If you have a can of Coca-Cola or any other refreshment, it is hard to keep it cool when left outside the fridge. The guys at Inside Science explain why the can looses its cool: it is due to the condensation of water vapors outside the surface of the can. Water in the form of vapors comes in contact with the walls of the can, they become liquid and then they increase the flow of heat from the surroundings to the can by more than 2 times.
That means that if the air is humid, then the can will lose its cool way faster. Via Inside Science:
The solution? Beer koozies not only insulate a can, but also keep condensation from forming on the outside, which is important for keeping the beverage cool.