Re: Clear Sky Detector Sees Clouds

[BARTHOLDI Paul <Paul.Bartholdi@obs.unige.ch>]

Hi Tom,

This is a nice idea.  I remember a similar instrument built in France around 
1950 and used in the Sahara to get very cold temperatures. Under clear
saharian weather, they could get more than 60 centrigrad (~33 Farenheit ?)
difference between to radiating part and the surrounding.

One trick they used was to have three concentric boxes, probably similar
to your foam/foil sandwich well, each one closed with a thin polyethylene
foil to stop convection. The distance between each box and the surrounding
one was about 1 inch if I remember right. Their field of view was very large
probably about 90 degrees.

I did myself the experiment in Switzerland, with a simple Hg thermometer
in a foam box covered with Al foils inside and ouside. The polyethylene foil
was very important to get the temperature down whatever the wind. We got
differences arounf 10-15 centigrades, it was very sensitive to even tiny
invisible clouds. What is measured is mostly the water vapor content in the
atmosphere. It reflects down the IR from the Earth.

I never had the time to build a more modern version as you did, but an idea
I had was to use a TEC in null mode, that is keep the radiating surface at
the same temperature as the other face and measure just the flux going out
of the radiating plate. The fact that all temperatures are the same would
reduce the effect of conductivity.

I am sure you would build that without effort. What do you think of the 
idea ?

Have a good week-end !     Regards,     Paul