Observing Snow

State Climatology Office
DNR Waters
January 2003

Daily observations are recorded on a form. That form is available as a Word (printable) data form.
Specific information for filling-in data forms is also available.

Snow Measurements
Based on ‘Snow Measurement Guidelines (10-23-96)’, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service (NWS), Office of Meteorology and ‘National Weather Service, Observing Handbook No. 2, Cooperative Station Observations’, Observing Systems Branch, Office of Systems Operations, Silver Spring, Md, July 1989 and ‘The Snow Booklet’ by Nolan J. Doesken and Arthur Judson, CSU, 1996.

At the beginning of each snowfall/freezing season:

• Put your snowboard out and mark the location with a flag or some other indicator so it can be found after a new snowfall.
• If you will measure ‘precipitation including melted snow’ in a cylindrical ‘rain’ gage, remove the funnel and inner measuring tube rain gage. Check your gage to make sure there are no leaks.
• Board and gage (if used) should be located in the vicinity of your station in an open location (not under trees, obstructions, or on the north side of structures in the shadows).

This measurement should be taken minimally once-a-day (but can be taken up to four times a day, see note below) and should reflect the greatest accumulation of new snow observed (in inches and tenths, for example, 3.9 inches) since the last snowfall observation. If you are not available to watch snow accumulation at all times of the day and night, use your best estimate, based on a measurement of snowfall at the scheduled time of observation along with knowledge of what took place during the past 24 hours. If you are not present to witness the greatest snow accumulation, input may be obtained from other people who were near the station during the snow event. If your observation is not based on a measurement, record in your remarks that the "snow amount based on estimate".

Remember, you want to report the greatest accumulation since the last observation. If snowfall occurred several times during the period, and each snowfall melted either completely or in part before the next snowfall, record the total of the greatest snowdepths of each event and enter in your remarks "snowfall melted during the OBS period".

For example, three separate snow squalls affect your station during your 24-hour reporting day, say 3.0, 2.2, and 1.5 inches. The snow from each event melts off before the next accumulation and no snow is on the ground at your scheduled time of observation. The total snowfall for that reporting 24-hour day is the sum of the three separate snow squalls, 6.7 inches, even though the snow depth on your board at observation time was zero. Snow often melts as it lands. If snow continually melts as it lands, and the accumulation never reaches 0.1 inches on your measuring surface, snowfall should be recorded as a trace (T) and record in your remarks that the "snow melted as it fell".

It is essential to measure snowfall (and snow depth) in locations where the effects of blowing and drifting are minimized. Finding a good location where snow accumulates uniformly simplifies all other aspects of the observation and reduces the numerous opportunities for error. In open areas where windblown snow cannot be avoided, several measurements may often be necessary to obtain an average depth and they should not include the largest drifts. In heavily forested locations, try and find an exposed clearing in the trees. Measurements beneath trees are inaccurate since large amounts of snow can accumulate on trees and never reach the ground.

If your daily schedule permits, you may wish to make a snowfall observation every 6-hours, beginning with your regularly scheduled time of observation. This is the procedure followed by National Weather Service Forecast Offices. Follow the same rules for a once-a-day observation, but the snow accumulation reported will be the greatest for the previous six hours instead of 24 hours. If you take your observations at this frequency, make sure that you clear your snowboard (or other measuring surface) no more than once every 6 hours. Record the frequency of observations during the day in the comments section of your report. Never sum more than four, six-hourly observations to determine your 24-hour snowfall total. If you use more than four observations, it would falsely increase snowfall totals.

While sleet or ice pellets should be included in snow amounts, freezing rain (or glaze ice) should never be reported as snowfall. This precipitation type is liquid precipitation and should be reported as such.

Precipitation (‘Rain, Melted Snow, Etc. – Ins & Hdths’)
Measuring the water equivalent of snowfall since the previous day's observation. This measurement is taken once-a-day at your specified time of observation using a standard 4-inch plastic gage that has been left to collect without using the funnel or inner tube.

• Melt the contents of your gage inside your home or other heated space. Let the gage stand OR add water using the inner tube to the large outer tube having first noted the amount in the inner tube being added. (Note: avoid using HOT water as the plastic gage may crack).
• Pour the combined melted snow plus any added liquid into the funnel and smaller inner measuring tube and measure the amount to the nearest .01 inch just as you use for measuring rainfall.
• Subtract any amount of water added from your measured total and record the result as ‘precipitation’.

Do not measure the melted precipitation directly in the large 4-inch outer cylinder. Make sure the inner measuring tube can't fall over when pouring the liquid back into it. If the melted water equivalent (including any added warm water) exceeds one inches and cannot fit into the measuring tube all at one time, then empty the full measuring tube and pour the remaining liquid from the large 4-inch outer cylinder into the emptied measuring tube. Then, add and record the water equivalent of the multiple measurements. If you added warm water to the gage to melt the snow, make sure you accurately measure the amount of warm water added before pouring it into the gage. Then, when you take your liquid measurement, subtract the amount of warm water added from the total liquid measurement to get your final liquid water equivalent of the snowfall.

As winds increase, gages collect less and less of the precipitation that actually falls. Generally speaking, the stronger the wind and the drier the snow, the less is captured in the gage. If you notice that less snow is in the gage than accumulated on the ground, or if the snowfall is deeper than the gage itself, or other condition that caused a non-representative amount to fall into the gage, you will need to take a core sample of the snow.

• empty any existing snow from inside the 4-inch cylinder which will be used to take a snow sample, sometimes referred to as "taking a core" or "cutting a biscuit".
• If drifting is a factor, find an area where drifting is minimal. This will usually be a flat area away from obstructions such as trees and buildings, although obstructions at some distance can help minimize drifting. Otherwise take the cutting from on top of your snow board.
• Invert the overflow can and force it down through the snow. The rim will cut a cylindrical vertical sample. If the snow is very deep, it may be necessary to push the can part way to the ground. Then, remove and empty the snow into a container, and insert the can in the same hole to obtain the rest of the snow. CAUTION: If the snow board was NOT used, do not push the can through snow that was measured at the previous observation, or its water equivalent will be counted in both measurements.
• Slip a piece of sheet metal or thin wood beneath the mouth of the can to prevent the snow from falling out.
• Take the snow indoors, melt it, and obtain the water equivalent as described on the previous page.
• If there is a question about the accuracy of the water equivalent of snow measured directly in the can, compare it with the amount determined by a core sample and use the larger of the two readings.

While the NWS standard is to report snow depth to the nearest whole inch, the State Climatology Office will accept snow depths reported to the nearest tenth of an inch. Frequently, in very hilly terrain, you will be faced with the situation where no snow is observed on south-facing slopes while snow, possibly deep, remains in shaded or north-facing areas. Under these circumstances, you should use good judgement to visually average and then measure snow depths in exposed areas within several hundred yards surrounding the weather station. For example, if half the exposed ground is bare and half is covered with six inches of snow, the snow depth should be entered as the average of the two readings, or three inches. When in your judgement, less than 50 percent of the exposed ground is covered by snow, even though the covered areas have a significant depth, the snow depth should be recorded as a trace (T). When no snow or ice is on the ground in exposed areas (snow may be present in surrounding forested or otherwise protected areas), record a "0".

When strong winds have blown the snow, take several measurements where the snow was least affected by drifting and average them. If most exposed areas are either blown free of snow while others have drifts, again try to combine visual averaging with measurements to make your estimate.

See also State Climatology Office and the Snow Rules! pages.

Contact Pete Boulay , Greg Spoden, or Jim Zandlo by or by phone at 651-296-4214.