On Saturday, 19 January 2013 07:41:48 UTC, Norman wrote:
Ken Cook wrote:



"Norman" wrote in message ...



It's been snowing in Tideswell all day with the temp between -2 and -4 and

with a Force 3-5 wind. The snow has no liquid water content and the action of

the wind has broken it up into a very fine powder which has drifted very

readily in the wind this afternoon and evening. There's now nothing that can

be described as a 'level depth' anywhere. Some areas have been blown almost

completely bare while in others it's getting close to a metre deep. This

evening I've guesstimated 15cm but, come daylight tomorrow, I suspect that I

might increase that a bit. Nevertheless, there'll still be quite a bit of a

guess in there.



The rainfall equivalent is an even more difficult matter. In a situation like

this the rain gauge is irrelevant. Also, there's no representative bit of snow

on the ground that I can 'capture' and melt. About the best I can think of is

to use whatever I decide as a representative depth of fresh snow and use a

10:1 ratio. Does anyone know if there's an 'official' way of dealing with

this.... Ken?...Bernard?



Hi, Norman,



For depth I do as you do and take an average / general / sensible depth of

several readings to the nearest cm. Some days this is only a rough

approximation as I don't think there is an answer! This throws the Met O AWS

snow depth instrument out of the window in my opinion and there is no

substitute for a human observer for snow depths. There is then the issue of

the fresh fall. I use a board placed on top of the snow, but where should it

be placed to obtain a further sensible reading for fresh falls. The Met O do

not ask for a fresh fall measurement any more incidentally.



As you say the rainfall equivalent is even harder. To use the inverted funnel

method you need to be sure that you are only capturing the snow since the

last reading, so you need a "fresh falls board". Where should it be placed?

You must be sure none falls out when you bring the funnel upright! The Met O

way used to be to do this three times and take an average - a long, laborious

practice in an average Copley winter.



In practice I normally just melt the snow that is in the gauge funnel. I

assume that the Met O (and Environment Agency) have placed the rain gauge in

the most suitable place to best record precipitation and go from there. I

also place the fresh falls board close to the gauge.



A couple of days ago I had a fresh fall of 2cm but nothing in the funnel as

the snow had sublimated! I entered "Trace" in the obs as I knew there had

been precipitation.



Incidentally, I use two funnels and bottles here in Copley. I bring one set

in with the snow and melted snow at the obs time and replace them with the

spare set. The next time there is snow to melt I replace with the original

funnel and bottle and so on.



AWS gauges now being used by the Met O record only the snow that melts from

the funnel so I suppose that is now the way. I know the Environment Agency

record in that way from their Copley logger gauge and some of our wettest

winter days are then the sunny, dry ones when the snow is melting from the

gauge! All our worries and efforts could be in vain.



Personally I would not use the "ratio method" as this alters so much with

different types of snow. I usually write a note at the end of the month such

as "precipitation totals unreliable in snow and wind". I do smile sometimes

when I see snow depths reported in millimetres!



Best wishes,

Ken

Copley





Thanks Ken.



You've confirmed my view that sensible estimation is the way forward. There's

almost nothing in the rain gauge funnel this morning. It has all blown out. A

'fresh falls board' wouldn't be a help because there's nowhere that's

'representative' to put it. Also, the snow is so fine and powdery that it

wouldn't stick in the funnel using the inverted funnel method.



--

Norman Lynagh

Tideswell, Derbyshire

303m a.s.l.

If you have a copy of the "Observer's Handbook" you'll see on P144, "Reliable measurements of snowfall in stronger winds are very difficult and depend much on the zeal and applied skill of the observer in following the guidance given." Your problems are not unique and are well recognized! Observers will have a "feel" for their site and should be able to estimate or, even better, measure any fresh snow depth. If the snow is very powdery and you know the depth you're attempting to measure you can cut a cross section in the snow, slide a thin sheet of plastic (I use the back of an old clip file) horizontally into the snow at the required depth and place your inverted funnel through the snow until it meets the plastic sheet. You can then lift funnel and plastic sheet together thus gathering all the snow in the funnel.. This is a good method for measuring any fresh snow.