In message , Martin Rowley
m writes
"Norman Lynagh" wrote

According to my Davis VP there was no precipitation today before
1230z. From that time it then shows steady moderate rain up to the
present time at a rate of about 1 mm/hr. Of course, what happened is
that snow collected in the funnel this morning while the temperature
was below freezing. It has then steadily melted through the afternoon
and is being measured as ongoing precipitation.

Does anyone know how this problem is dealt with in the automatic
weather stations in the Met Office synoptic network? Is the rain
gauge heated so that snow is melted as it falls? Or do these stations
suffer from the same problem that I have had with mine today?

I believe that the gauges are supplied with low-level heating elements
... this is what the 'official' word is from the document which relates
to Automatic Weather Stations (AWS), which you can find on the Met
Office web site (see link below) ....
" The gauge can be heated to ensure that it functions when the air
temperature falls below freezing. Although this does enable snow to be
melted, only

approximate values of frozen precipitation can be obtained in this way.
The low terminal velocity of snowflakes and the risk of lying snow being
blown into the

gauge make snow measurements unreliable. If the gauge is not heated it
can become frozen, giving a spurious reading of nil when in fact rain or
snow has

fallen. In these periods the CAWS missing data indicator (88888) should
be used. Snow should be cleared from the funnel each day. "

(Begs the question about what happens when the station, as so many
nowadays, is unmanned.)

http://www.metoffice.gov.uk/publicat...ons/index.html

(follow the link from 'More information on automatic weather
measurements', but beware, it is a large PDF document.)

Martin.

Thanks for that Martin. I've downloaded the document. I note that it
says the gauge "can" be heated. It doesn't say that all are heated.
Indeed it implies that not all are heated.

The reason why I asked the question is that measurements from AWS are
included in the archived national meteorological database on an equal
basis to observations from a manned site. If a rain gauge is not heated
the precipitation from a snow event could easily be assigned to the
wrong time period, as would be the case with my Davis VP measurements
today. This could be of great significance if the archived data were to
be used in litigation or in investigation of an iinsurance claim several
years after the event. By that time it might not easily be identified
that there could be a problem with the reliability of the rainfall
measurements.

Norman.
(delete "thisbit" twice to e-mail)
--
Norman Lynagh Weather Consultancy
Chalfont St Giles 85m a.s.l.
England