[wxqc] Measuring Snow

[dshelms at comcast.net]

Hi Neil,

The potential cross-section of the tube is very large relative to the catch area of the Peet Pro gauge which may account for the overcatch.  Maybe as snow fall diagnally (which is going to be the case at your elevation even with some tree wind blockage), the snow strikes the side of the tube, slides down the tube, and enters the Peet Pro gauge through the connection between the tube and the gauge.  

The other possibility is the Peet Pro gauge is hosed!

I am not a big fan of measuring "melt out" precip.  I see after-the-fact precip in Maryland from the unheated gauges days after the snow has fallen (why I advocate non-heated gauge owners to cover their gauges during winter precip situations).  For climate and hydrologic applications, the amount and timing of precip is very important so a timing error of hours and days will  cause problems which may be very difficult to identify once that are in the data record.  

I know your station's remote location and power constraints have pushed you towards you tube innovation (which is very creative solution); however, we should try to melt and measure within minutes of the actual snowfall whenever possible.  Hopefully, the group will offer some suggestions to this technology challenge (this is in fact a big issue for all remote precip measurement observing systems).  

Dave
CW0351



 -------------- Original message ----------------------
From: "Neil Hunt" <nhunt at tamarackmountain.com>
> Here's an interesting instrument design problem...  My rain gauge is
> modified to collect snow for passive solar melting, but now apparently
> over-reads by 2-4 *times*.  I'm looking for insight on my modification, or
> advice on how to modify it to collect snow for melting, but not over-read
> rainfall.  Details below.
> 
> I'm operating a remote solar powered weather station reporting via amateur
> packet radio (KG6PPD-9), on top a 7200 foot peak about 15 miles west of the
> Sierra crest.  The station is based on the Peets U2100 with the pro rain
> gauge (parabolic shaped collector with drip formation and counting).  The
> solar panels don't have sufficient power to heat the rain gauge, so I
> modified it by adding a passive solar melting tube to the gauge: pictures
> at: http://tamarackmountain.com/TamarackMountain/Weather/Pictures.html.
> 
> The solar melting tube is a piece of black plastic 3-inch drain pipe.  The
> Peets pro rain gauge has an 8-square-inch collection area (about 3.19 inch
> radius).  The inner diameter of 3-inch drain pipe is 3 inches, and the outer
> diameter is about 3.3 inches.  Some careful shaping yields a pipe with a
> razor-edge with the same 3.19 inches diameter as the unmodified rain gauge
> collector
> (http://tamarackmountain.com/TamarackMountain/Weather/Images/DSC_7821.jpg).
> Snow that collects in the pipe melts slowly when the sun comes out and the
> drips are directed into the raingauge by means of a 3-inch to 2-inch reducer
> fitting, and a 3-inch to 3-inch coupler makes a sleeve to prevent snowmelt
> or rain from the outside of the collector from tricking into the collection
> bucket:
> (http://tamarackmountain.com/TamarackMountain/Weather/Images/SnowCollector.j
> pg).  The lip of the gauge is about 12 feet above the ground, currently
> about 6 feet above the snow.  The site is generally not windy - average
> winds of 2-5 mph are common during major rain events.  The design of my
> collector was motivated by the CA dept of water resources sensors, which
> appear to be a similar vertical tube, albeit of much larger diameter and
> height.
> 
> 3 Teaspoons of water slowly dripped from a small nozzle yields a measured
> .11 or .12 inches of rain, as expected.
> 
> However, in a large storm, the gauge routinely reads 2-4 *times* more rain
> than nearby stations, often showing improbably high figures of 3 or 4 inches
> of rain in a day while my neighbors are reading 1.5 inches.  It's possible
> that the orography at the site really does extract a lot more water from the
> atmosphere, or it is possible that the design of the snow collector somehow
> funnels much more rain than the cross-section of the pipe might indicate.
> Note that it isn't a case of measuring 1/100 cms instead of 1/100 inches,
> since the 3-Teaspoon test comes out correct, and the multiplier between my
> measurement and neighbor's varies quite significantly from one event to the
> next.
> 
> Unfortunately, the site is about 3.5 hours drive away from home, and right
> now is about 30 minutes of snow-shoeing from the nearest vehicle access - so
> access is strictly limited.  But I was able to visit on one occasion while
> it was raining hard, and during that 30 minutes or so, the 2/10ths collected
> in a jar on the ground corresponded closely to the measurement from the
> instrument.
> 
> I made a temporary modification by adding a secondary collection vessel to
> the instrument drain.  Snowfall, freezing, and inability to access until the
> collection vessel overflowed made this calibration effort useless, but a
> side effect was that there was no path for wind to suck through the drip
> counter (since the drip tube was underwater).  The gauge still counted
> significantly more rain than nearby stations.
> 
> My station data can be viewed at
> http://findu.com/cgi-bin/wxpage.cgi?call=KG6PPD-9&last=240 (or, if you are
> patient, at http://tamarackmountain.com/TamarackMountain/Weather/Index.pl)
> while the nearest neighbor is at
> http://findu.com/cgi-bin/wxpage.cgi?call=CW4253&last=240.  Note that CW4253
> is not solar powered, and accordingly was subject to an extended power
> failure this past weekend.  That rain gauge isn't heated either.  We had a
> major snow event (4-6 feet) over the weekend, which has barely started
> melting through either of our gauges - judging from the readings.  The
> nearest CADWR sensor is about 5 miles East at
> http://cdec.water.ca.gov/cgi-progs/plotReal?staid=BLD.  This season I've
> measured 54 inches plus probably about 5 more still to melt, while the CADWR
> measurement is currently showing 34.4 - that's x2 for season total.
> However, on Dec 21, 22, 23, I measured a fantastic 9 inches, while BLD shows
> only 3.22 inches - which is x8!
> 
> Question: Can anyone share details of modification of rain gauges to collect
> snow for passive melting?
> 
> Question: Is this vertical tube design likely to somehow trap 2-4x as much
> rainfall as the same cross-section of rain-gauge?
> 
> Question: How likely is it that my site actually gets 3 or 4 inches of rain
> in a 24 hour period, while my neighbor 2 miles away only gets 1-2 inches in
> the same period?  My hilltop at 7200 feet is on the edge of the Stanislaus
> river valley as the first groud over about 6900 feet in the prevailing storm
> direction (from the SW); CW4253 is at a similar height, but is behind my
> ridge and one other also at 7200 feet.  BLD is also at 7200 feet, but is in
> an even more protected location about a mile East of a ridge of about 7300
> feet.
> 
> Neil/.
> 
> 
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