Snow Measurement Guidelines
10-23-96, replaces form WS TA B-0-26; 9-79
The following procedures were developed from previous National Weather
Service procedures and input from a broad array of expertise from climatologists,
snow specialists, weather observers, and data users. Some of the materials
have been extracted from "The Snow Booklet" by Nolan J. Doesken
and Arthur Judson, CSU, 1996).
It is essential for all observers to understand the importance of
taking standard measurements in the prescribed consistent manner. Inconsistent
observing and reporting methods result in incompatible data which can
result in profoundly incorrect differences between stations and observers.
Each season before the first snows come: Review these instructions
for measuring snow. It is easy to forget what needs to be measured,
especially in those parts of the country where snow falls infrequently.
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At the beginning of each snowfall/freezing
season, remove the funnel and inner measuring tube of the eight-inch
manual rain gauge to expose the 8-inch diameter overflow can
so that it can more accurately catch frozen precipitation. |
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Put your snowboard(s) out and mark
their location with a flag or some other indicator so they can
be found after a new snowfall. They 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). |
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Check your gauge to make sure there
are no leaks. If there are leaks, take appropriate action. |
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Once your equipment has been readied
for winter you are prepared for taking snowfall measurements. |
Observers should determine three values when reporting solid precipitation.
They are:
- Measure and record the snowfall ( snow, ice pellets )
since the previous snowfall observation,
- Determine the depth of snow on the ground at the normal
observation time,
- Measure and record the water equivalent of snowfall
since the previous day's observation.
Measure and record the greatest amount of snowfall that has accumulated
on your snowboard (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,
(every 6 hours) and should reflect the greatest accumulation of new
snow observed (in inches and tenths, for example, 3.9 inches) since
the last snowfall observation.
Snowfall amounts can be measured hourly or at any interval
as long as the snow measurement board is NOT cleared more frequently
than once every 6 hours.
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 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. 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.
Freezing rain (glaze ice) should never be reported as snowfall. This
precipitation type is liquid precipitation and should be reported as
such.
- 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 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, ice pellets,
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. Note: Hail accumulation
is not entered with snow and ice pellets. Hail accumulation is entered
in the “/remarks/” section with the amount and
diameter (inches and tenths) of the stones.
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 or mountainous
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.
- 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. Melt the contents of your
gauge (by bringing it inside your home or adding a measured amount
of warm water) and then pour the liquid into the funnel and smaller
inner measuring tube and measure the amount to the nearest .01 inch
(use NWS provided measuring stick) just as you use for measuring
rainfall. Do not measure the melted precipitation directly in the
large 8-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 two 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
8-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 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, you should first empty
any existing snow from inside the 8-inch cylinder, then use it to
take a snow sample, sometimes referred to as "take
a core" or "cut a biscuit" from your snow board with
the 8-inch overflow can. Melt the biscuit of snow, pour the liquid
into the small measuring tube to measure the water equivalent.
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