Friday, June 1, 2012


date: Mon Sep 22 14:12:56 2008
from: Phil Jones <>
subject: Re: Downward trend in relative humidity over land?
to:, Kate Willett <>

Thanks for the plots.
Kate didn't see much happening with RH, but the 1-1.5% reduction is
probably too small to be that clear. It may also not be statistically
significant either. Kate's series finished in 2003, so the last 3 years
may be a factor as you say.
The increase in q wasn't quite as large as expected from temperature,
but the thought at the time was that this was due to the air over land
not being saturated as it would be over the ocean. Thinking aloud, we
could do a simple back of the envelope calculation and see if the slight
reduction in RH explains the difference between the expected from T
changes in q and what Kate sees with the real q obs.
These are probably worth showing, to see if anybody next week has any
more thoughts. I guess you can't produce similar series for q?
Kate has received the proofs. I'll attach these. The password you need
to open this is JCLI2274. JCLI and then 2274. Kate is also
putting together another paper.
Glad to hear you'll have some time later in the year.
At 11:58 22/09/2008, Adrian Simmons wrote:

Phil, Kate
I'm afraid I've been hopelessly busy with my day job and standard GCOS
matters of late, so the surface humidity stuff was put to one side until
yesterday evening.
I'm in Boulder next week, and will be giving a talk on reanalysis at
NOAA/ESRL on the Friday. I thought I would include the surface humidity
comparisons between HADCRUH and ERA, and decided last night to set off
the data retrieval needed to plot relative humidity time series from ERA
(-40 and -Interim versions).
I've plotted the results this morning (for all land points, not just for
the grid boxes where there are HADCRUH data) and attach them for
selected areas. I hope the plots are self-explanatory. Time series are
adjusted to give zero mean value for the overlap period between ERA-40
and ERA-Interim (1989-2001).
These plots give a clear impression of a downward trend in relative
humidity (cf AR4, where it is stated "trends are uncertain, but suggest
that it [relative humidity] has remained the same overall" - perhaps you
wrote this Phil). This may not be that inconsistent with earlier
evidence - Kate's thesis summary does indicate a trend in specific
humidity that is slightly less than that expected for constant relative
humidity, although you state that trends in RH are statistically
indistinguishable from zero, and the indication of a downward trend in
the ERA results is strengthened by the low values analysed for recent years.
What do you make of this? I've used a quite standard set of programs to
do the calculations, so it's unlikely (but certainly not impossible)
that I've made a mistake. I would be wary about the quality of the
reanalysis for the tropics, but reasonably confident about it for much
of the extratropics.
I'll not have much time to work on this for the next five or six weeks,
but things may get easier in Nov/Dec, and by then ERA-Interim will be
close to the present day.
Best regards
Adrian Simmons
European Centre for Medium-Range Weather Forecasts
Shinfield Park, Reading, RG2 9AX, UK
Phone: +44 118 949 9700
Fax: +44 118 986 9450

Prof. Phil Jones
Climatic Research Unit Telephone +44 (0) 1603 592090
School of Environmental Sciences Fax +44 (0) 1603 507784
University of East Anglia
Norwich Email

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