MEASURING SNOWFALLS
By Lowell L. Koontz
Member, ACON - VA/NC/SC
One of nature's phenomenon that is enthusiastically enjoyed by
weather observers is the rare
occasion of a record-breaking
snowstorm. Great snowstorms happen
infrequently but they create problems that
frustrate even the most experienced
observer. You may be deprived of the
satisfaction of experiencing a blizzard or a
historic snowstorm if your data appears
erratic or imprecise. The snowfall may be a
new record for the area but can you be
confident in your methods and equipment?
Great snows are the greatest challenge to
measure. Ones like the blizzard of January
6 - 7th, 1996 along the East Coast consisted of
a cold, dry snow of great depth and strong
winds. I found accurate measurements
during this storm required methods to
account for variance in temperature, wind
speed, and snow compacting.
These are some methods, accompanied by
my commentary, offered by the Metro-
politan Washington Volunteer Weather
Network (MWVWN) for measuring snow
under particular weather situations.
Gauges to use when wind is less than 10
mph
Whether temperatures are above or below
freezing, melting the amount caught by a
Tru-Check (Tm) Rain Gauge will usually give a
reasonably accurate water equivalent when
winds are light. If you have a 4.5 inches
diameter Lake or Long-Term Gauge and
sleet is expected, the outer cylinder should
be used because the lip on the funnel of this
gauge is not deep enough to prevent some
sleet from bouncing out of the funnel.
Snow depth is most accurately measured
with a snow board.
Lay a large, flat, white board across two
bricks so that about two inches of air space
separates the board from the ground (this is
important when the ground is warm).
Measuring snow depth on this board rather
than the ground eliminates measurement
difficulties caused by ground warmth and
ground cover height. Without a snow board,
I have found the next best surface to
measure snowfall is the flat top of an
automobile if there is very little wind, the
vehide has been idle long enough to reach
ambient temperature, and is parked in an
open area away from obstrucfions such as
buildings and trees. Ample judgment must be
used; e.g., the car surface should be level
and measurements taken toward the center
and not along edges, and away from a
sloping windshield. It helps if you own the
car or your observation point may
unexpectedly drive off.
In reporting snowfall MWVWN would like
to receive two figures (measured to the
nearest tenth of an inch if possible): 1) the
snow depth on the ground at the time you
measure it; 2) an the maximum new snow
depth on the ground for the particular
storm. Sometimes the second value may
have to be estimated. For example, you
leave for work in the morning and during
the late morning it begins to snow. It snows
hard for a few hours, accumulating three
inches by 2 PM. Then the snow changes to
rain and the warmer temperatures begin to
melt the snow away. By the fime you get
home from work at 5 PM, there is only one
inch left on the ground. From talking to your
family and neighbors, you estimate that 2.8
inches was on the ground at your station at 2
PM before the snow changed to rain. In this
case, you would report 2.8 inches as the
estimated maximum new snow depth and
your recorded snow depth measurement of
one inch measured at 5 PM.
Gauges to use with mixed snow and rain and
winds greater than 10 mph
By far, this is the most common type of
snowstorm affecting the Washington area.
With winds greater than 10 mph, my
experience is the Tru-Check (Tm) Gauge will
not catch all the snow that falls due to
turbulence and eddy effect. To measure a
more exact water equivalent, MWVWN
recommends using a larger mouth gauge such
as the 4.5 inches diameter Lake or Long-
Term Gauge or, even better, a 20 centimeter
diameter (about 8 inches) standard
weather service gauge. This method is
recommended as a check against other
methods, particularly when situations are
difficult.
When to use a core sample for water
equivalent
A core sample is an accurate way of
measuring water equivalent only if snow or
sleet falls with a temperature below
freezing for the entire precipitation period.
Select an area where the snow has not
drifted (your snow board may be a good
site). Thrust the open end of the outer
cylinder Long-Term Gauge into the snow
until you reach the ground (or snow board).
Then, slip a thin piece of cardboard or
sheet metal under the mouth of the gauge to
avoid spilling, turn the gauge right side up
again and melt the snow in the gauge. This
melted amount is a core sample water
equivalent. Averaging several core samples
gives a more representative water
equivalent. Compare the core sample water
equivalent with the water equivalent
determined directly from the gauge and use
the larger of the two readings or do another
core sample for comparison.
How to measure wind-driven, dry, powdery,
drifting snow
This is the most difficult to handle of all
the snow situations mentioned. It is not very
common in the Washington area but
presents the greatest challenge. With 30
mph winds, the Tru-Check (Tm) Gauge will
catch less than 75 percent of the snow that
falls. Level-One Stations also have
measurement difficulties.
Larger gauges are recommended in this
situation. Gauges also should be positioned
so snow from near-by roofs does not blow
into the gauge. A slatted wind guard around
the gauge helps to cut wind turbulence.
Pavel and Legates stated in the February
1992 issue of The American Meteorological
Society : Only about 200 US rain-collecting
gauges have shields to prevent wind from
blowing snow away from them.
A core sample may still be good if you can
locate an area where the snow has not
drifted. The snow board should be used
with caution, since strong winds will often
blow accumulated snow off the top or may
drift snow onto the boards in sheltered
areas.
If the wind is greater than 30 mph and the
snow is severely blowing, a ground
measurement where the snow has not
drifted may be your best option for
measuring snow depth. Seek out a low,
reasonably flat terrain surrounded by trees
or houses, take your observation at a
distance of at least twice the height of
surrounding objects. This area could be a
park near your weather station.
How to measure snow depths greater than
five inches
When the snow is deeper than five inches
the snowfall loses depth due to compacting
under its own weight. It appears that snows
of higher snow-to-liquid ratios (greater
than 12:1 ratio) incur more packing. These
high ratio snows are often associated with
the deeper dry snows, strong winds, and
drifting snow.
The following example illustrates the
problem. Observer M routinely takes
observations at 2400 (midnight) and
observer S routinely takes observations at
1700 (sunset). A great snowstorm begins at
1900 on January lst and one inch of snow
falls each hour for the duration of the
storm, ending at 1600 January 2nd.
Therefore, observer M should record 5.0
inches of snow at 2400 January lst and 16
inches of snow at 2400 January 2nd. Observer
S observes no snow on January lst. The next
day observer S should record 21 inches of
snow, the total accumulation. However,
observer M actually recorded 5.0 inches on
January lst and 13.5 inches on January 2nd
for a total of 18.5 inches. Observer S
actually recorded a total of 16.2 inches at
1700 January 2nd. Observer S measured 2.3
inches less snow than observer M as a result
of increased settling due to the greater snow
depth before measurement.
(Graph A)
Snow measured on a snow board every hour
and then cleared and repositioned above
the old snow for the next hour's reading is
referred to as a running total. The method
of allowing snow to reach a depth not
greater than five inches and then clearing
the snow board to start new measurements is
referred to as a five inch clearing method.
This regimen gives measurements that are
more representative of the significant
snowstorms (greater than six inches). Not
surprisingly, the greatest errors in
measurement occur during historically large
storms. Storms that set snowfall records
exhibit the greatest range of observer
measurement for snow accumulation and
serve to amplify the importance of proper
procedures. My recent observations of a
recent storm illustrate the divergence point
in the measurement methods.
On 12 January 1996 a coastal low gave a
small snow-to-water ratio because there
was not an abundance of cold air. I found
there was no departure between the running
total method and the total snow
accumulation using a measuring stick until
the depth was greater than five inches.
(Graph B)
Another interesting observation was
made during the January 6-7th storm that
relates to the compacting of snow. I noticed
the large precipitation gauge, which
measures nine inches in diameter and stands
41 inches in height, had a snow depth
greater than the surrounding snow. The
snow falling into this gauge did not compact
as much due to the friction and added
support of the surrounding cylinder walls.
The snow surface in the gauge was sloped
due to the air flow across the opening and
was measured after the storm ended on
January 8th. The highest point of the slope
measured 26 inches, the lowest point
measured 19 inches, yielding an average
depth of 22.5 inches. This amount compared
favorably with the running total method
which was 23.5 inches. Even with the
support of the cylinders walls there was
one inch of compacting!
On February the 16th we obtained 10.8
inches of snow in a storm with a very high
snow to water ratio of 21:1 and there was
only 0.1 inch of compacting to a depth of 7.2
inches. But after this depth the departure
increased at a faster rate. With high ratio
snows
(Graph C) on page back the snow reaches a greater
depth before compacting is observed
because the snow does not have the weight
to compact the snow. It is also interesting
that time becomes a factor in relation to
snowfall rate. Observe that during the 13th
and 14th hour in
(Graph C) the snow
continued but the rate of settling equaled
the snow accumulation rate. The faster the
snow falls the less time for settling to occur.
The snow of February 16th was composed of
large snow flakes that trapped large
amounts of air but fell at a faster rate than
the January 6-7th snowfall. Thus the
departure in the running total and the snow
accumulation was comparable until the
snowfall rate decreased near the end of the
storm on the 16th. Between 14 and 15 hours
when only a trace of new snow was recorded
the snow accumulation decreases by 0.4
inches from settling therefore increasing
the departure.
It would make an interesting
experiment if a similar study was
performed where ratios are equal to or
greater than 30:1 as Colorado experiences.
It is my hypothesis that under these
conditions there would even be more
settling and greater departures in the two
methods of measurement.
How to measure snow equivalents for deep
snows
Have you ever noticed that snow-to-water
equivalents decrease generally as snow
depths increase? This is a two-fold result:
1) as the coastal low gets closer it pulls
warmer Atlantic air aloft and decreases
the snow-to-water ratio; and, 2) compacting
occurs with greater snow depths. Observers
normally take the total depth of snow and
divide it by the quantity of water melted
from the snow. This is a satisfactory
method if the snowfall is not a deep snow.
For example, in the blizzard of 1996 at
midnight January 7th, I recorded 1.95 inches
of water and a ratio of 10.7:1 (running total
method, snowfall depth of 20.9 inches).
The accumulation total method to that
time yielded 16.1 inches that even with
the small amount of sleet and graupel gave
an unbelievably small ratio of 8.3:1 and did
not represent the actual ratio. I used four
different precipitation gauges at this
station to obtain the melted snowfall
totals.
US Department of Commerce, Weather
Bureau current instructions on measuring
new snow
In the 1940s the Weather Bureaus Form
1009 gave straightforward but vague
instructions for measuring new snow that
had fallen in the preceding 24 hours.
Succinctly stated, a measuring stick was
used and where drifting occurred an
average was calculated from several points
of least drift and entered.
Today the instructions are more sophisticated
Two types of snow depth are
reported: 1) the depth of new snow having
fallen since the previous scheduled time of
observation; and, 2) the total depth of new
and old snow on the ground, reported to the
nearest whole inch. Measurement is still
done with the measuring stick, but the
instructions now advise on factors effecting
sample points, particularly where drifting
has occurred. Namely, seek a flat area
away from buildings and trees, and use an
average of places where the snow is more
evenly distributed. The instructions contain
advice on measuring fresh snow fallen on
old snow, but clearly states that snow
boards are the best method with most
situations. Subsequent text is devoted to
procedures to compensate for not having a
snow board.
Comments on reporting significant storms
The early records of snowfall were made by
using a yard stick when the snow had ended
or obtained its greatest depth. To compare
current snows with historic snows, we must
continue to use that method. However, we
can better profile significant storms today
by using more refined methods. The snow
board is a cheap and simple piece of
equipment and no serious weather observer
should be without one or more. A slatted
grate around your gauge is a little more
complicated but hardly more than a trip to
the hardware store. Considering the
improvement in observed data, it is a
wonder that these devices are not more
common.
To use the running total method would be
too time consuming for most observers.
However, use of the five-inch clearing
method takes little extra time and yields
greatly improved results. Snows greater
than five inches are not that common over
most of the United States and when they
happen we usually have that extra time
due to work and school closures. Even a 15
inch predicted snow would only require two
additional observations. These observations
could be placed under the remarks
column when snow of greater depths occur.
Seeing my data document the Blizzard of
1996 by use of these simple but improved
methods gave me a fuller experience of one
of nature's rare phenomenon. I encourage all
of you to use these methods and when
significant or great snowstorms occur, report
your five inch clearing method for snow
accumulations in addition to your observed
greatest depth on ground and new
accumulation since last observation.
Copyright 1996, Barcroft Hills Weather Station