Effective Sky Temperature

[Russ0448]

I'm trying to calculate the effective Sky Temperature at a given point on the earth. All that I have been given so far is a rule of thumb for locations around the 30 to 45 deg N Latitude to be 10 to 20 deg C below that of ambient for a clear sky day. For a cloudy day, the effective sky temperature comes out to be 1 to 2 deg C below ambient.I'm looking for something a little more exact.
Any help would be greatly appreciated.

 

[IRstuff]

Sky temperature for what purpose?

Since the sky radiance is a combination of many factors, there is no single "sky temperature". The photometric spectral sky temperature is something like 30,000 K, which reflects the fact that the sky is blue.

Sky temperature in the infrared is a function of elevation angle and can range from nearly ambient horizontally to 250 K at zenith.

TTFN

 

[ko99]

Russ,

This may not be what you're looking for, since it won't help with radiation calculations, but I found a little program that calculates air properties (temperature, pressure, density, viscosity, sound speed) at any altitude up to 40,000 feet, for a given set of sea level conditions.
If you're interested and I can send you a copy.

ko

 

[Russ0448]

The radiation of the surface that I'm looking at (from my understanding) is a by product of the surface temperature and the effective sky temperature and not the directly surrounding ambient, My assumption for this problem is that the emissivity and the absorptivity of the atmosphere are equal being that the wavelengths are fairly close. The equation that I'm using is (epsilon*sigma*(Tsuf-Tsky) Where Tsky is the effective sky temperature. I've pulled this information from Incropera and Dewitt's fourth edition "Fundamentals of heat and mass transfer".

I think that the temperature that I looking for is actually an average temperature over which the high density of CO2 and H2O lie in the atmosphere (where the radiation is actually absorbed by the atmosphere).

I agree, with the above statement that this is a product of the elevation angle. I will be considering the “stow” position only. Zero degree look angle relative to the Azimuth Axis (looking directly up)

 

[IRstuff]

There is a Mathcad library file that perhaps might shed some light on this subject:

http://www.mathcad.com/library/LibraryContent/clearnightfrost.mcd

TTFN

 

[aviat]

Your best bet is to contact your local meteorologist. They should have that information or know where to find it.

Cheers

 

[jimwilson]

The ASHRAE Fundamentals Handbook (Chapter on Solar Energy) has 3 charts which plot average night sky temperature below ambient for the US. They reference the original work done in the early 1980's and observe that not much more practical work has been done in this area.