I followed up with this query:
Thank you. I'll post on note on my blog with your reply.
Was there a particular rationale for using 16 instead of 15?
They have now replied. Unfortunately, the reply doesn't really shed any light on the situation because they don't say why 16 vs. 15 just that they were calculated separately:
Thank you for your email.
The normals and standard deviations were calculated separately and the limits (15 years and 16 years) were set independent of one another.
I wonder why? Brohan et al. 2006 clearly says 15 is the limit:
(the requirement now is simply to have at least 15 years of data in this period)
I just might be possible that the Met Office isn't telling my why because the why could be a bug. Since there are two programs this could be one of those classic off by one errors that crop up in programming all the time.
It's not hard to imagine the normals program doing
if ( number_of_years >= 15 )
and the standard deviation program doing
if ( number_of_years > 15 )
Equally that's almost groundless speculation on my part and perhaps there's some other good reason that the Met Office decided not, or hasn't taken the time, to tell me about.
Either way the six stations can be used.