Editor's note: This story originally was published Jan. 5, 2010.
When it snows 10 inches, does that actually equal one inch of rain?
I’m sure you’ve heard that claim. It is a belief that seems to get regurgitated every time it pours cats and dogs or snows feet.
But, is the legend true?
The quick answer: sometimes.
When the temperature is around 30 degrees, one inch of liquid precipitation would fall as 10 inches of snow -- assuming the storm is all snow.
But the amount of moisture in each snowflake differs depending on the temperature. That's the variable that changes the snow-to-rain ratio.
For example, our big December (2009) snowstorm occurred with temperatures closer to 25 degrees. During that storm the snow ratio was closer to 15 inches of snow to one inch of rain. We had 1.75 inches of "liquid equivalent," yet ended up with 23.2 inches of snow, not 17.5 inches of accumulation.
In fact, I took this into account when forecasting 15 to 25 inches from the Philadelphia area southward for the storm. We even showed a graphic on-air explaining those estimates.
We've had storms with snow closer to 20 degrees -- moving the snow ratio closer to 20-to-one. And, when it's warmer (35 to 40 degrees), the ratio moves to 5-to-1.
So, when someone claims that it's 10:1, PERIOD, you can tell him or her that they're wrong.