This is related to the process of water evaporation.
A dry bottle loses heat when the energy of the chaotic vibrations of its molecules is transferred to the air. The rate of this process depends on the surface area of the bottle and the temperature difference between it and the surrounding air.
When the vessel is wrapped in a wet cloth, an additional pathway for heat dissipation is created: water molecules, which typically have high energy, detach from the surface and carry heat away with them. Furthermore, due to the irregularities of the fabric and the water film between the fibers, the surface area for heat dissipation is increased. However, this effect should not be overestimated: studies show that wet cloth accelerates cooling by only 10–20%.
