Thursday, April 23, 2009

Measuring Snow

The other day I had the question from Texas about a 25" snowfall that only accumulated to 4" deep.

So how did they know 25" of snow fell?

Snow measurement definitions and practices have changed and evolved over the years. There are some differing opinions on how you should handle different situations, such as measuring with wind, or when the temperature is hovering around freezing.

At one time in NWS history, observers were taught to estimate the accumulation of snow as if it were not melting when it reached the ground.

This was normally done by estimating an approximate snow to water ratio, then measuring the amount of precip that fell from the melting snow, and then multiplying that amount of precip but the inches of snow per unit amount of water to get a total snowfall estimate.

In the 1990s, many observers and users of snow data decided that was not an appropriate way to report snowfall.

Since then, we report only snow that can be seen to accumulate. If it melts on contact, it does not contribute to a reported accumulation.

So more than likely, in the case of the 25" snow that only measured to be 4" on the ground, they were estimating the accumulation.


  1. Actually, it is entirely possible that the snow was measured, not estimated as you stated. The observer probably measured the snowfall every 6 hours during melting situations, as suggested in the CoCoRaHs training seen here: or in the In Depth CoCoRaHs Snowfall training PDF (see Example on pg 56). These 6 hours measurements probably totaled to 25", but when the official observation was taken at 7am the next morning, the total snow depth on the ground was 4" after melting and compacting.

    Another site for snow measurement guidelines is on the NWS COOP page here:

  2. Nevermind, after finding the original question, I agree that the snow was estimated and probably shouldn't have been.

  3. After looking at the links Steve F posted I noticed this in bolded font on the NWS link...

    Snowfall amounts can be measured hourly or at any interval as long as the snow measurement board is NOT cleared more frequently than once every 6 hours.

    What are the reasons behind not clearing the board?