than what we would see if we had received 0.2" to 0.5" of rainfall ...  in part because the run-off into the streams and rivers is further slowed by the need for the snow to melt before it moves!

Jay Grymes
Chief Meteorologist
WAFB Storm Team