On 09/06/2014 12:50, Ian Bingham wrote:

Just bought a new (Met 01) Stevenson Screen. The interior is black
instead of white as before. Does anyone know the scientific principle
behind this - why should a black interior be superior to the usual
white? It seems a bit of a contradiction as the black surface is shiny.

ISTR they have determined empirically that it works better to prevent
ingress of solar heat when the unit is in the sun.

In long wave thermal band IR anything that isn't a shiny polished
metallic surface is "black" to a very good approximation.

It is possibly due to the amount of scattered light getting in but these
things are incredibly finely balanced. I'd also consider that making
the slats out of the foil backed polystyrene foil facing inwards might
also reduce the thermal radiation getting into the enclosure.

Smooth mirror finishes are poor radiation emitters which probably
explains the interior gloss finish.

Observatory domes are now painted a semimetallic grey white to prevent
supercooling at night which is a problem with pure whites. The cold air
then drips into the dome aperture and creates turbulence damaging image
quality. Modern observatories are actively cooled during daytime.

I'd hazard a guess that super cooled air also runs down the side of a
Stephenson screen at night and slightly biases the readings lower
overnight. I guess for a weather station the thermal load in sunshine is
a much more serious consideration.

--
Regards,
Martin Brown