What Is Dew Point?

News
By published
(Image credit: Dreamstime)

The dew point is the temperature at which air is saturated with water vapor, which is the gaseous state of water.

When air has reached the dew-point temperature at a particular pressure, the water vapor in the air is in equilibrium with liquid water, meaning water vapor is condensing at the same rate at which liquid water is evaporating.

Dew point is closely linked to relative humidity, which is the ratio of the pressure of water vapor in a parcel of air relative to the saturation pressure of water vapor in that same parcel of air at a specific temperature. Relative humidity (RH) is expressed as a percentage.

The relative humidity is 100 percent when the dew point and the temperature are the same. If the temperature drops any further, condensation will result, and liquid water will begin to form.

Compared to relative humidity, dew point is frequently cited as a more accurate way of measuring the humidity and comfort of the air, since it is an absolute measurement (unlike relative humidity).

Most people are comfortable with a dew-point temperature of 60 degrees Fahrenheit (16 degrees Celsius) or lower. At a higher dew point of, for example, 70 F (21 C), most people feel hot or "sticky" because the amount of water vapor in the air slows the evaporation of perspiration and keeps the body from cooling.

Follow Marc Lallanilla on Twitter and Google+. Follow us @livescience, Facebook & Google+.

Marc Lallanilla
Marc Lallanilla
Live Science Contributor
Marc Lallanilla has been a science writer and health editor at About.com and a producer with ABCNews.com. His freelance writing has appeared in the Los Angeles Times and TheWeek.com. Marc has a Master's degree in environmental planning from the University of California, Berkeley, and an undergraduate degree from the University of Texas at Austin.