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Measuring Snowfall
Name: R. P.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 2001
Question:
I am teaching a unit on weather and I was studying up on
the material. As I was reading the textbook, I started wondering about
how snow is measured. The book stated that it is measured using a ruler,
but is this possible? When snow starts to melt and then new snow falls,
how accurate is a ruler?
Replies:
Dear R.P.
The book is correct in stating that the most common way of measuring snow
depth is with a ruler. This measurement is an indication of snow on the
ground, not how much "snow that fell." You are correct in that many times
snow does melt as it falls, or after falling, melts on the ground. So, the
"snow depth" is continually changing, as new snow falls, melts, or compacts.
Another way snow is measured, is by its "water equivalent." Snowfall is
collected in a gauge that contains antifreeze, so it all melts as it falls.
The increase in the amount of liquid in the gauge indicates the water
equivalent of the snow. This varies with different kinds of snow, and some
snows are characterized as "wet snows," and some as "dry snows." The water
equivalent of wet snows may be as low as 6 or 8 to 1. This means 6 inches of
snow equals one inch of liquid. A dry snow may have a ratio of 20 to 1, or
even as high as 50 to 1 in very cold arctic regions. This means it would
take 50 inches of snow to equal one inch of liquid. Water equivalent can
also be determined by pushing a hollow tube through the snow, and melting
the core. This is the way that water equivalent is measured in mountainous
areas. Water equivalent is important in determining snowmelt and water
runoff in the spring. Forecasts of river and stream flooding during the
spring thaw utilize water equivalent as the primary indicator of the snow
pack.
Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
R. P.,
You have asked a very valid question.
The best way to measure snowfall is with a
ruler. You should average about a dozen measurements
taken over a large area that is free from
snow drifts. However, be careful to not
penetrate the top of a layer of previous
snowfall; sometimes that old layer is crusted at the top
from thawing and re-freezing, making this easy.
To prevent the problem of penetrating previous
snowfalls while making the measurement, people
at the National Weather Service use surfaces that
they clean off after each snowfall or each hour of
snowfall. You could do this also, even in your
yard.
If the temperature is near or above freezing,
melting of the snow, even over the period of an
hour can lead to underestimated snowfalls. I know
of no way to combat this than to take measurements
over a sufficiently short period of time that the
compaction from melting is minimal.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
You are correct that measuring snow fall by "inches" is very dependent on
the density of the snow, and the freeze thaw cycle compounds the
uncertainty, and of course drifting has a big effect. The rough rule of
thumb is that the ratio of snow / water // 10 / 1.
You are correct, it is not very accurate.
Vince Calder
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Update: June 2012
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