Monday, March 26, 2018

Issues with Snow Measurement

Snow observations are very difficult to make well and even when they are done following "protocol" they still have uncertainties and ambiguities.

Let's take the latest storm as an example.  The ground prior to the storm was quite warm, and this leads to variations in accumulation depending on the characteristics of the surface.  Let's suppose, for example, that you decided to head out and measure the accumulation on the grass this morning.  Here's what you would have found this morning. 

First, the grass on most lawns is beginning to grow and is of variable height.  This in turn affects the transfer of heat from the relatively warm ground to the snow, leading to small mounds and variations in snow height and depth. 

I was able to measure anything from 2 to 4.5 inches of snow on the lawn depending on where I placed the ruler (apologies for not having my metric ruler handy!). 

This is one of the reasons why it is recommended that snowfall measurements should be made on a snowboard, which is typically a piece of wood (painted white) that is at least 16" x 16".  Ideally, snow should be measured as close as possible to the end of the storm, although one might also measure it in intervals that are no more frequent than four times per 24-hour period.  For more information, see this CoCoRAHS site

Standardizing snow measurements in that way provides uniformity, but there are still problems.  For example, it tells you little about what is happening on street and road surfaces, which is where many of the impacts of winter storms occur.  Sadly, most snowfall records provide little information on these important details of winter storms. 

I sometimes wonder if instead of trying to get one number (e.g., 4.5 inches) from each location on snowfall amount, if we should be better characterizing the distribution of snowfall across a wide range of surfaces.  Certainly providing a snowfall observation to the nearest tenth of an inch implies a level of precision and accuracy that is not representative of the snow-measurement uncertainty or local snowfall variability.  In our digital era, we might be able to do much better than a single number and perhaps consider collecting records of accumulations on several different surface types. 


  1. Does snow settling ever start to affect measurements? In other words, suppose a storm lasts 48 days. If you measure totals every 12 hours, would that ever give a larger total than one big end measurement?

    1. Yes.

      The frequency and timing of measurement matters a lot. More frequent measurements means a greater total. Official records do not recognize snow totals based on more than 4 measurements in a 24-hour period.

      In general, the lowest frequency of measurements for freshly fallen snow is once-per-day. This is a common measurement frequency for weather volunteers. The measurement might be taken at a particular time (e.g., 7 am) or as close as possible to the end of the storm. The highest is every six hours. Many snow-safety groups do twice a day.


  2. There used to be a great deal of emphasis on precision with snow amounts and full data profiles in the snow and avalanche world. As an avalanche forecaster here, I tend to broad-brush snow amounts and snow structure due to overall spatial variability. It's key to have the big picture and not trip over the minutia. Unless the minutia is the big picture. Hardesty

  3. Timing of snowfall (day vs night) is a major factor in spring with a high sun angle. The solar energy that gets through the clouds and through thin snow cover to the ground surface causes a lot of heating, and melts the snow from underneath.