How to Measure Snow

[DaculaWeather]

As part of CoCoRaHS, I get the occasional email about how to correctly measure things. This one was pretty good as it pertains to measuring snow in a blizzard... not that we're gonna see one anytime soon!

Snow Measurement tips for Blizzards and badly behaving snow.

Here in Fort Collins, we haven't had a flake of snow in over 4 weeks.

But that's not been the case for many parts of the country. Several

thousand of us have had snow to measure and some have had a LOT!.

There are times when measuring snow and its water content are fairly

easy. When the winds are light and temperatures are well below

freezing, it's pretty easy to stick the ruler in the snow to get the

snowfall amount and bring in the snow in the gauge to get the melted

water content. But throw in some sleet, some freezing rain, and a whole

lot of wind and things get really messy in a hurry. Even experienced

professionals struggle.

Here are a few tips on measuring snow under duress -- 16 tips to be

exact. Read at your leisure or as the next snow storm approaches. If

you have other questions, please ask.

1) When in doubt, follow the instructions :-) There are many

training resources on the CoCoRaHS website to help you learn how to

measure snow and its water content. Use them.

2) Don't put your body and health at risk just to try to get a good

measure of snow and it's water content. Yes, the data are important,

but so are you. So be careful. As easy as it may normally be to go out

and check your gauge, in a blizzard or an ice storm, it's a whole

different story.

3) Remember that Precipitation (water content) and Snowfall are two

separate measures. Don't enter your snowfall amount (e.g. 3.5") as your

precipitation amount. We've had a lot of those lately so we programmed

the computer so that you can't enter the precipitation amount and the

snowfall amount to be the same.

4) If you can't or prefer not to get a measure of the water content --

that's OK. For example, if you measure 12" of new snow but you didn't

melt it down to get a precipitation amount, then just enter NA for "Rain

and melted snow" and enter 12.0" for the snowfall amount.

5) If there is a lot of blowing and drifting, you may not find any

ideal place to measure the accumulation of new snowfall. Your favorite

snow measurement surface may be blown clear or may be buried by a

drift. Plan to take several measurements and compute an average.

Under extreme blizzard conditions you may need to simply provide an

"educated guess" based on reasonable measures.

6) When you compute an average, don't include the depth of the largest

drifts in your sample/

7) If there are many buildings and roofs in your neighborhood, keep in

mind that the snow often blows off the roofs and collects below. As a

result, we sometimes see measurements that are likely too high from

densely populated neighborhoods. Keep that in mind as you measure.

8) When there is considerable wind, your gauge will likely catch only a

fraction of the snow that fell. What you find in your gauge may be very

unrepresentative. For example, there were places on the East Coast on

Monday who had snowfall amounts up around 15-20" but who had

precipitation measurements less than 0.75". This is certainly possible

for dry snows with lighter winds, but wind-driven blizzard snows are

usually compact and dense and will likely have between 0.07" and 0.12"

for each 1.0" of new snow.

9) If you suspect undercatch (less snow landing in your gauge than what

actually fell) then take a representative core sample of the new snow on

the ground.. Take a core at a location where the snow is an average depth.

10) Your outer cylinder is 12" tall (unless you are an official NWS

Cooperative Observer, in which case your gauge is 24 inches tall) You

would think that a 12" gauge would be able to handle a 12" snow but it

usually can't. The gauge begins to fill to the top and begin spilling

after only about 6" of new snow has fallen.

11) The rain gauge outer cylinder is for collecting snow to measure its

water content. Do not measure the new snowfall in the outer cylinder.

Measure new snowfall on the ground at one or more locations.

12) It is really, really handy to have an extra outer cylinder for

helping with snow measurements. You can order an extra cylinder by

itself for a reasonable cost from www.WeatherYourWay.com and possibly

from other companies, too. Also check with your state or regional

coordinator in case there are any spares available.

13) Before submitting your daily report, always do a "reasonableness

check". Does each measurement make sense and do the set of readings

match up? For example if you measured 10" of new snow and 2.05" of

water content is that reasonable? It might be if the snow was extremely

wet or if there was a lot of rain mixed in with the snow. If it was only

snow with a fairly typical density, then this report would be

unreasonable -- either the snowfall is too low or the water content is

too high. While snow rarely has an exact ten to one ratio of snow to

water, that may be a good first guess for a wet or wind compacted snow.

14) (similar to 8) Always check your snow- to-water-content ratio..

You can tell a lot about snow just by walking through it, shoveling it,

squeezing it, etc. You may be able to make a good estimate of the water

content just by lifting a shovelful of snow. Five inches of wet snow

may have 0.50 to 0.60" of water content and it will feel very heavy when

you lift it. But a dry, low density snow of 5" may weigh very little

and may only contain 0.15 - 0.30" of water content or even less.

15) Report your total depth of snow on ground. That is a separate

measure from your daily snowfall. Report total depth to the nearest

0.5" whenever there is any snow on the ground even if no precipitation

fell today or in recent days. Knowing how much snow remains on the

ground and watching it settle from day to day is very helpful

information for many applications.

16) Snow depth decreases quite rapidly after recent large snowfalls.

(this is called "settling", "compaction" or "densification". Find a

favorite representative location and measure in that same area each day

if the snow is no longer blowing and drifting An average of several

measurements may be necessary to get a good reading of the total depth

of snow on ground. Watch the snow settle. And measure the water

content (Snow Water Equivalent) too. It is fascinating to see that as

snow depth goes down, its water content may or may not go down.

OK, that' s more than enough about snow for one evening. I can't wait

until we start doing CoCoRaHS training "webinars" next year. It will be

easier to demonstrate than to describe in words.