Water vapor turns into a liquid whenever the air in which the vapor is located is cooled enough to cause the vapor to reach its dew point. Cooling air, such as the night air that cools at ground level, causes water vapor to condense, forming water droplets. This results in the formation of fog or dew. Similarly, when humid air comes into touch with a cold surface, such as a mirror, the vapor droplets condense and form a coating of water droplets. This is referred to as condensation in general, and it occurs whenever water vapor in the air hits a surface that is cooler than the dew point.