On 6 Jan, 20:50, "Richard Griffith" wrote:
I have rainfall records for my site back to 1983 (24yrs) to calculate my
percent of average rainfall I have been using the date range
1983-2004.(21yrs). For the length of records I have (not that long I know
compared with some stations !) would this date range be the most appropriate
or should I now be looking at say all years 1983-2007 to calculate a new
percent of average. (Until I hopefully reach 30yrs of records in 2013).
Would be interested to know what other observers do for semi short record
periods.

Best Wishes

Richard

Richard, by far your best approach would be to obtain the rainfall
records from at least one, preferably two or three, long-period sites
which overlap at least several years with your own records. Compare
the overlapping periods of record with your own, and evaluate your
totals as % of those sites. Average these by site over a number of
years (the longer the better, but as little as 5-7 years overlap can
give a pretty robust answer) and then adjust the observed long-period
average (ideally 1971-2000) for that site by the correction factor/s
for each site. Bingo - you've a reliable 30 year normal. Doing it this
way will probably get you a figure within a few per cent of your true
average, provided you choose stations with similar characteristics
(i.e. don't choose a hilltop site to compare with a valley record,
etc).

It's sensible where possible to use the same average period as
everyone else, in most cases 1971-2000 (except the Met Office, which
for arcane reasons still mostly use 1961-90) rather than your average
derived purely from your own period of record - which can be biased
owing to short-term fluctuations (an average from 2000 to 2007 would
probably be quite wet, for example.)

Once the figures are entered into a spreadsheet, the processing and
comparison steps are very easy. You can do this on annual totals only
although obviously doing it monthly will give you monthly means as
well as an annual average.

Where do you get the data from other sites? Well, you could try the
Met Office archives, although most of their paper records only extend
to about 1988 - everything since then is on computer. A Good Thing,
you might say, except they'll probably charge you heavily for it. You
should get a reduction/for free as a long-term co-operating rainfall
observer, though - ask! You might also try your local Environment
Agency, particularly if you also provide them with data. You can also
obtain monthly and annual totals in the old Met O publication that
replaced British Rainfall, although only up to when it cased
publication in 1991 unfortunately. There's a few COL stations quite
close to you wuth long records which could probably also assist, but
it's important to check the instruments and site are standard.

If you do visit MO Archives, you'll find in the 'ten year books'
monthly rainfall totals for every site back to the 1860s. (These are
the original British Rainfall Organisation archives.) With a few hours
to spare, and some money for the copier, it's easy to assemble a
continuous monthly record back 140 years or more. I did this about 20
years ago and found about 25 sites operating within 10 km of here back
to 1862, one just 6 km away with an unbroken daily record from 1871,
another less than 2 km away operating from 1900 to 1947. With a bit of
data entry into spreadsheets and comparisons between sites, it's quite
straightforward to assemble a 'homogenised' monthly rainfall total
representative of your own site 100 years, which gives you a good
long-term perspective with which to compare your own records.

Hope this helps.

Stephen Burt
Stratfield Mortimer, Berkshire