How to Take Snow Measurements
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How to Take Snow Measurements |
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 | CoCoRaHS approved! |
Snow measurements described here include:
Snowfall: Depth of new snow that fell in the last 6 or 24 hours. Snow Water Equivalent of Snowfall: Water equivalent of the new snowfall. Total Snow Depth: Total amount of ALL snow on the ground, old and new. Snow Water Equivalent of Total Snow Depth: Water equivalent of the total snow depth
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, ‘The Snow Booklet’ by Nolan J. Doesken and Arthur Judson, CSU, 1996, ‘MANOBS: Manual of Surface Weather Observations ’, Environment Canada, Meteorological Service of Canada, June 1996, and the Minnesota SnowRules! website.
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 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 equipment, and remember to have fun conducting this important public service. Volunteers are encouraged to enter any or all of the following observations, there's no requirement to enter all of the possible observations in order to participate.
Data can be entered in either English or metric units. The reporting precision requirements listed below are specified for both kinds of units.
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.
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 0.001 inches (0.002 cm) 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 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. 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.
If you are not going to measure the Snow Water Equivalent of Snowfall,
after you have finished making the Snowfall measurement clear the snow
off of the snowboard. After completely clearing snow and ice from the
board, reposition it on the surface of the existing snow and mark it with
a flag or other marker so you can find it at your next observation time.
Never leave a snow measurement board in a depression in the snow as it will then
tend to drift over and provide unrepresentative data.
Do not measure the melted precipitation directly in the large
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 inch (4-inch gauge) or
2 inches (8-inch gauge) 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 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 gauge to
melt the snow, make sure you accurately measure the amount of warm
water added before pouring it into the gauge. 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, gauges 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 gauge. If you
notice that less snow is in the gauge than accumulated on the ground,
or if the snowfall is deeper than the gauge itself, or other condition
that caused a non-representative amount to fall into the gauge, you
will need to take a core sample of the snow. To take a core:
Empty any existing snow from inside the 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 above. 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.
Once you have finished measuring both Snowfall and Snow Water Equivalent
of Snowfall, clear off the snowboard as described in the last paragraph
of the instructions above for Snowfall.
*Many people make the assumption that gauge catch goes down
as wind speed increases because of the angle at which the snow
approaches the gauge which leads to a reduced "catch area". However,
the real reason is the deflection of the precipitation particles
around the gauge and sometimes updrafts created over the top of the
gauge. These cause flakes that were headed toward the orifice to be
deflected.
Report the snow depth with precision of at least 1 inch (1 cm). If
you are reporting only to 1 inch (1 cm) precision, values of less than
0.001 inches (0.002 cm) should be reported as a trace (T).
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. From the perspective of water resources, flood prediction, and
engineering, the most important observation (and one not taken by most
weather observers) is the "Snow Water Equivalent of Total Snow Depth".
This measurement is very important as it represents the amount of
water contained in the total snowpack that is available to melt and
run off when temperatures rise. Thus it can be used to forecast
future water supplies and assess the potential for flooding. It can
also be converted to weight per area. This number is critical for
monitoring the weight of snow on roofs. For example, if there are 10
inches of old packed snow on the ground, this may contain 3.0" of water
content which equates to nearly 15 pounds per square foot if
uniformily distributed on the ground and roofs.
Last modified October 15, 2009.
Snow Water Equivalent of Snowfall
Measure the water equivalent of snowfall since the previous
observation. This measurement is taken at your specified time of
observation using a rain gauge that has been left to collect without
using the funnel or inner tube. Melt the contents of your gauge inside
your home or other heated space. Let the gauge stand OR add water using
the inner tube to the large outer tube having first noted the EXACT amount
in the inner tube being added. (Note: avoid using HOT water as a
plastic gauge may crack). Pour the combined melted snow plus any added
liquid into the funnel and smaller inner measuring tube and measure
the amount to at least the nearest 0.01 inch (0.2 mm). Subtract any
amount of water added from your measured total and record the result.
Total Snow Depth
Determine the total depth of snow, ice pellets, or ice on the
ground. This observation is taken once-a-day at the scheduled time of
observation. 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, ice pellets, and glaze ice on the ground at your usual
measurement site (not disturbed by human activities). Observers
should always make sure they are measuring just the depth of snow and
not the height of the grass. A thick turf can hold several inches of
heavy snow up in the air. Measurements from rooftops, paved areas,
and the like should not be made.
Snow Water Equivalent of Total Snow Depth
Measure the water equivalent of ALL snow on the ground...old and new.
The snow water equivalent is obtained by inverting the rain gauge can
and pushing it through the entire depth of snow on the ground that
represents the average snow depth. Make sure that snow does not fall
out of the can when you remove the snow core. It is very helpful to
have a firm, thin sheet of metal to slide between the surface of the
ground and the rim of the can to cleanly gather the entire sample of
snow. The snow is then melted and measured as described above in the
instructions for Snow Water Equivalent of Snowfall.