Question Snowfall versus Rainfall

Discussion in 'Alpine & Snow' started by RayC, Jul 5, 2016.

  1. RayC

    RayC A Local

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    Point of disclosure: I am not a weather tragic but I do enjoy reading the comments on these pages. So here is my dumbass question. What is the relationship between reported rainfall and snowfall.
    The Bom reports 10 mm of rain since 9am and Perisher reports 10 cm of snow over the same period.
     
  2. ojisan

    ojisan station master Ski Pass: Gold

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    a fair bit of 'wet snow' this morning me thinks
     
  3. Legs Akimbo

    Legs Akimbo Grumblebum Ski Pass: Gold

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    Rule of thumb is that amount of snow on the ground is 10 times greater than the measured precipitation, which is recorded as liquid water. Plus or minus. What is reported is consistent with the rule of thumb.
     
  4. RayC

    RayC A Local

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    Thanks Legs
     
  5. Legs Akimbo

    Legs Akimbo Grumblebum Ski Pass: Gold

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    In fact if you find a rain gauge in the area you will find that they are heated so that any snow that falls melts and flows to the collector as liquid water. A fact I discovered at Munyang Power Station once when we caused anomalous readings by extended testing of what happens when you put snow into a rain gauge. Sorry BoM and Snowy Hydro.
     
  6. The Snow Gauge

    The Snow Gauge Hard Yards

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    Prefer The Snow Gauge ;)
     
  7. Trail Blazer

    Trail Blazer Part of the Furniture

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    Certain members of this forum insist on converting closer to 8:1, that may be the case if the snow is wet but generally 10:1 is consistent with actual snow obs
     
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  8. Vermillion

    Vermillion Pool Room Ski Pass: Gold

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    I almost completely discount AWS rainfall data during a storm or snowfall. I dont trust that an accurate representation of what is actually falling from the sky makes it to the heated plate, and then melts and falls in. When it's a blizzard or particularly windy (often), I think this is even less accurate.

    Ski resorts take their depth measurements from the same place every day and it's checked by a human. Whether or not you think they embellish the depths (it only seems to snow in 5cm increments in NSW) is up to you, but i'll take the overall depths the resorts report vs. the AWS data.

    At the end of the day, the amount of reported snow that falls is irrelevant in 2016. This isnt the 1990s where you relied on a number in the newspaper to tell you if the conditions were good or not. Just load up a snow cam or go on social media and you can see how much snow there is around the place.
     
  9. Sandy

    Sandy Dark Sith Lord of the Pool Room Moderator Ski Pass: Gold

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    This general rule of thumb is dependent on the temperature & the shape/size of the snow. My general rule of thumb is 10:1 +- 50%.
    So in a much colder place overseas, 10mm of precipitation can get you 15cm+ of very light powder. In some cases you get snowflakes the size of 20cents pieces, and this combined with low temperature produces a lot of space between the settled snow, making it even deeper for a given amount of precipitation.

    Because of the relatively high temperatures in Australia, wet snow most probably settles at a ratio of maybe 6:1 or 7:1. Remember even if it falls at a ratio of 10:1 on the top layer, the water content weight will make it settle much more compressed.

    As an example of the difference between Aussie snow and snow in colder places, when you remove snow from your car in Australia, it's often quite heavy (just the weight of it). In Japan, when removing snow from my car, I will often take a plastic snow shovel, slide it underneath and lift a 50cm deep "block" of snow and toss it EASILY.... try that in Australia, and you may hurt your back or break the plastic shovel!!!
     
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  10. Richard

    Richard Maintenance Dept Administrator

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    I can't recall exactly where Ian and I saw one of these heated bowl things in somewhat blizzardy conditions during our years of the snow cam safari's, but any casual observation with a smattering of scientific objective laid over it can only conclude;

    'that ain't accurate'
     
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  11. mr

    mr Part of the Furniture Ski Pass: Gold

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    from my obs, once it hits zero the hotham AWS says 'calm' and doesnt record rain at all until it is well above zero (and then it all melts and we get a big reading), where as falls creek on does record wind and moisture ongoing. baw baw seems the most accurate of the lot in terms of real time moisture, but the wind vane freezes at around -4.

    more importantly today - does 1mm of rain wash away 10mm of snow? i reckon it would be more like 1:5.......any thoughts??
     
  12. Vermillion

    Vermillion Pool Room Ski Pass: Gold

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    It's not far from being snow, so the rain wont be damaging anything that's been packed and/or groomed too much.
     
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  13. ice_man

    ice_man One of Us

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    Depends on how warm the rain is. Rain at 0.5 degrees C isn't going to be as problematic (low thermal energy) as rain at 10 degrees C. Think of it as melting ice cubes in your sink by pouring hot water on them vs. cold water.
     
  14. Sandy

    Sandy Dark Sith Lord of the Pool Room Moderator Ski Pass: Gold

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    No, it wouldn't. Think about it... If 1mm precipitation produces 10mm snow, and gets 1mm rain, then the water content is double. If it's unpacked snow, the I would expect 1mm of rain to pack it to maybe 5mm of snow, if it's just above freezing
     
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  15. warrie

    warrie Addicted

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    Warning, physics follows! Imagine 10 mm of fresh powder snow at zero Celsius, covering an area of 1 square metre. Allowing this to melt will produce 1 litre =1000 grams of liquid water still at zero. This will require 334 kJ of heat. Let us assume that the only heat available is from rainfall with a temp of 5 degrees. So the rain cools from 5 to zero and the ice stays at zero but melts. The specific heat of water is 4.18 Joules /gram / degree. Every mm of rain that falls on the area has a mass of 1 kg so to drop 5 degrees it will release 20.9 kJ. Therefore it takes 334 divided by 20.9= approx 15 mm of rain to melt the snow. Thus it takes a lot of rain to melt snow. Of course in reality other heat sources come into play.....W
     
  16. mr

    mr Part of the Furniture Ski Pass: Gold

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    i think all that that science tells me i will still be skiing at hotham this weekend!! cheers
     
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  17. warrie

    warrie Addicted

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    If it's sunny at Hotham on Saturday and the snow is at an angle on a SE facing slope such that only 334 of the suns 800 watts hits our square metre from the above post then it will only take 1000 seconds to melt our 1 cm deep sample. That's a little over 15 mins. So in say 8 hours of usable sunshine 32 cm could melt. Ouch. Fortunately snow has a high albedo and reflects 97% of incoming light/heat/UV. This brings the daily melting back to just 1 cm Phew!! And then there's heat from the air - calm v windy - and conduction through the snowpack itself. We need Cedar Lodge to keep their ruler in the snow and measure decreases as well as increases.. LOL.....W
     
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  18. Sandy

    Sandy Dark Sith Lord of the Pool Room Moderator Ski Pass: Gold

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    There's a couple of errors ;)

    - If the temperature is zero degrees, it's not fresh powder snow!! ;)
    - It has been assumed that 10mm of snow is produced from 1mm of water. At just under 0 degrees, the ratio is likely to be more like 5:1, so the amount of water would be closer to 2 litres for 10mm of snow, requiring 668kJ of heat to melt it.

    Taking this into account, with 5 degree rain, 668/20.9 = approx 30mm rain to melt the snow.

    OR if we assume it IS powder, then the snow might be at -5 degrees, the ratio is more like 10:1
    Taking this into account, with 5 degree rain, 334/20.9 = approx 15mm rain to melt the snow.

    ;) :nerd:
    :out:
     
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  19. glengary

    glengary One of Us Ski Pass: Silver

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    All this is very interesting for a physics teacher. When it's cold enough it snows. Rain is measured by volume, snow by depth. In the good old days an average for Aus. conditions was one inch of rain = 6 inches of snow. I.E. 25.4mm of rain = approx. 150mm of snow.
     
  20. mr

    mr Part of the Furniture Ski Pass: Gold

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    Water being one of the few things that expands when colder. Hydrogen bonds, says my Year 9 Physics

    What else gets bigger, when colder?

    Hi! @JoeKing
     
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  21. warrie

    warrie Addicted

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    98 mm of rain at Baw Baw in 40 hours on what was a decent snowpack. Check out the cams this morning for the damage Physics causes .LOL... W
     
  22. glengary

    glengary One of Us Ski Pass: Silver

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    Shocking, petty, it was looking good until this happened. No snow base will stand up well to that much warm rain.
     
  23. Red_switch

    Red_switch Old n' Crusty Ski Pass: 30 Day

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    In terms of the damage to the snowpack by rain, think in terms of latent heat fluxes. It doesn't take much rain to destroy a cold, dry snowpack.

    In fact, it doesn't even need to rain. It's common in NZ for humid air following a snowfall to have a pretty detrimental effect on the snowpack. If the snowpack is "drier" than the atmosphere above then there will be a vapour pressure gradient towards the snowpack, and mass transfer of moisture from the atmosphere to the snowpack.

    As for the performance of heated rain gauges in the snow, if it's not windy, they should work fine. But there are many examples in NZ where even shielded gauges are believed to only capture 30% of local incident precipitation.

    @Centago will likely have some thoughts, and might be able to articulate my mass flux ramblings more eloquently.
     
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  24. Red_switch

    Red_switch Old n' Crusty Ski Pass: 30 Day

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    97% is a pretty exceptional albedo. For fresh snow I'd raise my eyebrows at anything greater than 95% for fresh snow, it will usually exceed 90%, but then relatively quickly degrade to 80 something %, and lower if it's long enough before the next snowfall bumps it up again. That said, it does depend on how you actually measure the albedo in the first place (usually measuring albedo is straightforward, but for highly reflective and highly sensitive materials like snow, it quickly gets complicated).
     
  25. warrie

    warrie Addicted

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    Sounds reasonable so with the mid point being 85% reflection this will give 5 times more absorption than my estimate and 5 cm would melt in a day. 6 sunny days and there's a foot gone. And in Oz we don't have too many feet to play with. And as for snow gauges being inacurate in wind - it's windy more often than not LOL......W
     
  26. Sandy

    Sandy Dark Sith Lord of the Pool Room Moderator Ski Pass: Gold

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    There's another point too....
    If you assume that the first day of sun, it's fairly dry fresh packed snow with less water content and more space in between. After the first day's melting, and if we assume it's the same max temperature each day(let's say 0 degrees, to take air temperature out of the equation) and the same min temperature each night (lets say -5), then the frozen top few cm of the snow will be more dense with more water content (having absorbed the previous day's melt). That means it will take more energy to melt the same depth of snow.
    So all up, with 5cm of decrease on the first day, each day's melt due to sunlight, will be less decrease in depth each day..... so in my view, it won't be 30cm decrease over 6 sunny days, if the temperature doesn't get above freezing.
     
  27. Red_switch

    Red_switch Old n' Crusty Ski Pass: 30 Day

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    Don't forget to factor in sublimation.
     
  28. warrie

    warrie Addicted

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    Still on topic but reversing to make it a depth gain with this upcoming cold outbreak and potentially 50 cm of snowfall over 2 days. After it's all fallen by Wed and inaccurately measured by our non-wind friendly gauges LOL. Will its powderiness rival the stuff Sandy has in Japan and pass the plastic shovel test he described above? As it falls in say 5 cm increments an observer could measure compaction on one rail/table at Cedarwood lodge while wiping off another. As the next 5 cm is built up on the second rail he checks the total depth on the first rail This could discount sublimation and solar insolation since it will be snowing and overcast. Then as it fines up later in the week keep measuring. But no one will do this as they will all be out there enjoying the freshies LOL.....W
     
  29. Sandy

    Sandy Dark Sith Lord of the Pool Room Moderator Ski Pass: Gold

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    Sounds like a good plan!!! :D
     
  30. Astro66

    Astro66 Still looking for a park in Thredbo Ski Pass: Gold

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    I took the time one season to track mm of precipitation recorded at PV weather station Vs the subsequent increase in snow level at Spencers Creek. And Sandy is spot on. Came in around 10mm of precipitation gave about 7-8cm of snow.

    Yes I know settling would reduce ratio, but quite often the reading was taken close to when the snow fell. So on average it's a reasonable measure.
     
  31. Cuddles

    Cuddles One of Us

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    I concur on recent events Around 90% of fall ends up as measure after settling. Wish we had a heated device up higher.