The evaporation of fuel in the carb cools the mixture and below ~ 50 °F, and if the humidity is high enough, the wall temperature in the carb will drop below freezing and exterior frost can be seen. Frost can also form in carb venturi (from the humid air), and over the transition port and butterfly and mess up the mixture control. It might run ratty until the manifold heats up enough to prevent this. It"s a problem with leaner mixtures so Detroit added the heated intakes on engines in the late "60"s and early "70"s when the first emmission controls came in on cars. Adding Heet additive to the fuel will not prevent this only a heated intake will. That"s why aircraft engines have carb heat. It"s obviously more critical for them.