I'd be interested to know if there's a "normal" way to measure the
duration of rainfall as well as the total amount that falls. Obviously
with the discrete measurements of 0.01" from the tipping device in my
Davis VP it might actually have been raining lightly for some time
before the bucket tips, so I appreciate that whatever value I get for
the "duration" is only going to be approximate. But it would be nice to
be able to quantify somewhat such sweeping statements as "it's been
raining all day".

--
Jonathan Stott
Canterbury Weather: http://www.canterburyweather.co.uk/
Reverse my e-mail address to reply by e-mail

In message , Jonathan Stott
writes
I'd be interested to know if there's a "normal" way to measure the
duration of rainfall as well as the total amount that falls. Obviously
with the discrete measurements of 0.01" from the tipping device in my
Davis VP it might actually have been raining lightly for some time
before the bucket tips, so I appreciate that whatever value I get for
the "duration" is only going to be approximate. But it would be nice to
be able to quantify somewhat such sweeping statements as "it's been
raining all day".

The tipping bucket gives very little guide to rainfall duration. For
example rain falling at a steady rate of 0.5 mm/hr is continuous
moderate rain but would result in only 2 tips of the bucket per hour. A
tilting syphon gauge is what is needed to measure duration, and even
that has limitations in very slight rain.

Are there any electronic instruments that will measure duration? I
haven't come across any but I haven't looked for any.

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

Norman Lynagh wrote:

Are there any electronic instruments that will measure duration? I
haven't come across any but I haven't looked for any.

There are some that measure 'wetness' (i.e. drops lowering resistance
of an electrical grid) which is not the same as rainfall duration of
course - think of a spell of intermittent drizzle in fog over many
hours in a mild winter warm sector, which might just give 0.2 mm but
could show almost continuous 'wetness'.

There's no easy solution. However, as Norman points out, the current
method relies on manual measurement of the slope of a tilting-siphon
rain recorder (TSR) paper chart record; these instruments are becoming
rarer by the day and the availability of records must be (excuse the
pun) drying up rapidly.

Since I commenced AWS/tipping bucket records (Feb 1993) I've maintained
a record of 'number of hours with 0.2 mm or more (i.e. one tip). I'd be
the first to state that this is also _not_ the same as rainfall
duration measured from a TSR, but it does have the great advantages of
being easy to tabulate automatically, and increasingly available from a
wide range of sites.

For what it's worth, my annual average of 'number of hours with 0.2 mm
or more of rainfall' 1994-2005 is 839 hours; this compares with an
annual average of 575-600 hours rainfall duration measured by
conventional methods. The monthly averages vary between 94 hours in
November and 41 in July.

I'm not aware of any site undertaking a direct comparison side-by-side
between the two methods, I'd be interested if anyone does have access
to these or can generate them. The greatest differences will be in the
winter months (longer, lower-intensity rainfalls) and obviously the
tip-count will always be higher (as effectively every hour is counted
as 1.0 hour, vs fractional hours from the chart measurement methods).

Stephen Burt
Stratfield Mortimer, Berkshire

Norman Lynagh wrote:

Are there any electronic instruments that will measure duration? I
haven't come across any but I haven't looked for any.

There are some that measure 'wetness' (i.e. drops lowering resistance
of an electrical grid) which is not the same as rainfall duration of
course - think of a spell of intermittent drizzle in fog over many
hours in a mild winter warm sector, which might just give 0.2 mm but
could show almost continuous 'wetness'.

There's no easy solution. However, as Norman points out, the current
method relies on manual measurement of the slope of a tilting-siphon
rain recorder (TSR) paper chart record; these instruments are becoming
rarer by the day and the availability of records must be (excuse the
pun) drying up rapidly.

Since I commenced AWS/tipping bucket records (Feb 1993) I've maintained
a record of 'number of hours with 0.2 mm or more (i.e. one tip). I'd be
the first to state that this is also _not_ the same as rainfall
duration measured from a TSR, but it does have the great advantages of
being easy to tabulate automatically, and increasingly available from a
wide range of sites.

For what it's worth, my annual average of 'number of hours with 0.2 mm
or more of rainfall' 1994-2005 is 839 hours; this compares with an
annual average of 575-600 hours rainfall duration measured by
conventional methods. The monthly averages vary between 94 hours in
November and 41 in July.

I'm not aware of any site undertaking a direct comparison side-by-side
between the two methods, I'd be interested if anyone does have access
to these or can generate them. The greatest differences will be in the
winter months (longer, lower-intensity rainfalls) and obviously the
tip-count will always be higher (as effectively every hour is counted
as 1.0 hour, vs fractional hours from the chart measurement methods).

Stephen Burt
Stratfield Mortimer, Berkshire

"Stephen Burt" wrote in message
oups.com...

Norman Lynagh wrote:

Are there any electronic instruments that will measure duration? I
haven't come across any but I haven't looked for any.

There are some that measure 'wetness' (i.e. drops lowering resistance
of an electrical grid) which is not the same as rainfall duration of
course - think of a spell of intermittent drizzle in fog over many
hours in a mild winter warm sector, which might just give 0.2 mm but
could show almost continuous 'wetness'.

There's no easy solution. However, as Norman points out, the current
method relies on manual measurement of the slope of a tilting-siphon
rain recorder (TSR) paper chart record; these instruments are becoming
rarer by the day and the availability of records must be (excuse the
pun) drying up rapidly.

Since I commenced AWS/tipping bucket records (Feb 1993) I've maintained
a record of 'number of hours with 0.2 mm or more (i.e. one tip). I'd be
the first to state that this is also _not_ the same as rainfall
duration measured from a TSR, but it does have the great advantages of
being easy to tabulate automatically, and increasingly available from a
wide range of sites.

For what it's worth, my annual average of 'number of hours with 0.2 mm
or more of rainfall' 1994-2005 is 839 hours; this compares with an
annual average of 575-600 hours rainfall duration measured by
conventional methods. The monthly averages vary between 94 hours in
November and 41 in July.

Stephen, I kept TSR and AWS records at Hampstead for three years,
not always complete wrt the TSR, but near enough. For annual
figures I found that TSR rainfall duration = AWS "wet hours" * 0.7
almost exactly for each of the three years. Although for personal
use I also apply the same figure for monthly data, the figures vary
quite widely although I couldn't really detect a seasonal cycle which
surprised me.

It is interesting that your figures also give 0.7 + or - 0.015. One would
expect the multiplier to be fairly consistent over lowland Britain, but
would probably be drift the further north and west you go (I'm not
even sure in which direction the drift would be ...!)

Of course for daily records such a multiplier is just not appropriate.

I aim to re-start side-by-side records at another site shortly :-)

Philip