Eastern Snow Density, Snowfall Measuring Methods

Resort and backcountry skiing and snowboarding in eastern US and Canada, including our famous reader-submitted No-Bull Snow Reports.

Eastern Snow Density, Snowfall Measuring Methods

Postby Tony Crocker » Fri May 21, 2010 6:34 pm

As we have discussed before, simple measurement of snow water content (total snow divided by total water) in the East are contaminated by rain events. It is necessary to utilize daily records to determine incidence of snow density. Fred Lavenberg has provided me with at least 25 years of records from the Mansfield Stake, for which I have used December-March data. I have compared these to November-April data from 22 years of Mammoth ski patrol and also the last 6 years posted online from Alta.

SnowDensity.JPG
SnowDensity.JPG (51.75 KiB) Viewed 16323 times


The data indicates that Northern Vermont snow density is by far the most volatile. The most frequent events have snow density lower than the coastal but higher than the intermountain regions of the West. The super light snow events of <5% density are at the same frequency as Alta (7%). But as we know on the other end 14% of December-March precipitation is of the pure liquid variety, with another 10% mixed.
http://bestsnow.net
Ski Records
Season length: 21 months, Nov. 29, 2010 - July 2, 2012
Days in one year: 80 from Nov. 29, 2010 - Nov. 17, 2011
Season vertical: 1,610K in 2016-17
Season powder: 291K in 2011-12
User avatar
Tony Crocker
 
Posts: 10442
Joined: Thu Sep 23, 2004 10:37 am
Location: Avatar: Charlotte Bay, Antarctica 2011
Location: Glendale, California

Re: Eastern Snow Density

Postby J.Spin » Sat May 22, 2010 9:28 pm

That’s a really informative analysis Tony. It’s far more useful than simply total snow divided by total water, and it’s actually a necessity for ski locations that receive any sort of liquid precipitation if one wants to get a sense for typical snow densities.

I do have one very large concern about the validity of the Mt. Mansfield data though. If the snowfall numbers used to calculate snow density are derived from the evening snow accumulations that get reported from the stake, then the Mt. Mansfield data are not likely to be comparable to the other two locations, nor are they a very good representation of what skiers are going to encounter on the slopes at the start of the ski day. If the analysis uses the stake snowfall numbers, and those numbers accumulate to the 230 annual inches you have posted on your website, then we know that the snowfall used in the analysis is a far cry from reality. This type of calculation would inflate the resulting water content numbers substantially. Using Stowe’s annual snowfall number of 333 inches, and assuming that the 10% H2O peak number shown in your data is the mean of a roughly normal distribution, that would center Stowe’s distribution at ~6.9% H2O, or presumably the 7% H2O bracket in your table. That value actually makes much more sense than the current peak at 10% H2O.

To add some practical weather perspective on the issue, the rule of thumb used by forecasters to estimate snow totals from synoptic storm precipitation around here in the valleys is 10 or 12 inches of snow to 1 inch of water. That means average snow in the 8.3% H2O to 10.0% H2O range. That’s for the valleys, and as far as I know, that’s the rule of thumb they even use down on the coastal plain in southern New England. So, if 8.3% H2O to 10.0% H2O is the average snow density for those types of locations, even they (assuming a relatively normal distribution) would have a peak of frequency at, or assuming an average of 9% H2O snow, below the peak of frequency shown on your table for Mt. Mansfield. Since most valley locations, especially those well away from mountains, lack both the colder temperatures of the mountains, and the mesoscale upslope/orographic events and upslope/orographic portions of storm cycles that often produce extremely dry snow, then the average snow density in places such as the higher elevations of the Northern Greens should be much lower than it is for the typical valley locations in New England.

I’ll provide two additional arguments to suggest that the actual average snowfall density for the snow falling on the upper elevations of Mt. Mansfield (especially when the analysis is focused on just December through March) is even lower than the ~7% H2O number calculated above.

1) While we know that the annual snowfall number that results from summing the snow totals from the Mansfield stake is depressed due to the collection method, the snowfall deficit is likely not depressed equivalently across all storms. The 100+-inch deficit in snowfall is most likely derived disproportionally from the storms with the driest snow, the upslope/lake-effect style snow that settles very quickly after falling. Denser snow (such as we’ve seen in some of the storms we’ve had this spring) does a lot less settling during the day than fluffier snow. With the snowfall deficit being derived disproportionately from the driest snow, most of the correction in snowfall numbers must be made there, resulting in most of the drop in snow density being obtained there. That is going to push the mean/mode for snow density further downward.

2) This second modification will depend on how the 230-inch annual snowfall number that you have on your site was derived, but if it is for the entirety of November through April, then the differential between the stake numbers and the actual snowfall is going to be a bit more than 103 inches. This is because Stowe stops recording snowfall when they close around the middle of April. The additional snow contributing to the increased differential is probably not going to be super dry most of the time since it is the second half of April, so the differential between what was recorded at the stake and what would have been recorded by the resort is not going to be as huge, but if anything it will push the snow density number still lower.

With regard to correction number 1 above, I guess the perfect way to do it would be to devise an equation that would gradually factor in the 100+-inch loss of snowfall due to settling in a linear (or perhaps non linear if that is how settling behaves) manner with respect to snow density. I’m not sure if there are any empirical data on the rate of snow settling with respect to density, so barring that, one could simply choose a cutoff value, a point at which snow is dense enough that it is assumed to settle minimally after falling, and distribute the 100 inches of missing Mt. Mansfield snowfall proportionately to all the categories in the table below that value. In people’s experience, what density of snow marks the point at which settling becomes negligible? I’m going to throw out a value of ~12% H2O based on experience, but I’d be interested in hearing what other people think because it’s not something I think about too often in my snow analyses.

So, based on the above analyses, in Tony’s table format I would put the peak for Mt. Mansfield’s snow somewhere below 7% H2O, presumably in the 6% H2O to 7% H2O range with the additional two corrections made above, although it’s hard to say just how dramatic correction #1 will be. Looking at where the peak of Alta’s snowfall density is located in the table, I would say that Mt. Mansfield’s peak would be in a similar location, but probably a bit denser. I’d also argue that if in the initial analysis Mt. Mansfield and Alta were running around the same frequency for super light snow events of <5% density, then Mt. Mansfield actually receives a higher frequency of such events compared to Alta with the refined analysis, especially since those super light snow data should be corrected to the greatest degree in the Mt. Mansfield analysis. That would fit in well with the volatility aspect that Tony Mentioned for Mt. Mansfield’s distribution of snow density.

With the greater snow density, I suspect it’s not quite as critical at Mammoth as it is on Mt. Mansfield and at Alta, but how are the snowfall data at the ski areas collected? Are 24-hour snowfall totals obtained from a single snow and liquid collection in the morning, or are they summed from two (or three of four) collections during the day? Obviously the analysis will be best (apples to apples) if the three ski areas collect their snowfall data in the same manner, because there can be substantial settling effects with regard to the drier snow. I’m sure we can find out from Powderfreak exactly how Stowe’s data are collected.

-J
User avatar
J.Spin
 
Posts: 918
Joined: Wed Sep 22, 2004 9:54 pm
Location: Waterbury, Vermont - Image: Skiing Bone Mountain in the Bolton Valley backcountry, VT

Re: Eastern Snow Density

Postby Tony Crocker » Sun May 23, 2010 2:28 am

I should have reminded readers (as I now do on my website http://bestsnow.net/eastnet.htm) that the Mansfield Stake data is collected at 4PM vs. the very early morning nearly everywhere else. Given my estimate of Jay at 336, I would guess that the comparable number at Stowe is close to but unlikely any more than 300. But JSpin's point is well taken that the 4PM measurements would overstate density relative to areas that measure in the morning. I'm not sure if I agree about a big density difference in density between valley and mountain snow. Once you take the rain factor out I think it boils down to mainly a quantity of precipitation difference from orographic uplift. This is perhaps my western bias. Mammoth's altitude confers many advantages over Tahoe and the PNW, but snow density is not one of them. The 1979-1995 Westwide Network data showed Alpine Meadows and Stevens Pass having similar 13% water content numbers as Mammoth.

Here's the chart again with Mansfield daily densities multiplied by 80%
SnowDensity8.JPG
SnowDensity8.JPG (49.56 KiB) Viewed 16290 times

While this may be overall more accurate, I suspect the "4PM measuring bias" may affect some storms/densities more than others.
http://bestsnow.net
Ski Records
Season length: 21 months, Nov. 29, 2010 - July 2, 2012
Days in one year: 80 from Nov. 29, 2010 - Nov. 17, 2011
Season vertical: 1,610K in 2016-17
Season powder: 291K in 2011-12
User avatar
Tony Crocker
 
Posts: 10442
Joined: Thu Sep 23, 2004 10:37 am
Location: Avatar: Charlotte Bay, Antarctica 2011
Location: Glendale, California

Re: Eastern Snow Density

Postby Harvey44 » Sun May 23, 2010 5:42 am

Guys... I'll be honest I don't totally get all this, but it is very cool.

Here's a dumb question. Density is higher (more settling?) at 4PM because .... most snow falls overnight? The impact of the sun?

Also are you quoting an average density for each area? If so how is that figured?
NYSkiBlog.com
User avatar
Harvey44
 
Posts: 1265
Joined: Tue Dec 20, 2005 3:37 pm
Location: North River, NY
Location: North River, NY

Re: Eastern Snow Density

Postby J.Spin » Sun May 23, 2010 8:17 am

Harvey44 wrote:Here's a dumb question. Density is higher (more settling?) at 4PM because .... most snow falls overnight? The impact of the sun?

It's not a dumb question, it might be the core of what makes the snowfall numbers obtained from the Mt. Mansfield stake so low (just 230 inches of snowfall are reported at the stake annually according to Tony's website, when Stowe itself reports 333 inches at a similar elevation, Jay Peak to the north reports 355 inches, Bolton to the south reports 312 inches from a lower elevation, and even Smuggler's Notch to the northeast/east reports 288 inches). I don't believe there's any real relationship around here with regard to snow falling overnight vs. during the day, the low snowfall accumulations from the stake are thought to arise from the fact that the snow sits there during the warmest part of each day before it gets measured, vs. being measured in the early morning before whatever sun/warming hits. The only other thing I can think of that would produce the low numbers (vs. other areas in which snow is collected at 24-hour intervals) would be if the accumulations are simply taken as the difference observed in the snowpack instead of the accumulation on a clean, hard, non-compressible surface like a board. Unless there is a very solid crust on the snow, measuring accumulations as simply the increase in snowpack each day would also result in low numbers because the underlying snow is compressed as the new snow falls on top of it. Looking at the Mt. Mansfield data from each storm, it sometimes seems as though that could be the case, but at other times it doesn't. I wonder if Powderfreak has ever watched how the stake guys do their analyses. I guess there's always the chance that snowfall for the stake report is just collected in a spot that is somehow sheltered and doesn't receive much snow.

-J
User avatar
J.Spin
 
Posts: 918
Joined: Wed Sep 22, 2004 9:54 pm
Location: Waterbury, Vermont - Image: Skiing Bone Mountain in the Bolton Valley backcountry, VT

Re: Eastern Snow Density

Postby Tony Crocker » Sun May 23, 2010 6:04 pm

JSpin wrote:the low snowfall accumulations from the stake are thought to arise from the fact that the snow sits there during the warmest part of each day before it gets measured, vs. being measured in the early morning before whatever sun/warming hits.

That has been my assumption. Yes, it would be good for powderfreak to get up there and find out some details and whether our assumptions are correct.

Harvey44 wrote:Also are you quoting an average density for each area? If so how is that figured?

That is usually done by taking total water divided by total snow using seasonal or monthly data, as I did for my original 1995 Powder magazine feature: http://bestsnow.net/snoqlnet.htm. Rain is so pervasive in eastern winter monthly weather data that in order to get at the question of density of just snow you have to analyze daily data.

I have never asked for daily data but occasionally it has been provided to me. I just stored it even though I had no immediate use for it. 3 years ago I realized the daily data would be useful to get at the frequency of powder days. The eastern density issue intrigued me after seeing much pictorial evidence from powderfreak, JSpin etc. that some of that snow is pretty light and dry. As with many statistical questions, this is one where averages provide inadequate information and you need to see the whole distribution to understand what's happening.
http://bestsnow.net
Ski Records
Season length: 21 months, Nov. 29, 2010 - July 2, 2012
Days in one year: 80 from Nov. 29, 2010 - Nov. 17, 2011
Season vertical: 1,610K in 2016-17
Season powder: 291K in 2011-12
User avatar
Tony Crocker
 
Posts: 10442
Joined: Thu Sep 23, 2004 10:37 am
Location: Avatar: Charlotte Bay, Antarctica 2011
Location: Glendale, California

Re: Eastern Snow Density

Postby Harvey44 » Sun May 23, 2010 7:58 pm

What Jspin said makes TOTAL sense to me. The snow stake is an annual total.

The snow keeps piling up ON TOP of other snow, not a board like the one J has in his yard.

Which I assume you sweep off every time you measure.

So unless it's boilerplate - you get (Seinfeldian?) SHRINKAGE. :-D

Does it matter that the guys measuring the snow stake probably aren't in marketing?

EDIT: Jspin I tried to feed your blog on Harvey Road, but it doesn't seem to work. The message I got was that the feed was not enabled?
NYSkiBlog.com
User avatar
Harvey44
 
Posts: 1265
Joined: Tue Dec 20, 2005 3:37 pm
Location: North River, NY
Location: North River, NY

Re: Eastern Snow Density

Postby J.Spin » Mon May 24, 2010 6:22 am

Tony Crocker wrote:
JSpin wrote:the low snowfall accumulations from the stake are thought to arise from the fact that the snow sits there during the warmest part of each day before it gets measured, vs. being measured in the early morning before whatever sun/warming hits.

That has been my assumption. Yes, it would be good for powderfreak to get up there and find out some details and whether our assumptions are correct.

I sent out an email to Powderfreak yesterday to let him know about this thread, but delivery failed, and that's the only email I've ever used in our communications. So I sent him a private message through EasternUSwx.com, and I'm hoping that will get through. I'm sure he'll be very interested in Tony's analysis, and although I believe Powderfreak is usually on the mountain in the morning, while the stake guys are on the mountain in the afternoon/evening, he may have some insight into their methods.

-J
User avatar
J.Spin
 
Posts: 918
Joined: Wed Sep 22, 2004 9:54 pm
Location: Waterbury, Vermont - Image: Skiing Bone Mountain in the Bolton Valley backcountry, VT

Re: Eastern Snow Density

Postby Tony Crocker » Mon May 24, 2010 12:28 pm

I just sent an e-mail to my source at the Mansfield Stake, referring to this thread and following up on the questions raised here. I knew JSpin would be all over this. :lol: That's what peer review is all about.
http://bestsnow.net
Ski Records
Season length: 21 months, Nov. 29, 2010 - July 2, 2012
Days in one year: 80 from Nov. 29, 2010 - Nov. 17, 2011
Season vertical: 1,610K in 2016-17
Season powder: 291K in 2011-12
User avatar
Tony Crocker
 
Posts: 10442
Joined: Thu Sep 23, 2004 10:37 am
Location: Avatar: Charlotte Bay, Antarctica 2011
Location: Glendale, California

Re: Eastern Snow Density

Postby bobbutts » Tue May 25, 2010 8:20 am

Cool chart.

After the snow falls it seems to behave differently too.
One thing I noticed when living in Tahoe is how the snow would maintain a soft creamy consistency even following multiple freeze-thaws. In NE the same weather seems to produce hardpack.

How much does atmospheric pressure affect the snowpack?
How much does humidity affect the snowpack?
Is the lower pressure and humidity at higher elevations a significant factor in the softer western conditions?
bobbutts
 
Posts: 3
Joined: Wed Feb 21, 2007 6:52 am
Location: NH

Re: Eastern Snow Density

Postby J.Spin » Tue May 25, 2010 11:07 am

Harvey44 wrote:What Jspin said makes TOTAL sense to me. The snow stake is an annual total.

The snow keeps piling up ON TOP of other snow, not a board like the one J has in his yard.

Which I assume you sweep off every time you measure.

I actually don’t know that accumulations of new snow at the stake are reported on top of the old snow, it’s just one theory to explain why the numbers might come out so low. I’ve seen quotes from Powderfreak on how even getting just a double digit (as in 10 inches) accumulation of snow at the stake is pretty infrequent. To think of the number of times during the winter season that Mt. Mansfield/Stowe gets more than 10 inches of snow during the span of an entire 24 hours, rarely getting double digit snow accumulations at the stake is very strange. Perhaps it is all settling of the snow that’s doing it, but at times it almost seems like there has to be more to it than that.

Measuring snow accumulations off of a hard surface, and starting fresh by clearing the accumulated snow after the measurement is an important concept in snow collection. One wants the accumulations to be on a level playing field so to speak, without influence from the compressibility of the snowpack below (which can vary a lot).

Snowboard clearings are typically done every 6, 12, or 24 hours, depending on how the setup is monitored. I try to measure the snow on my board and then clear it every 6 hours, but most of the time my intervals end up being 12 hours (6:00 A.M. and 6:00 P.M. clearings) because the other potential clearing times fall during the middle of the ski/work day and the middle of the night. If I’m around and awake though, I’ll try to hit those intermediate clearings, especially if there’s a big storm in progress. Some information about my setup and procedures is on my Waterbury snowfall page.

One other note on clearing and measuring off of snowboards with regard to your comment above – you can actually measure the snow whenever you want, it’s only the clearing that needs to be done at certain intervals. Typically I measure just before I clear the board, but below is an example of why one may want to measure at a different time or multiple times. The goal according to the NWS snow measurement guidelines, is to report the greatest, or total, accumulation since the last observation/clearing, and I’ve pasted in some of their text with an example below:

“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".”

So while 90% of the time I simply measure the snow on the snowboard, and then clear it, if I happen to be around during the middle of a collection cycle and see the way that things are melting, I’ll do my best to record that highest accumulation before it melts. During most of the season that’s not an issue, since it’s cold enough that there isn’t any melting, but a great example of the process is from the Mother’s Day storm a couple weeks back. I woke up and found 0.2 inches of snow on the board, cleared it at 6:00 A.M., and then another 0.2 inches fell. Not long after that, the warming of the day melted those 0.2 inches. But then, much heavier snowfall came in and beat out the warming of the day, leaving an additional 0.6 inches on the board (all before clearing). The end result was 1.0 inches of snow accumulation from that event at that location, even though we never had 1.0 inches of snow accumulated at any one point.

Harvey44 wrote:Does it matter that the guys measuring the snow stake probably aren't in marketing?

It might, in that they are presumably impartial. Personally though, now that I actually partake in observations myself and appreciate how much snow falls (especially compared to how much the casual observer thinks actually falls) combined with the fact that observations from Bolton, Stowe, Smuggler’s Notch, and Jay Peak, taken by folks like Powderfreak, nicely corroborate each other with a beautiful trend of snowfall increasing to the north, and falling off a bit to the east as topography would suggest, I find it hard to believe that everyone could somehow be in cahoots to inflate snow totals. If we can accumulate over 200 inches of snow at our house as we did in the winter of 2007-2008, then there’s little doubt that the mountains can do far better than that.

Harvey44 wrote:EDIT: Jspin I tried to feed your blog on Harvey Road, but it doesn't seem to work. The message I got was that the feed was not enabled?

I actually wasn’t aware that I had a blog, but fill me in and I’d be happy to try setting one up. I would assume that it’s possible to set one up at my web page?

-J
User avatar
J.Spin
 
Posts: 918
Joined: Wed Sep 22, 2004 9:54 pm
Location: Waterbury, Vermont - Image: Skiing Bone Mountain in the Bolton Valley backcountry, VT

Re: Eastern Snow Density

Postby Tony Crocker » Tue May 25, 2010 12:25 pm

JSpin wrote:Measuring snow accumulations off of a hard surface, and starting fresh by clearing the accumulated snow after the measurement is an important concept in snow collection. One wants the accumulations to be on a level playing field so to speak, without influence from the compressibility of the snowpack below (which can vary a lot).

The Mansfield Stake snowfall is described as being measured from a "bucket." So I think it's OK in terms of being off a hard surface. Whether a bucket would produce different measurements from a board is is a different question, could depend upon size and material composition of the bucket and wind effect.

JSpin wrote:Snowboard clearings are typically done every 6, 12, or 24 hours, depending on how the setup is monitored....... The goal according to the NWS snow measurement guidelines, is to report the greatest, or total, accumulation since the last observation/clearing

In my conversations with Mt. Baker after NOAA certified their 1998-99 snowfall record I was told that NOAA accepts snowfall counts that are measured at least once but not more than twice per day. We do know the Mansfield Stake is measured only once a day, so that is a source of understatement but again we don't know by how much.

JSpin wrote:observations from Bolton, Stowe, Smuggler’s Notch, and Jay Peak, taken by folks like Powderfreak, nicely corroborate each other with a beautiful trend of snowfall increasing to the north, and falling off a bit to the east as topography would suggest

I'm inclined to agree with JSpin's comment about consistency among the ski areas, aside from my adjustment for Jay to be consistent with historical data. We know the Mansfield Stake is impartial and internally consistent over a very long time frame, but its measuring method is not the same as most ski areas, East or West.

bobbuts wrote:How much does atmospheric pressure affect the snowpack?
How much does humidity affect the snowpack?
Is the lower pressure and humidity at higher elevations a significant factor in the softer western conditions?

Not sure about the first question, but the answer to the 2nd and 3rd questions is yes, a lot IMHO. We've all seen these reports, most recently from admin last weekend, about undisturbed new snow in the West "drying out." I've even observed rain crust (Island Lake Snowcat near Fernie, 2003) break up and dry out over about a 300 vertical foot range over the course of 3 days. Then there is preservation of dry winter snow through sustained drought, and on north facing steeps through warm weather. As most of you know California precipitation is volatile, yet I've seen winter snow conditions on nearly half the mountain at Mammoth after an entire month with no new snow. And I've seen that same winter snow hold up on Snowbird's steeps in March when it's been warm enough to ski in a T-shirt. By contrast my 3 March days in northern Vermont hit the first warm-up after 2+ months of below freezing weather. On day 1 ~50F at Stowe low angle groomers in the sun (Perry Merrill, etc.) were already sticky by midday. Day 2 ~60F at Mad River was similar to a typical late May to June day at Mammoth in snow surface, somewhat lucky as that's how I like it for mogul skiing. And when the temps went back into the 30's on day 3 at Jay naturally most of the snow turned to boilerplate.

The altitude/humidity are the likely reasons for East vs. West snow preservation, because as noted above the Vermont snow is actually lower in water content than the Sierra's when it's fresh. At some eastern areas you can argue that a lot of the snow being manmade contributes to the hardpack. But my recent experience at Sun Valley suggests that even manmade snow can preserve well with favorable altitude/humidity.
http://bestsnow.net
Ski Records
Season length: 21 months, Nov. 29, 2010 - July 2, 2012
Days in one year: 80 from Nov. 29, 2010 - Nov. 17, 2011
Season vertical: 1,610K in 2016-17
Season powder: 291K in 2011-12
User avatar
Tony Crocker
 
Posts: 10442
Joined: Thu Sep 23, 2004 10:37 am
Location: Avatar: Charlotte Bay, Antarctica 2011
Location: Glendale, California

Re: Eastern Snow Density

Postby bobbutts » Wed May 26, 2010 12:01 pm

I found this chart
http://wiki.xtronics.com/index.php/Pres ... sion_Table
that says air pressure at
820' is 14.26 psi
4101' is 12.64 psi
8202' is 10.83 psi
11483' is 9.54 psi

So there's a significant difference even within the elevation range at a single resort with enough vertical.. how much that affects the snowpack, I have no clue. Since so many other factors change with elevation in the real world it would seem hard to isolate just the effects of pressure.
bobbutts
 
Posts: 3
Joined: Wed Feb 21, 2007 6:52 am
Location: NH

Re: Eastern Snow Density

Postby Tony Crocker » Wed May 26, 2010 3:45 pm

I think it's mostly humidity. I do notice at Mammoth in spring that wind results in more sublimation of snow into the air and less melting and refreezing on the surface. Lower air pressure could also cause more sublimation, but I wouldn't know how important a factor that might be.
http://bestsnow.net
Ski Records
Season length: 21 months, Nov. 29, 2010 - July 2, 2012
Days in one year: 80 from Nov. 29, 2010 - Nov. 17, 2011
Season vertical: 1,610K in 2016-17
Season powder: 291K in 2011-12
User avatar
Tony Crocker
 
Posts: 10442
Joined: Thu Sep 23, 2004 10:37 am
Location: Avatar: Charlotte Bay, Antarctica 2011
Location: Glendale, California

Re: Eastern Snow Density

Postby J.Spin » Thu Jun 17, 2010 8:25 am

Tony Crocker wrote:The Mansfield Stake snowfall is described as being measured from a "bucket." So I think it's OK in terms of being off a hard surface. Whether a bucket would produce different measurements from a board is a different question, could depend upon size and material composition of the bucket and wind effect.

OK, that clears things up a lot; at least we now know that measurements are taken off a hard surface so that snowpack compressibility is not an issue. However, your concern about potential differences between snow collection on a board, and snow collection in a bucket is warranted. I have firsthand experience with that issue through simultaneous collection of snow on snowboards and in a rain gauge (bucket). I do not have the exact same type of gauge/bucket that is used on Mt. Mansfield, which, according to what Fred from the Mansfield stake crew said in the email you sent me, is that standard NOAA issue stainless steel unit, pictured below:

Image

Those gauges are 24 inches high as Fred indicated, and have a top opening that is 8 inches in diameter. Unfortunately, in terms of direct comparison, my rain gauge is not that type, it is the standard 4-inch rain gauge used by CoCoRaHS (although also by NOAA etc.). Both gauges are described in a document from NOAA. If you look at that document, you’ll see my type of gauge referred to as an 11-inch rain gauge, which I believe refers to the height of the gauge, not the diameter of the opening. While the opening on my gauge is a bit smaller than those NOAA ones, so not a perfect comparison to how theirs might perform, once I installed it and the snow started to fly, it was quickly evident that I was not going to be using it to get accurate measurements of snowfall. I have observed at least a few issues with trying to measure snowfall in a rain gauge:

1) The first issue is the effect of wind reducing the amount of precipitation entering the gauge. If the precipitation is falling straight down, one would expect it to fall in the gauge and accumulate just as it would on other flat surfaces (barring effects of the rim of the gauge, which I will describe in issue #3 below). Now imagine some wind, and the precipitation coming in at a 45-degree angle as it falls to the opening of the gauge. This precipitation that comes in at that angle is still technically falling on the same amount of surface area created by the circular opening at the top of the gauge, so one would expect to get the same amount of precipitation into the gauge. In practice, I don’t think this is quite the case for snow due to effects of the edge of the gauge, but again I’ll talk about that in #3 below. Take the angle of the precipitation to the extreme however, and imagine a very strong wind in which the precipitation is essentially passing horizontally past the gauge; no precipitation is going to enter the gauge in that situation. There is obviously going to be a continuum between how much precipitation falls into the gauge when the precipitation falls perfectly vertically (hopefully all of it) and perfectly horizontally (essentially none of it) with an increasing loss of captured precipitation skewed toward those angles closer to horizontal. So, the stronger the wind is, the more precipitation that is going to be missed by the gauge. Now let’s focus on snow, because that seems to be where the major trouble lies in terms of wind. In terms of collecting rain, which is quite dense compared to most snow, it’s going to take quite a bit of wind to have that rain falling nearly horizontally (think hurricane etc.). But with snow, especially the dry upslope snow that we often get in parts of Northern Vermont, it doesn’t actually take much of a breeze to get that snow falling at a substantial angle. Thus, even a little wind is going to mess with the ability of one of these gauges to collect snowfall accurately, and the amount of snow caught is going to be reduced. Obviously NOAA is aware of the issues with wind and attempting to catch snowfall in these types of gauges, as you’ll see in the linked document above. They talk about it more on pages 2 and 3, but already on page 1 they state “Wind shields can be used in areas where snow constitutes 20 percent of the total catch.” The bottom line is that the more wind there is, the more precipitation that is likely to be missed with a rain gauge, and it is an especially big problem if the precipitation that you are trying to catch is snow.

2) The second issue I’ve seen is related to the first. With wind, not only is the accumulation of the snow going to be reduced, but as you’ll see in one of the images I have below, if snow is falling into the gauge under the influence of wind, the snow is not likely to accumulate evenly within the gauge, and that’s going to provide inaccurate readings of snowfall. Now, depending on how one tries to measure that potentially slanted/inconsistent accumulation of snow in the gauge, it could mean reporting more or less snow than what actually fell, but it’s certainly going to produce error.

3) Relative to my gauge, on which the walls of the cylinder are probably ¼” thick and flat on top, this third issue is probably less of a factor in the metal gauges, because the walls are thin. However, it is still a cause for concern in any collection unit with walls. The issue here is snow collecting atop the wall of the cylinder. This may seem totally insignificant, but strange things happen when you deal with upslope snow. I have seen my gauge clog after just a few inches of fluffy snow have fallen. All that is required to create an issue is for a few dendrites land on the rim of the cylinder. Then, they start grabbing onto their friends that are falling in close proximity, including those trying to fall into the cylinder near its edge. The accumulation of snowflakes on the rim of the gauge grows upward, outward, and inward, and due to the ones growing inward, before you know it the whole thing can be clogged and snow is not getting into the cylinder at all. I’d argue that NOAA may be aware of this phenomenon as well, as on page 4 of that linked document they state, “The collector’s upper edge must remain “sharp” to get an accurate catch and excess paint may round this edge.” As I mentioned above, this may not be a very big issue with the big, 8” metal gauges because they have a thin wall, but it is definitely something to watch out for if one is trying to measure snow accumulations inside containers. When it comes to dry, upslope snowfall composed of certain flakes, I’ve literally seen the snow accumulate sideways (not due to the wind, but simply due to something seeding snow accumulation and the dendrites attaching to each other). A couple of sample images are shown below. The first image is an example from the snowboard, showing how the snow is actually accumulating out into space beyond the edges of the board. For perspective on how far outward that snow accumulation grew, the depth of snow on the board is 4.3 inches, so I’d say the snow grew outward a couple of inches. Unfortunately I wasn’t calculating snow densities at that point, but just from looking at the picture you can tell that it’s very fluffy snow. The second image below is from a small metal pole that I had out for measuring snow depth. The top of the pole is a flat surface probably 1 cm or less in diameter, and all it took was a tiny bit of accumulation to seed the growth of that cotton ball on the side, and that fluffy ball of snow is a couple of inches in diameter. If that 1 cm surface of a metal rod can form an accumulation like that from just 4.3 inches of snow, imagine what could happen over the entire surface of a rain gauge. Unfortunately I don’t think I have a web picture of my gauge closed off with snow, but I’ve seen it happen to varying degrees on more than one occasion. I think it’s easy to see how it could happen, and with the dendrites closing in from all sides, it happens quite quickly. Additional pictures and details from that snowfall can be found in my report from April 5th, 2007.

Image

Image

4) My fourth point is simply another one of accuracy. It’s much harder to get an accurate measurement of the fallen snow if you have to reach down into one of these opaque-walled gauges with a ruler, and then try to look down on the ruler at an angle from above. Since the type of gauge I have is transparent, that actually makes it more practical, but one of these opaque gauges is certainly going to be less accurate than looking direction across at your ruler on an open, elevated surface like a snowboard. That’s why an elevated snowboard is so nice, although if you’re willing to bend over and stick your head right down to snow level, you can also get a good reading off of a ground-based snowboard. With a snowboard I can easily measure new snow accumulations to tenths of an inch, which is presumably more difficult from inside an opaque gauge. I could see reaching into the gauge and marking with your hand the depth of the snow on the ruler, then pulling the ruler out to read it, although one would still have to worry about disrupting delicate accumulations of snow to get the most accurate reading. The stake measurements of snow are done only to the nearest inch as far as I know, so this may not be a huge issue. An example of just how delicate these accumulations can be, is shown below in an image I took while measuring off my snowboard:

Image

Now I’ll show a comparison of how snow accumulated in my gauge and on my snowboard during the same snowfall. This example occurred during the 2008-2009 snowfall season, in a snowfall event that took place from December 19th through December 21st, 2008. The weather details can be found in the link above for that snowfall event, and for those that want to see all the pictures of the snow and the resulting skiing, those are found in the December 20th, 2008 skiing trip report. On Friday evening, December 19th, I measured the first accumulation from that storm, which turned out to be 7.9 inches of 5.1% H2O snow. I happened to post pictures of the way the snow had accumulated both on the snowboard and in my rain gauge, and it’s a perfect example of how superior the snowboard is to the gauge in terms of accurate snow measurement. In the first image below, we see how the snow accumulated on the board in a fairly square shape. As Powderfreak says, when the snow stacks straight up like that (instead of forming a trapezoid like sand, dirt, rocks, and various other substances would), it’s a sign that you’ve got nice dendrites and “dry”, high-quality powder. Many types of smaller flakes, plates, needles, granules, graupel, sleet, etc. will accumulate in various trapezoidal formations like the image I linked above, but to really get snow that stacks straight up you need those fluffy dendrites as shown below in the picture of the snowboard from that evening:

Image

As I’ve mentioned before, our house is in a very sheltered location that doesn’t get much wind, and that’s important for accumulating fluffy snow. If wind is present, accumulations can be blown away, or flakes can potentially even get smashed into other objects and broken up as they are falling. The accumulation shown on the snowboard above formed with little wind as usual, but if you look at the accumulation in the rain gauge below, you can see what even the tiniest bit of wind can do to snow accumulations in there. The rain gauge is mounted on the boy’s swing set, only about 50 feet away from the snowboard, so there should be little difference in the wind there. As a control for the wind though, you can also tell that it was very light by the way that the snow accumulated evenly on top of the bar of the swing set itself. As the snow accumulates to enough depth, the accumulations on the bar can certainly affect what goes into the gauge, but clearly the gauge did not collect a nice flat accumulation of snow, even with very minimal wind:

Image

In the past I have also placed one of my snowboards over near the gauge to compare how it worked in that location, and confirm that I’m getting similar amounts of snow around our property, and the accumulated snow in that location was the same as I get on my main snowboard.

I’ll finish with another thought, and that is about total precipitation. If a location is trying to document the total precipitation that falls over the course of a year and a significant portion of that precipitation is snow, relying solely on a cylindrical gauge may not be the best way to go. In snow events, I have compared the amount of liquid equivalent collected by my gauge, and the amount of liquid equivalent collected on my snowboard(s) many times. In the case of the snowboard, the liquid is obtained by taking a core sample from the stack of snow, melting it down, and measuring the water that is there (this is how the snow density is calculated). On some occasions with snowfall, the gauge has caught the same amount of liquid as the board, but most of the time, the amount of liquid collected by the gauge falls well short of what was collected on the snowboard. The gauge just doesn’t collect all the snow most of the time, even with very light wind. In mixed precipitation events (or rain events) the gauge is necessary, and does a great job, but those are situations where the precipitation is denser and likely falling closer to vertically.

After the clear deficiencies I’ve seen in how my cylindrical gauge collects snow, I always use my snowboards to collect snow and get core samples to determine the liquid equivalent and snow density. I’m glad to hear that the accumulations at the stake are done on a hard surface, although it’s interesting to find out that a cylinder gauge is being used for snow collection. It’s possible that this method could be contributing to part of the deficiency in observed snowfall.

-J
Last edited by J.Spin on Thu Jun 17, 2010 9:53 pm, edited 1 time in total.
User avatar
J.Spin
 
Posts: 918
Joined: Wed Sep 22, 2004 9:54 pm
Location: Waterbury, Vermont - Image: Skiing Bone Mountain in the Bolton Valley backcountry, VT

Next

Return to Eastern North America

Who is online

Users browsing this forum: No registered users and 7 guests


All content herein copyright © 1999-2017 First Tracks!! Online Media

Forums Terms & Conditions of Use