Water droplets form on the outside of a glass of ice water due to condensation. The temperature of the glass is cold enough to cool down the water vapor in the surrounding air, so it turns from gas to liquid. Therefore, drops form more readily during warm weather than cold weather.
Condensation is the basic transformation of a substance from gas to liquid. All air in the atmosphere contains some degree of water vapor. If the air is cooled past the dew point of water, which depends on pressure and concentration, condensation will occur. Condensation will occur more easily in saturated air than dry air.