On 31 Mar, 19:25, Graham Easterling
wrote:
On 31 Mar, 18:13, Steve J wrote:





On 31 Mar, 15:46, "ronaldbutton" wrote:

A questions for the experts...would it not be better to construct Stevenson
screens from polishes aluminium or any reflective metal rather than a white
painted surface ,which in my experience absorb some heat especially when
they get dirty in urban situations...?.

Ron

The new screen that the Met Office have put into our our enclosure is
white plastic; meanwhile our own double sized screen needs painting -
a job that I loathe, and have been putting off for a while.

I like the plastic personally!

Steve Jackson
Bablake Weather Station
Coventry UKwww.bablakeweather.co.uk

Steve, as you've got a plastic and a wood screen, is there any
difference in the readings?

Graham
Penzance- Hide quoted text -

- Show quoted text -

There was a Met Office comparison run at Beaufort Park in (I think)
2000/2001 over about 8 months which showed no significant difference.
I don't have it to hand, but the differences were slight between the
new plastic screens and wooden Stevenson screens of the same size - as
I recall there was more of a difference from the large-pattern
Stevenson screens, almost certainly because of thermal inertia/larger
thermal mass/lower ventilation, etc.

I still use a large-pattern wooden screen, and have done so for 25
years. My current one is only 15 years old, but I will use it until it
rots, then I'll buy a plastic screen. I too hate the maintenance. But
I have also seen many of the plastic screens turn a rather dirty grey
over time - I have two uPVC AWS screens which have gone this way
although they are only 5-6 years old, and the one at Kew Gardens can
only be described as 'mid grey' although it was new in late 2003. I
believe this is due to UV curing, but I wonder what the long-term
impacts are on readings as the screen gets gradually less reflective
to inbound solar radiation. I also don't know whether a regular wipe-
over minimises this colour change - I suspect not as both my small
screens are regularly wiped down. I do know that the sites used for
Central England Temperature stats are mandated to continue with wooden
Stevenson screens for now until the long-term impacts, if any, are
clearer - after all, these screens have been in use for less than 10
years today. But compared with the gradual deterioration of screens
over the years I'm sure this is a big improvement.

Incidentally, early trials of these screens in the Antarctic proved
less than successful, as the uPVC became brittle at low temperatures
and shattered with even a gentle blow, such as shutting the door a bit
too hard. I believe the formulation of the plastic has now changed and
they are better, although not perfect. I guess this wouldn't be a big
problem in Penzance though, Graham ... !

--
Stephen Burt
Stratfield Mortimer, Berkshire