What is dew point?

Unsaturated air to keep the water vapor partial pressure unchanged (ie keep the absolute moisture content unchanged) lowering the temperature, so as to reach the saturation temperature called the "dew point". The temperature dropped to the dew point, wet air, there will be condensation of water droplets precipitate.
Wet air dew point is not only related to temperature, but also how much of the moisture content in wet air. The water content of the dew point and less water content and low dew point.