I saw some of the posts about drought definitions while I
was away.

Can I emphasise that the usage of terms such as
"Absolute Drought", "Partial Drought" and "Dry Spell"
(and the more rarely-used equivalents at the wet end of
the spectrum) were discontinued officially in 1961 when
emerging computer power allowed a much more flexible
approach to rainfall analysis.

Those definitions were useful as a sort of book-keeping
exercise in the pre-computer era when all climatological
analysis was carried out manually by dozens (at least) of
clerks at Dunstable/Bracknell as well as by observing
staff at outstations.They helped to highlight periods of
rainfall shortfall, but their hydrological, water-supply, and
agricultural relevance was very limited.

It's OK for people to refer to them, or for u.s.w. to
resuscitate them if they/we wish, as long as we bear in
mind the history and the limited relevance.

With that in mind, however, it may be useful to point out
that the original threshold for the rainfall amount for
breaking an absolute drought was 0.01 inch (which is as
near as dammit 2.5mm). As the categories were dropped
at a time when rainfall analysis (at least in the publication
"British Rainfall") was still carried out in inches, I don't
believe the definition was ever formally changed to a
metric equivalent: if it had, it would have been more
logical and more precise to change it to "more than
0.2mm" rather than "0.2mm or more". (I have a very
vague memory of having seen "more than 0.2mm" in
print somewhere, but I may just be remembering a
long-forgotten rationalisation of my own from 25 years
ago or more).

Whatever, for anyone wishing to use these categories
in the modern era of tipping-bucket rain-gauges, it is
advisable to allow recordings of 0.2mm to pass without,
as it were, breaking the drought. A tip of 0.2mm is
very often the result of an accumulation of several
smaller quantities - indeed tipping-bucket tips may be
induced by vibration or by thermal expansion/contraction
of the rain-gauge housing or by insect life, when one of
the buckets is almost full. Relatively rarely is an isolated
0.2mm tip the result of 0.2mm of rain.

Philip

"Philip Eden" philipATweatherHYPHENukDOTcom wrote :

With that in mind, however, it may be useful to point out
that the original threshold for the rainfall amount for
breaking an absolute drought was 0.01 inch (which is as
near as dammit 2.5mm).

0.25mm, of course. Just testing, you understand.

pe

On Apr 26, 2:46 pm, "Philip Eden" philipATweatherHYPHENukDOTcom
wrote:
"Philip Eden" philipATweatherHYPHENukDOTcom wrote :

With that in mind, however, it may be useful to point out
that the original threshold for the rainfall amount for
breaking an absolute drought was 0.01 inch (which is as
near as dammit 2.5mm).

0.25mm, of course. Just testing, you understand.

pe

Philip

I may have missed the point, but are you now saying there is no such
thing as a drought ? - or that we dont make a note of extended dry
spells ?

sorry just a bit confused.

Paul
Brampton
www.bramptonweather.co.uk

Philip Eden wrote:
"Philip Eden" philipATweatherHYPHENukDOTcom wrote :

With that in mind, however, it may be useful to point out
that the original threshold for the rainfall amount for
breaking an absolute drought was 0.01 inch (which is as
near as dammit 2.5mm).

0.25mm, of course. Just testing, you understand.

or 0.254mm to be as precise as possible ;-)

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

On a tatty piece of sub-ether at said...
On Apr 26, 2:46 pm, "Philip Eden" philipATweatherHYPHENukDOTcom
wrote:
"Philip Eden" philipATweatherHYPHENukDOTcom wrote :

With that in mind, however, it may be useful to point out
that the original threshold for the rainfall amount for
breaking an absolute drought was 0.01 inch (which is as
near as dammit 2.5mm).

0.25mm, of course. Just testing, you understand.

pe

Philip

I may have missed the point, but are you now saying there is no such
thing as a drought ? - or that we dont make a note of extended dry
spells ?

sorry just a bit confused.

Paul
Brampton
www.bramptonweather.co.uk


I think Philip is saying that there now is no official definition of what
constitutes a drought. Right???
--
Rob C. Overfield
Hull

wrote in message
ps.com...
On Apr 26, 2:46 pm, "Philip Eden" philipATweatherHYPHENukDOTcom
wrote:
"Philip Eden" philipATweatherHYPHENukDOTcom wrote :

With that in mind, however, it may be useful to point out
that the original threshold for the rainfall amount for
breaking an absolute drought was 0.01 inch (which is as
near as dammit 2.5mm).

0.25mm, of course. Just testing, you understand.

I may have missed the point, but are you now saying there is no such
thing as a drought ? - or that we dont make a note of extended dry
spells ?

Paul ... just that those old definitions had a very limited value,
and haven't been used in any official capacity (or, as far as I
am aware, in any serious met/clim journal) for 46 years.

As I said, anyone can use them if they want, and FWIW I have
no view either way.

The point, really, was in the bit of the original post that
you didn't copy :-)

Philip

On Apr 26, 12:18 pm, "Philip Eden" philipATweatherHYPHENukDOTcom
wrote:

Whatever, for anyone wishing to use these categories
in the modern era of tipping-bucket rain-gauges, it is
advisable to allow recordings of 0.2mm to pass without,
as it were, breaking the drought. A tip of 0.2mm is
very often the result of an accumulation of several
smaller quantities - indeed tipping-bucket tips may be
induced by vibration or by thermal expansion/contraction
of the rain-gauge housing or by insect life, when one of
the buckets is almost full. Relatively rarely is an isolated
0.2mm tip the result of 0.2mm of rain.

Philip

I have a tipping bucket raingauge (Davis VP 2) and was recently
annoyed to see, what would have been a 26-day drought "spoiled" by
falls of 0.2mm. There were only three recorded rainfalls between 22nd
March and 16th April - 0.2mm on 23rd March, 0.2mm on 6th April and
0.2mm on 15th April. So there was never more than 13 consecutive days
without recorded rain.

I often see the bucket has tipped to give 0.2mm, when it appears that
no rain has fallen, and in these cases I just clear the total and do
not record it. However in each of the above instances I was satisfied
that rain had actually fallen. There was approx 1 hr of very light
rain/drizzle on the evenings of both the 23rd and 15th, which wet the
ground and could easily have amounted to 0.2mm. I did not see any rain
on the afternoon of the 6th and was going to ignore that 0.2mm but
someone else was able to confirm that there had been a very brief
heavy shower, even though there was no sign by evening that any rain
had fallen.

I do not know of any other occasion during the 26 day period when rain
fell but I was away in Cornwall, from 7th to 14th April, and, even
when I'm here I won't notice every trivial rainfall, so it's certainly
possible that none of those 3 individual falls did actually amount to
0.2mm. Of the three falls I think the one on the 6th is least likely
to have amounted to 0.2mm. I was very close by at the time and neither
heard nor saw any rain and the person who confirmed the shower is
known to exaggerate rainfall rates (such as saying it is pouring rain
when the gauge is only registering 1mm/hr).

In view of the uncertainty of 0.2mm falls I think I may for my own
records regard a drought as 15 or more days without a fall greater
than 0.2mm. In any case 0.2mm of rain could hardly be regarded as
useful so it's hardly as if it's breaking a drought.

Richard Slessor,
Hazlehead, Aberdeen.