The dew point is the temperature at which water vapor in air will condense because the air is saturated with water. Warm air is able to hold more moisture than cooler air so as the temperature decreases the relative humidity rises. (Relative humidity is how much water the atmosphere holds compared to the maximum amount it can hold at a given temperature). There might be a situation where air is not saturated with moisture, but as the temperature goes down it becomes saturated. Once the temperature is below the dew point water will begin to condense out of the air. Depending on how cold the air is, we will either see dew or frost form. When the temperature is below freezing the dew point is called the frost point.