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