The ability to fly hot air balloons in the winter is limited mostly by the ability of the participants to withstand the cold.
The balloons themselves fly well in cold air. Because the temperature difference between inside and outside the balloon, not the absolute inside temperature, determines the lift it develops, a much lower internal temperature is sufficient to fly in cold weather.
However, if the liquid propane in the fuel tanks is too cold (0 °C/32 °F or less) it does not generate sufficient vapor pressure to adequately feed the burner(s).
So you can very well fly a hot air balloon in the winter. This is definitely our slow season, though, just for the fact that not many people like to be freezing high up in the sky on a cold, wintery morning.
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