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Posted at 10:07 AM ET, 02/17/2012

Accurately measuring snow: a critical winter weather challenge


A snowfall measurement taken about one third of the way into Snowmageddon (Kevin Ambrose)
With any prediction of a possible snowstorm, the primary focus is on “how much?” That is the bottom line of all forecasts. The path leading to this end game, as with the current situation, is quite wide (but hopefully narrowing) indicated by the variety of solutions in forecast models.

But for the sake of argument, let’s assume there will be measurable snow on the ground by Monday morning. How do we most reliably figure out “how much?”The answer to the question is not as straight forward as one might think.

Sure, one can grab a yardstick, stick it into the snow to verify accuracy of forecasts and, more likely than not, determine whether the storm was an under- or over-achiever. But it’s also likely that casual measurements are not especially accurate. And they may be biased. For example, some may artificially inflate totals to report more snow.

There are many problems and challenges in measuring snowfall. For example, amounts can vary considerably from place to place even in near calm conditions, as well as being blown and redistributed into drifts. Between measurements, snow can melt on contact with the ground, and melt and/or settle after resting on the ground, or evaporate once the snow has ended. An additional complication is distinguishing new from total snow depth if there was old snow from a previous storm

The importance of reliable measurements of snow (as opposed to general curiosity) stems, for example, from emergency managers’ strategy for mobilizing snow plows or deciding on whether to close highways and airports during major storms.

Additionally, the amounts and areal distribution of total snowfall, including equivalent water content, is critical when it comes to assessing the threat of floods from melting snow.

In the western U.S. officials require detailed assessments of snow packs to anticipate chances of avalanches and evaluate the water available from spring and summer runoff (Note: the snow pack on the Sierra Nevada Mountains accounts for one-third of the water supply for all of California).

The National Weather Service uses automated equipment (not overly reliable) often located at airports, but relies on a team of almost 15,000 volunteers across the country that makes up the Cooperative Observer Program (COOP). The observers use a standardized way of measuring the accumulation of falling snow, a standardized way of selecting sites to measure, and standardized equipment to maintain control on the information. Water content is obtained by simply obtaining and then melting a cylindrical core of the accumulated snow.

Notwithstanding the dedication of COOP observers, their efforts are often inadequate for capturing snow totals that may vary even within a single field or neighborhood.


NCAR scientist Ethan Gutmann examines a laser instrument for measuring snow. (UCAR)
Now, scientists at the National Center for Atmospheric Research (NCAR) in Boulder, CO, and other institutions are finding that prototype devices that use light pulses (lasers), satellite signals and other technologies offer the potential to almost instantly measure large areas of snow. Without going into technical details, suffice it to say that, in a little more than an hour, snowfall measurements are obtained at more than 1,000 points across an area almost the size of a football field. If experiments are successful, the instruments will reveal a continually-updated three-dimensional image of the snowpack, including density (water equivalent) and its variations in depth.

Ideally, one would like to detect falling snow and measure surface snowpack information from the vantage point of space. Admittedly, NASA acknowledges we have a long way to go for this to be possible. To assess the possibilities, a team of scientists from NASA and Environment Canada are running the large GPM Cold-season Precipitation Experiment (GCPEx) in conjunction with the Global Precipitation Measurement (GPM) satellite mission.

Having said all of that, if and when snow starts to follow this weekend, be assured I’ll be out there with my trusty yardstick...

Here are a couple websites with measuring tips:

Basic measurement tips

Detailed measurement tips (including slide show and video)

We have been informed this website provides the most current, accurate guidelines for measuring snow (the websites crossed out are not up to date):

Snow Measurement Guidelines

By  |  10:07 AM ET, 02/17/2012

Categories:  Latest, Winter Storms, Science, Tracton

 
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