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What colour is best for inside a Stevenson Screen?

PostPosted: 2013 May 10, 08:44
by Niall Dollard
I have a small wooden screen and have continually maintained both the inside and outside with white paint. More recently I see that some screens (the plastic version) are coloured black on the inside.

Obviously white is best for the outside but what is best for the inside or does it indeed make any difference?

Re: What colour is best for inside a Stevenson Screen?

PostPosted: 2013 May 11, 09:15
by Len Wood
I think black has come into favour because it will not reflect the diffuse light that gets into the screen.

As regards infra red radiation, I believe the emissivity of the white surface is very close to the black surface so it doesn't matter.

I assume the Met Office have done research on this with different screens but I have not seen the paper.

I have heard that it is more difficult to read the thermometers with black inside the screen.

Re: What colour is best for inside a Stevenson Screen?

PostPosted: 2013 May 11, 18:25
by Stephen Burt
There's quite a bit of confusion about the new plastic screens with a black interior.

In various trials conducted by the Met Office and other national met services, plastic screens with black interiors - both Stevenson-type and 'stacked plate' AWS models - generally gave slightly better results (i.e. less radiational warming) than those with white interiors, but the reasons why this was so were not rigorously determined and are not covered in any detail in the various published papers on the comparison trials. Despite considerable detective work I have been unable to find the underlying rationale for this to be so. The most likely reason appears to be that the white plastic used for the screen interiors, although opaque in visible wavelengths, was actually slighly transparent or translucent in the infra-red, whereas the black formulation plastics were opaque in the infra-red (or at least more so than the white plastics).

It is surprising that this important determinant was not clarified before the screens were widely introduced, because with the right equipment it would not be difficult to test this hypothesis. However, it does appear that the matter is now largely academic, as these screens are now UK Met Office (and Met Eireann) standard - I included some discussion on them in my recent book, with references to numerous published comparison trials.

For existing wooden screens, the interior should continue to be painted white, per existing practices. Painting it black inside probably won't make much difference to the penetration of infrared radiation, which will be determined by the radiative transmission properties of the wooden louvres rather than their surface colour, but my own experience shows that a (Stevenson-type) screen with a black interior begins to heat up much more quickly when the screen door is open, particularly when the sun is shining. As my screen is only opened briefly once per day, for the morning observation, this has only a limited effect, but if the screen was opened, say, hourly or for a noon or mid-afternoon observation, the effect on maximum temperatures in summer would be quickly apparent. In any case, it would be unfortunate to change the existing properties of the screen (i.e. by painting the inside black) when this would likely undermine the homogeneity of the existing record. I think it would be fair to say, however, that painting the inside of a wooden Stevenson screen would not generally need to be undertaken as frequently as painting the outside - perhaps every second time would be sufficient.

I have also heard comments that reading the thermometers is more difficult in a screen with a black interior, owing to the reduced visual contrast, but I have not personally found this to be a problem when I replaced my large wooden Stevenson screen with a Metspec plastic model last year, even though I overlapped both screens for 7 months to assess whether there were any significant differences.

Hope this helps.

SB

Re: What colour is best for inside a Stevenson Screen?

PostPosted: 2013 May 11, 18:26
by Graham Easterling
I read something in a PDF file I found on the MetO site (though I can't find it now) which stated that black was the preferred inside colour for plastic screens, but white was OK for wooden ones, which seemed a bit odd.

I understand that black would not reflect the diffuse light, but by absorbing more wouldn't it also heat up to some extent? Has anyone (Stephen springs to mind) done any comparisons between screens with white & black interiors?

UPDATE - Stephen replied a few seconds before I posted the above - I must psychic!