In article ,
Richard Griffith writes:
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.

What's your reason for not having included the last few years in
deriving your average before now? There seem to be two plausible
approaches for averaging: (1) use as many years as possible, to minimise
the impact of random fluctuations, or (2) use the most recent N years on
the grounds that the climate may be changing. At the moment you are
doing neither. The "professionals" do the latter by taking a thirty year
period, but they only update the period every ten years because of - I
assume - the work involved and the difficulty of getting everybody to
use the same reference period..
--
John Hall
"Honest criticism is hard to take,
particularly from a relative, a friend,
an acquaintance, or a stranger." Franklin P Jones