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What depth of snow equals 25mm of rain?

All things being equal except for the ambient temperature, what depth of snow could we expect in lieu of 25mm of rain?
David Watton from West Midlands (Age: 55+)

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3 Responses

  1. Hi :).
    Snow melts to provide about 10% or less liquid water. That is really only an estimate since immeasurable factors affecting compaction and accumulation significantly alter the density of any ‘fall’ making it impossible to give a universally precise figure. The volume of water which is represented by a precipitation of rain measured as 25mm is, given the many influences which may dictate the outcome in an individual scenario, possible to estimate for a particular occurrence, I believe. This is something that I have attempted and if you have the time to read what follows I will try to justify my ‘10%’ estimate and to answer your question by a back to front analysis…
    Using my front and back gardens as test beds after a recent fall of snow finished early in a cold morning I used a ‘paired sampling/averaging’ statistical design with randomisation to obtain an unbiased estimate of composition density. I collected my samples by gently skimming the top layer of snow with a flat, broad wooden spatula then I transferred this material into a 2 litre plastic jug until this jug was filled. I chose open, but calm areas which were distributed over spaces of about 15 square metres in each garden. I took each sample indoors, thawed it in my microwave and recorded the ‘snowmelt water’ that was obtained. I compared these figures with the maximum jug volume which I had measured earlier. Using averaging and rounding, with due regard for sampling errors etc. I analysed the results to obtain a best estimate of the water / snow volume when collected in this way. To summarise, my conclusion was that a ratio of 1:10 existed on that occasion. The reproducibility was more than 95% and the impression was that an accurate measurement had been achieved.

    Re analysing the data in a ‘back to front’ way, I calculate that each 40 mms square of land would have received 1ml of water. A volume of water equal to 100 millilitres has, from my data, the potential to create 1 litre of snow. Thus a rainfall of 25mms depth would in these conditions have the potential to produce 1 litre of snow for each 200 mms square of land. This high value may partly explain why an enormous amount of snow appears to fall for the depth it that it creates on the ground. Other major influences include environmental temperatures, wind speed and pattern, compaction, melting / run-off and topography etc….

    I hope you will conclude, as I do, that this experiment provides a meaningful estimate of the potential for rainfall to have produced snowfall under similar conditions and that I have given an answer to your question. I wonder if anyone else has observations to add….?
    Good Luck.
    Rockno3.

  2. many thanks for your hard work. I have puzzled over this one for many many years without finding the right person to ask.

  3. Hi Dave,
    Your welcome 🙂
    We had another good snowfall today so I rechecked that ratio for loose snow to snowmelt water and again found it to be 10:1. However I would like to tidy up the expression I gave for the relationship between a rainfall depth of 25mm and the potential for that to have produced a quantity of snow.
    Developing from the 40 square millimetres of land (which under these conditions would recieve 1 millilitre of water) it follows that 1000 times that area of land would receive 1 litre of water, Application of the ratio therefore suggests that 10 litres of snow cf the equivilent composition evaluated could have been created in appropriate conditions. Sorry to use so many qualifications in this answer. I felt, on checking through my previous attempt, that I had failed to make the calculation add up. I hope this helps.
    Good Luck.
    Rockno3

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