The question mark is not because I am agonising over whether my July 2004
rainfall of 38.9mm differs significantly from my (estimated) 1971-2000 average
of 40mm. However, using that benchmark alone, you could say we have had the
second consecutive drier than average July.

The reason for this posting is to alert us to the unrepresentative nature of
the 1971-2000 period over most of England and Wales.

Let me illustrate this with the Kew rainfall series, compiled directly from Kew
Ob. for 1840s to 1980, and from a selection of other nearby rainfall sites
(carefully adjusted and highly regarded) for 1697 to 1840s and after 1980.

Using this series as a base, 50 year averages of July rainfall are as follows:

1991-04 40.6mm
1971-00 41.3

but this is a misleading 'benchmark' ? We then have.....

1951-00 48.3
1901-50 59.9
1851-1900 60.5
1801-50 65.0
1751-1800 63.2
1701-50 55.9

There are no short downwards blips prior to 1970 comparable to what we have
experienced since then (I'll resist the temptation to post decadal means!).
[Data from 1981 are derived from Kew Gardens adjusted values and post 2001 from
my own data (my average matches Kew Ob.). You get the same results by adjusting
other local sites].

Julian

Julian Mayes, Molesey, Surrey - where the grass verges are now getting 'browned
off' again.

"JJCMayes1" wrote in message
...
snip

The reason for this posting is to alert us to the unrepresentative
nature of
the 1971-2000 period over most of England and Wales.

Let me illustrate this with the Kew rainfall series, compiled
directly from Kew
Ob. for 1840s to 1980, and from a selection of other nearby
rainfall sites
(carefully adjusted and highly regarded) for 1697 to 1840s and
after 1980.

Using this series as a base, 50 year averages of July rainfall
are as follows:

1991-04 40.6mm
1971-00 41.3

but this is a misleading 'benchmark' ? We then have.....

1951-00 48.3
1901-50 59.9
1851-1900 60.5
1801-50 65.0
1751-1800 63.2
1701-50 55.9

It's an extraordinary change, isn't it? Both the Kew and the E&W
series
show July was, more often than not, the wettest month of the year
during
the 18th and early 19th century ... now it is effectively the
driest. The
drying trend is also detectable in August rainfall in both series,
though
it is smaller and largely confined to the last 30 years. So it seems
to be
a high summer feature rather than just a statistical artifact
confined to
one particular calendar month. June, meanwhile, appears to have
become significantly wetter during the last 100 years or so (this is
more noticeable in the E&W series than in the Kew one).

Some of July's drying may be attributable to broadscale circulation
changes ... see www.climate-uk.com/indices07.htm where the
downward acceleration in rainfall (from c.1970) was associated
with rising pressure and declining westerlies. The westerlies
continue to decline, but pressure has been falling again during the
last 10-15 years and it now lower than it was during the wet
1950s and 60s. In any case, most July rain (at least in London)
does not come from westerlies.

Those big falls of widespread heavy rain that some of us grew up
with ... Julys like 1966, 1968, 1969 and 1973 stick in the mind ...
hardly seem to happen now. We had one last month, but that's all.
Superficially, with higher SSTs to the SW of the UK one might
have expected them to become more intense if not more
frequent. Perhaps the frequency with which unstable polar air
engages what we now call Spanish plumes has declined. Has
trough extension/disruption between longs 10 and 20W become
less frequent in July? Has polar air become less polar (ie
less cold, less unstable)?

Now ... the 71-00 average, whatever else it represents,
is still representative of the last 30 years ... and like recent
temperature changes, it gives us the closest thing we are ever
likely to get to a "normal" (in the statistical sense)
representative
of "now" (whatever that might mean).

Philip Eden