"John Hall" wrote in message
news
In article ,
Brendan DJ Murphy writes:
I've always wanted to know how much precipitation (in rain mm) is
required to produce 1 foot of snow.

I've heard statements such as "1 foot of snow is equivalent to 1 inch
of rain" (ie: 12:1 ratio)
Ive also heard "1mm of rain is equivalent to 1cm of snow" (ie: 10:1
ratio)
Which one is correct?
snip

The answer is both/neither. The "drier" the snow, the more air it
contains and so the lower the water content. Dry snow will have the 10:1
or 12:1 ratio you quote. Really wet snow, close to turning to sleet/rain
can have a ratio as low as 4:1.
--
John Hall
"It is a very sad thing that nowadays there is so little useless
information."
Oscar Wilde (1854-1900)

Harold Brooks in earlier post said

"Several years ago, some colleagues and I looked at the snow/liquid ratio
for 1650 snowfall events from a number of sites in the US. The ratio
ranged from ~2:1 to ~50:1. Our mean was 15.6:1 and the median was 14:1
(we used 6-hour snow accumulation obs-the ratios would be smaller for
longer time periods). The paper is online at "

http://www.nssl.noaa.gov/~brooks/pap...reetal2006.pdf

Harold
--
Harold Brooks
Head, Mesoscale Applications Group
NOAA/National Severe Storms Laboratory

this shows the huge range -- 2:1 to 50:1 in1650 events - all over 6hr
accumulation periods.
Obviously there are many variables - as have been mentioned, apart from the
fact that the longer the snow lies the greater it is compacted.
His median figure of 14:1 seems about the best on offer as an extremely
approximate ratio.
With snow lying the 'water equivalent' is very important in assessing risks
of flooding when it melts. In years past this value was routinely measured
as part of the observational routine...I don't know whether it still is?
Cheers
John