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

My students while attending a physics/engieneering competition encountered 
a simple question for which i have no answer. 

Will it require less fuel to fly at and maintain a given altitude in a hot 
air balloon on a hot day or a cold day? (notice that it would be the same 
altitude in both cases)

My first thought was that on a cold day, the balloon would radiate heat to 
the surrounding environment faster than on a hot day, when the temperature 
inside the balloon was closer to the surrounding air temperature. However 
I am not certain and would like to find the answer to this question which 
has vexed me for quite some time. Thankyou for any help.

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