On Friday, 18 January 2013 23:18:07 UTC, 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



On Friday, 18 January 2013 23:18:07 UTC, 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.

I'm sure the Met Office would like a huge network of manual observations, but like everyone else, they have no funding for that. As long as the AWS observations aren't used climatologically then they are a good indication of snow depth.


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.


Paradoxically, the best place for a rain-gauge is often where drifts form!



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.

If the gauges are registered with the Met Office i.e. they have a six figure rain-gauge number, then all data from that gauge is subjected to quality control (QC) by the Met Office. As part of that QC, ALL data from a rain-gauge is checked against its neighbours and the neighbouring data checked against its neighbours and so on and so on. This means that any anomalies are readily spotted. These anomalies are then carefully checked against chart data, radar data etc, etc. If rainfall is shown against a day when there was no precipitation reported and especially in previous snow conditions, any precipitation amounts are apportioned to the days of snow. The amount isn't just carved up but apportioned according to intensity reported on the snow days. If you've thought about it don't you think that the Met Office has worked it out given that it's been going for more than 150 years!



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