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12 December 2003

The Science of Measuring Snowfall

by Paul Murphy
The Ozarks Weather Observer

The science or art of measuring snowfall has changed over the years in order to more accurately preserve the climatological records which are the basis for global warming studies. Here at the National Weather Service in Springfield,MO, snowfall records date back to the late 1800s, and the techniques of measuring snowfall have been passed down from observer to observer. Many factors, including wind, water content, and snow melt, can make snow measuring a difficult task. Hopefully, the following set of instructions will provide you, the observer, with the necessary skills to provide the National Weather Service with accurate snowfall information this coming winter season. Don't worry if your snow measuring equipment doesn't match what is listed in the instructions. Simply alter the instructions to fit your needs, and remember to have fun conducting this important public service.

Each season, before the first snow: Review the following instructions for measuring snow. It is easy to forget what needs to be measured. Even NWS forecasters need a refresher every year, especially after a low snowfall season.

At the beginning of each snowfall/freezing season, remove the funnel and inner measuring tube of the eight-inch manual rain gauges, (or other type gauges) leaving only the 8-inch diameter overflow can so that it can more accurately catch frozen precipitation. When it rains, simply pour the contents of the can into the inner measuring tube to report rainfall.

Next, put your snow board(s) out and mark their location with a flag or some other indicator so they can be found after a heavy snowfall. They should be located in the vicinity of your station (home) in an open location (away from trees, obstructions, and the north side of structures in the shadows). A snow board can be a simple piece of plywood. It serves as a flat open surface where snowfall can be accurately measured.

Check your precipitation gauge to make sure there are no leaks. If there are leaks, take appropriate action. Plastic rain gauges often crack and break if water becomes frozen inside. Once your equipment has been readied for winter you are prepared for taking snowfall measurements!

Observers should determine two values when reporting solid precipitation (snow, sleet, ice). They are:

Measure the amount of NEW snowfall (snow, sleet, and ice) since the previous snowfall observation. Determine the depth of snow on the ground at the normal observation time.

1. Measure the greatest amount of new snowfall that has accumulated on your snow board (wooden deck or ground if board is not available) since the previous snowfall observation.

This measurement should be taken minimally once-a-day (but can be taken up to four times a day) and should reflect the greatest accumulation of new snow observed (in inches and tenths, for example, 3.9 inches) since the last snowfall observation. When calling your daily snowfall into the National Weather Service, make sure to distinguish between new snowfall over the last 24 hours, and the total snowfall on the ground.

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 landed."

It is essential to measure snowfall and snow depth in locations where the effects of blowing and drifting are minimal. By finding a good location where snow accumulates uniformly, you can simplify all other aspects of the observation and reduce the numerous opportunities for error. In open areas where wind-blown 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.

2. Determine the total depth of snow, sleet, or ice on the ground.

This observation is taken once-a-day at the scheduled time of observation with a measuring stick. It is taken by measuring the total depth of snow on exposed ground at a permanently-mounted snow stake or by taking the average of several depth readings at or near the normal point of observation with a measuring stick. When using a measuring stick, make sure the stick is pushed vertically into the snow until the bottom of the stick rests on the ground. Do not mistake an ice layer or crusted snow as "ground." The measurement should reflect the average depth of snow, sleet, and glaze ice on the ground at your usual measurement site (not disturbed by human activities). Measurements from rooftops, paved areas, and the like should not be made.

Report snow depth to the nearest whole inch, rounding up when one-half inch increments are reached (example 0.4 inches gets reported as a trace (T), 3.5 inches gets reported as 4 inches). Frequently, in 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 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.

HOW MUCH SNOW HAS FALLEN: One of the most frequently asked questions is, "Where do I measure the snow depth?" The answer is simple. Based on your estimation of the conditions at your station, you determine an average depth, excluding drifts and spots blown bare by the wind. Your estimate is the official snow depth for your station.