Source: Calculator.net

Dew point is defined as the temperature at which a given volume of air at a certain atmospheric pressure is saturated with water vapor, causing condensation and the formation of dew. Dew is the condensed water that a person often sees on flowers and grass early in the morning. Dew point varies depending on the amount of water vapor present in the air, with more humid air resulting in a higher dew point than dry air. Furthermore, the higher the relative humidity, the closer the dew point to the current air temperature, with 100% relative humidity meaning that dew point is equivalent to the current temperature. In cases where the dew point is below freezing (0°C or 32°F), the water vapor turns directly into frost rather than dew.

While perception varies between people, and people on some level can acclimatize to higher dew points, higher dew points are generally uncomfortable because the humidity inhibits proper evaporation of sweat, making it more difficult for a person's body to cool down. Conversely, lower dew points can also be uncomfortable, causing skin irritation and cracking, as well as drying out a person's airways. The US Occupational Safety and Health Administration recommends that indoor air temperatures be maintained between 68-76°F with a relative humidity of 20-60°.

Dew point is also considered in general aviation to calculate the probability of potential issues such as carburetor icing as well as fog. In some cases, devices known as dew point meters are used to measure dew point over a wide range of temperatures. These devices consist of a polished metal mirror that is cooled as air is passed over it. The temperature at which dew forms on the mirror is the dew point.

See also Humidity.

Here is a link to a Dew Point Calculator.