The atmosphere generally contains water vapor. This is what we call humidity. The amount of water vapor that the atmosphere can hold depends on temperature, and decreases as temperature decreases. The dew point is the combination of temperature and humidity at which water vapor in the air will condense. When hot air comes into contact with a cooler surface the amount the air that is cooled will no longer be able to hold as much water vapor. If it contained more water vapor than it can hold at the new temperature it will reach its dew point and water vapor will condense. Air that contains the maximum amount of water vapor that it will hold at a given temperature is said to be saturated. Any time that air is saturated with water vapor and the temperature is lowered water will condense.