Re: Does it take less fuel to fly a hot air balloon on a hot day or a cold day

Dear Mr Pegelow

Thank you  for the interesting question.

The gas flame has to maintain a certain temperature difference 
dT=T_out-T_in between the inside of the ballon and the outside air so that 
the "buoyancy" can work properly. The chief heat transfer mechanism 
convection (and also heat conduction) depends only on the temperature 
gradient dT. For them, T_out doesn't matter, no effect. For heat radiation, 
however, the Stephan Bolzmann Law says that the heat loss is 
dQ=Q_Emission-Q_Absorbtion=sigma.area.(T_out+dT)^4-sigma.area.T_out^4. With 
the Binomi formula one finds out that the radiation heat loss is roughly 
proportional to the absolute temperature T_out (in Klevin).  This means 
that radiation losses are bigger on a warmer day. 

(There might be another effect. The gas flame is located under the opening 
of the ballon sphere. The hot air raises and fills the ballon. I reckon 
that the heat losses produced by turbulence are smaller on a cold day.)

It might be interesing to know what someone who knows the  balloning 
practise says.