"Eskimo Will" wrote in message
...

"Len Wood" wrote in message
...
On Thursday, 29 October 2015 11:25:16 UTC, wrote:
"Len Wood"



Hi Jim,
If you look at the record of annual rainfall for Plymouth 1874 to 2014
you
see how variable it is with no longterm trend, but clearly trends on
the
decadal timescale.

You can see an increasing trend after the drought of the mid
seventies,
and then a decrease and now slight increase again.

I have also put in dropbox the link to the graph of summer and winter
rainfall.
There is a significant increasing trend in winter rainfall and a less
convincing decrease in summer rain. Hence no trend in annual rainfall
over
the longterm.

https://www.dropbox.com/s/u0s7p9kroj...02014.jpg?dl=0

https://www.dropbox.com/s/9lfjie7ko4...0rain.jpg?dl=0

If I was you Jim I would hold fire, but if the water usage has gone up
in
recent years and/or the surface morphology has changed then you are
stuck.



Hi Len, interesting stuff, but I have some questions about the graphs?

1. How consistent was the raingauge site over the years.

My experience of looking at Dartmoor gauges has revealed that just a
small
change in location (even a few hundred metres) can make a statistically
sig.
difference to rainfall. Cowsic is a case in question, due to changes in
aspect from one side of a valley to another.

2. In the 19th Century rainguage height was not consistent, in fact a
lot of
gauges were elevated. Scientifically this was proven to produce lower
rainfalls than rainguages on the ground due to turbulence effects.

In my Dartmoor study I have been very careful to only use data where the
raingauge location has been consistent over the decades and where the
height
of the gauge above ground has been consistent. I think that is
important.

3. Have you done a statistical Student's T test to measure significance
of
the changes, I suspect that they are very sig (esp. winter). but with
the
provisos of my points one and two above.

Jim, you need to look at your local area rainfall over the decades too,
if
you can of course?


You are of course right Will to question the homogeneity of the record.
1874 to 1979 is for Plymouth Hoe Observatory. 36 m asl
1980 to 2014 is for Plymouth Mount Batten. 45 m asl
Mount Batten is about 1.8 km to SE of the Hoe site.

In 1991 I was asked by the Association of British Climatologists (now
defunct) to give a talk at their meeting in Durham on the 'History of
Observatories' about Plymouth Hoe Observatory.
I did a fair amount of research for this, from local libraries, but also
a trip to the Met Archive in Bracknell.
There is an element of vagueness about the early record because there are
never any photos of the site in early times, although the location as a
name remains the same.

2)There was no elevated rain gauge at the Hoe as far as I know.

The dear old Met Office provide the record from 1874 and label it
Plymouth Mount Batten! Mount Batten opened in 1921 when measurements
began. The Met enclosure move up the Mount Batten peninsula in 1940
during WW2.

A comparison of measurements on the Hoe and at Mount Batten shows there
is not a big difference.

https://www.dropbox.com/s/z52svn3b63...red.j pg?dl=0

https://www.dropbox.com/s/2xovemf7r8...tions.doc?dl=0

Will, have you seen my 2004 paper in Weather on Regional trends?

https://www.dropbox.com/s/lx3lrc47c0...0NAOI.pdf?dl=0

I have quite a bit of info on the Plymouth record if you want it.

3)And finally, yes, the increasing trend in winter rainfall is
statistically significant.

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Hi Len, no I haven't seen that paper. Unfortunately I cannot seem to
download from dropbox as it is asking for my account and I don't use
dropbox. Could you e-mail it to me, it looks interesting? Thanks.

One thing about Dartmoor, Graham has pointed out that gale frequency has
dropped right off and it is possible that the decrease in wind speed has
decreased the orographic enhancement (which is where we get a lot of our
rain). So it is perfectly possible for rainfall to decrease or stay the
same on high ground and increase on the coast, especially in winter!
Have winds speeds decreased in Plymouth like Penzance?


Hi Len, again!

Having just read Jim's excellent response and queries in this thread, I'm
getting interested now in my own hypothesis which I'd dearly like to test.
If I gave you a list of "orographic enhancement days" (moist SW'ly winds),
would you be able to extract the Plymouth rainfall (and wind) data for me?
If we do this well it could end up as a super paper? What do you think?
Could be a lot of work for me initially though to find the days, but that's
science - hard work.

Will
--
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
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