Sunshine differences between Reading and Wokingham

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Sunshine differences between Reading and Wokingham

Postby greg_gruner » 2015 Jun 05, 15:02

Looking at the May figures (which will appear in the COL report in due course) I see there is quite a large difference in the May sunshine total between Reading University and Wokingham:

Reading University: 211.9 hours (electronic) or 210.1 hours (Campbell-Stokes) [taken from ... aries.html]

Wokingham: 166.1 hours (this is electronic) [taken from Bernard Burton's website]

Normally these figures are within 10% of each other, but in May it's a whopping 28% higher at the university (and these sites are only around 8km apart).

I'm wondering whether this is an instrumentation issue at one of the sites, or a real difference in sunshine. If the latter, it is quite a point of note.
Greg Gruner
Farnborough, Hampshire
Posts: 224
Joined: 2012 Feb 04, 17:28

Re: Sunshine differences between Reading and Wokingham

Postby Stephen Burt » 2015 Jun 05, 21:33

Yes, these differences are puzzling. But the plot thickens!

- The Kipp & Zonen total from the university's own unit was 211.9 h as Greg states, however the total from the Met Office K&Z sensor about 20 m distant and with a similar exposure was only 183.2 h. The university's Campbell-Stokes recorder, about 300 m distant and on the roof of the Met department, recorded 210.1 h. On the face of it therefore, the MO K&Z unit looks to be wrong, low, possibly because of missing data. However, that was checked and found not to be the case.

- The Met Office K&Z unit at the university (183.2 h) is supported by my own total, also from a logged K&Z unit, 10 km distant of 186.7 h. Meanwhile Roger's C/S unit in Maidenhead showed 181.8 h, contrasting with Bernard's figure for Wokingham (part way between Reading and Maidenhead, and an Instromet sensor) of 166.1 h, as again quoted by Greg.

So, from bottom to top, and all within 20 km or so, we have -

Wokingham (Instromet) 166.1 h
Maidenhead (C/S) 181.8
Reading University (MetO K&Z) 183.2
Stratfield Mortimer (K&Z) 186.7
Reading University (C/S) 210.1
Reading University (UoR K&Z) 211.9

The Reading University C/S cards have been checked and found to be accurate within the variations expected from individual analysis (typically 5%). But that doesn't explain why the MetO K&Z unit read so much lower. Which is right? Unfortunately the pyrheliometer at the university, which would provide a definitive answer, is currently out of operation. However, Roger and I are looking very closely into the three university figures, and comparing the hourly totals from the university, Stratfield Mortimer and Wokingham in an attempt to get to the root of the differences. If we come up with a hypothesis, I'll post it here!

Stratfield Mortimer, Berkshire - central southern England
51.4°N, 1.0°W, 60 m AMSL, station grade A - AAAA47R
Records commenced here 1987 - local records available back to 1862
The Weather Observer's Handbook:
Stephen Burt
Posts: 247
Joined: 2011 Dec 02, 19:36
Location: Stratfield Mortimer, Berkshire; a well-exposed rural site, 10 km SW of Reading

Re: Sunshine differences between Reading and Wokingham

Postby greg_gruner » 2015 Jul 06, 19:41

June seems to have produced a similar sunshine discrepancy to May:

Reading University: 242.9 hours (electronic) or 241.5 hours (Campbell-Stokes)

Wokingham: 183.6 hours (this is electronic)
Greg Gruner
Farnborough, Hampshire
Posts: 224
Joined: 2012 Feb 04, 17:28

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