January 7, 2001

SNOWFALL and FROZEN PRECIPITATION MEASUREMENTS
By Nick Stefano
 

By far, snowfall is the most difficult of all types of precipitation to measure. Drifting, compressing, melting etc. make this sometimes a real chore to get an accurate measurement.

Below are some "basic" rules of thumb to use when recording your snowfall measurements.

1. Since snow does not usually accumulate on the level, one needs to take at least ten (ten) measurements in various locations in the designated snowfield area. Take your total new fallen snowfall and then divide by ten. (for our purposes, 5 measurements added together and divided by 5 will serve as an average, but 10 is the preferred number of measurements)
2. Make sure that there is no previous snow on the ground. If there is, DEDUCT that amount from the new total.
   (I recently received a call from an observer who complained that I posted a much smaller amount of snow from his township then actual. It was later learned that he did not deduct the amount of snow that was already on the ground. , which decreased the total snow by 5 inches!)
3. The use of a snowboard is preferred since the grass on the surface can actually "add" to your total from the little air pockets between the grass and the dirt surface. Snowboards are fine, however I just mentioned previous that we need to take at least 10 (ten) measurements. During some storms, a snowboard in one location is representative of the snowfall. First, start out with the board and
    then move out should conditions warrant. This means that at least ten (10) snowboards must be placed around an area. Something no one does. I prefer the old fashion way. Over the ground or over the old snow using the strict guidelines mentioned above. (Again, for our purposes, one snowboard that is sheltered from the wind is a good start. The snow board should be 20"x20" rough surfaced white painted plywood. The rough surface helps Snow adhere to it and the white coloration decreases solar effects. )
4. New NWS guidelines suggest taking measurements every 6 (six) hours (but no more then every six hours) and then clearing your snow area or snowboard. Once the storm has ended, add the amounts from your six-hour measurements.   (During heavy snow like the Storm of the Millennium, hourly measurements are needed to give NWS an idea as to how quickly snow is accumulating.)
5. When melting snow for a liquid equivalent, you need to be using a rain gauge  (with the inner tube removed) so you can collect the new snowfall inside. Then, when it is time to melt the snow, bring the collector inside and first:

   a- Fill the inner tube, which usually is graduated in hundredths of an inch of liquid, with a known  amount of warm to hot water. Write down this amount on a piece of paper because you might have to add more hot water if this does not melt down the snow completely.  

   b- Pour this known amount into the larger outer tube, which collected the snowfall. Swish it around  until the snow inside is completely melted. If this does not melt the snow completely, add more hot  water and swish that around as well. Remember to ALWAYS write down the amount of hot water  in hundredths of an inch poured into the larger collector.

   c- Once the snow is completely melted, slowly pour the liquid into the funnel, which fits on top of the  smaller graduated inner tube. Without overflowing the tube, pour some in and then write that  number down. Then pour the amount you just measured out. Repeat the same procedure until  all the melted snow liquid is empty.

   d- Now, add your total and DEDUCT the amount from the total of the warm to hot water you  added initially to melt the precipitation
   
   An even better way would be to take a core sample of the snowfall that is representative of  the current event. Capture a core sample by pushing your collector through the representative snow area down to the snowboard surface, ground surface or to the beginning of the existing  snow on the ground prior to this event. Turn the core tube right side up slowly but make sure  you do not let any captured snow fall out. You can do this by using a flat surface cardboard etc. and slipping it under the tube opening before you turn it right side up. Then proceed with the melting process.
    

6. To get your snow to liquid ratio, simply take your total snow measurement and divide it by  the total liquid measured i.e.
   
   Example: 17.3" of new snow
                   1.44" of melted liquid = 12 to 1 ratio
   
   One way to establish if an observer is measuring the snow and liquid equivalent correctly is to  watch the snow to melt ratios reported. The New Years' Eve Millennium storm of 2000 was  pretty equal over a larger area with respect to liquid equivalent ratios. If most people are within a 14 to 1 ratio and another in the same area comes in with an 18 or 20 to 1 ratio, they are probably measuring the snow incorrectly.

As mentioned, snowfall measurement is not an exact science. Keep up the excellent work.  This is a great public service from everyone.

Nick Stefano
Sussex County Weather Network
nick@sussexcountyweather.com