Atmospheric infrared radiation at night
 

Hi all.

I need an equation for the calculation of atmospheric infrared radiation from the clear sky at night to the earth surfaces in terms of irradiation angle, wavelength, temperature and humidity or at least an atmospheric irradiation equation for a specified humidity.

My questions are presented as follows :
1- Is there a database/reference for such a formulation ?
2- How can I assign such an equation at night in Fluent Commercial software? For example, should I implement such an equation as a boundary condition?

Thanks a lot for any advice or help.