Dew Point

Dew point (ger: Taupunkt; fr:point de rosée)
or condensation point is the temperature at which a parcel of air must be cooled at constant pressure and humidity mixing ratio until it reaches saturation, and at which condensation of water vapor forms either as dew, cloud droplets, ice crystals, mist or fog.

All air has some water vapor in it. Water vapor is an invisible gas and the water can only be seen when it condenses. The amount of water vapor air can "hold" varies with temperature, with warmer air having the ability to hold more water vapor. If air cools down sufficiently (to the dew point temperature), it can no longer hold the moisture forcing the water vapor to condense.

The dew point may be measured directly by a dew-point hygrometer, or indirectly from wet-and dry-bulb thermometer readings. A wet-bulb thermometer is a regular thermometer with it's bulb stucking in a wet wig or sock. The dew point is calculated then at constant pressure and humidity mixing ratio. Dew point temperature (or dew point as it is often called) is reported at many weather observing stations. The dewpoint temperature is also used to infer the relative humidity of an air parcel.