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Types Of Snow & Snow Ratios


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Posted
  • Location: New Ash green 150M / 500 FT
  • Location: New Ash green 150M / 500 FT

    Grab my Graupel mentioned posting this one as a topic-

    Just picking up on this point its something this forum or TWO never discuss is the snow variety we expect to see & its density type-

    The NOAA are talking about Snow density & snow ratios-

    The accepted Snow ratio from the weather community is around 10:1- although this is more geared towards the US than the UK-

    The Climo for the NE US snow ratio is around 11:1-

    This means with a ratio of 10:1 if you melted down 10 cms of snow you find 1cm of rain, or on the mm Scale 1mm = 1cm of snow-

    I would hazard a guess that the UK sits closer to a snow ratio of 3-5:1- ( thats between 3 & 5 ratio to one)

    this is typical of a polar Maritime track- one like we are going to see over the Weekend-

    Values of the ratio I would guess rise in Polar Northerlies to 5-8:1-

    The best Snow ratios are seen in situations where the surface air is very cold & the uppers are very cold & the the RH levels are very low ( low dewpoints)- think 1987,1991,1996-

    Snow ratios that exceed 10:1 for the UK are only really seen in these circumstances...

    The fluffy snow the NOAA talk about referes to Stellar Dendrites ( the type of snow) only found in the very cold Upper air conditions around 5-10F

    the 6 sided snowflakes are generally accepted as the best for aggregation & combined with perfect upper air conditions the snow ratios can get to 30:1- even 40:1

    If your old enough & Im sure some of the oldies will empathize with this- If you can remember feb 1991 & ever stood in falling snow & swept of a car bonnet to see what type of snow was falling & I did ) then you will have probably seen 'stellar dendrites)-

    these are the biggest & best shown on this Snow morphology diagram-

    morphologydiagram.jpg

    & one reason why we never see such decent flaks in the UK any more because the upper air profiles arent cold enough-

    Just something off topic & never discussed in this forum....

    Stellar Dendrites------ GONE FOREVER???????????????

    I will try to update some more on this later- However this weekend is a good example, not much chance of the Stella Dendrites, however the smaller dendrite shape is on the cards for Northern parts of the UK & Ireland

    S

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    Posted
  • Location: Steeton, W Yorks, 270m ASL
  • Location: Steeton, W Yorks, 270m ASL

    Great post Steve. I recall during the blizzards (and I'm using that word very deliberately and carefully) of 1979 the prisms piling up against the bottom of the window: often the accumulation on the sill would get up to 6" and more.

    Re conversion ratios the rule of thumb is 10:1, though drier snow may stack more than this (it traps more air and compacts less immediately at low masses) and wet snow rather less. They type of flake also matters: prisms will arrange themselves more tightly than plates, particularly where the snow is wet and chunky.

    Such is the stuff of avalanche modelling by the way: instability between the layer boundaries brought about by the different crystal structure - all of which further compounds with temperature variation. This is why ski resorts like heavy early snow pre season followed by substantial rain (without washing the base away) - the soaking and freezing helps lock the pack.

    Like you I recall stading in the snow looking at small plates on my sleeve: until I was 8 or 9 I thought snowflakes were the big gloopy aggregations that we associate and imagine - the reality is more awesome when the air is cold enough to prevent any cohesion. Marvellous.

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