When moist air is cooled and cooled down to a certain temperature, the water vapor in the air begins to condense and form dew. The temperature at this point is called the dew point, and the humidity at the dew point is 100%. In other words, the dew point is the temperature of saturated moist air having a partial pressure of water vapor equal to its partial pressure of water vapor. Therefore, if we know the partial pressure of water vapor or absolute humidity, we can determine the dew point directly from the moist air diagram.