On Thursday, December 12, 2019 at 5:54:28 PM UTC, Steve Jackson wrote:
Someone will know the answer to this question; my money would be on Julian Mayes!

Be careful where you put your money, Steve!

When in doubt I'm afraid I look at Wikipedia and the entry https://en.wikipedia.org/wiki/Decade
is 'disputed'.

It seems to come down to the idea of the ordinal decade having years starting with a 1 as there was no year 0. So, in climatology we are being consistent with this principle - maybe when 30-year averages started in the late C19th people adhered to this rule? Mind you, I've just picked up British Rainfall for 1896 at random and see averages there for 1880-1889. They maybe were just more 'correct' when starting the 1881-1915 averages, though not when ending it!

As you say, precedence is the governing issue nowadays otherwise we'd be double-counting a zero-year if we changed. The COL book of averages comes to mind with its decadal averages.

At the Millennium some people said we were celebrating it a year early - but I suppose most of us go with the flow and consider that a new decade starts on 1/1/20. But yes, let's hope everyone keeps with the 1 rule in climatology for consistency.

Julian

Molesey, Surrey.