Dew point

Definition

Dew point is the critical temperature at which a parcel of air will become saturated with water vapor if it is cooled at constant pressure and constant water vapor content. Dew point is a measure of humidity, or atmospheric vapor content, since the higher the dew point temperature of an air mass, the greater the water vapor content. Compared to warm air masses, cold air masses have smaller water-holding capacities and therefore lower dew points. Dew point is also related to evaporation and condensation. The closer the dew point is to actual air temperature, the lower the rate of evaporation. When actual air temperature cools to the dew point, condensation will occur.

Significance for Climate Change

A rise in the concentration of greenhouse gases (GHGs) in the atmosphere makes available more energy at the Earth’s surface. This additional energy could heat the atmosphere by way of the sensible heat flux, increasing temperature, or it could evaporate water at the Earth’s surface via the latent heat flux, increasing humidity. The latter course of events would lead to higher dew points. When water is freely available, the latent heat flux will always dominate, meaning that energy otherwise available to heat the atmosphere will be used in the evaporative transpiration of moisture from the surface to the atmosphere.

Most of Earth’s surface is either water (approximately 70 percent is occupied by oceans, seas, and lakes) or land that is well supplied with precipitation. Thus, most of the additional energy at the surface due to an increased concentration of GHGs in the atmosphere will enhance the latent heat flux. The resulting warming of the atmosphere would be less in this case than if all the additional available energy were accounted for by an increase in the sensible heat flux alone. The dew point would then rise accordingly.

Water vapor is by far the most important GHG in the atmosphere, so a rise in global dew point would add significantly to the greenhouse effect, leading to enhanced warming and further enhanced evaporation and transpiration, and so on. This self-reinforcing cycle is known as a positive feedback effect. On the other hand, a rise in dew point could result in increased cloudiness as moisture is added to the Earth’s atmosphere. Clouds, especially low clouds, act to reflect incoming energy from the Sun, energy that would otherwise be absorbed at Earth’s surface. The result is to reduce the energy available for heating the air and for the evaporation and transpiration of moisture. This negative feedback, or stabilizing effect, would only partially compensate for the warming effect of higher dew points.

Brackett, Ron and Jonathan Belles. "Weather Words: Why the 'Dew Point' Temperature Matters." The Weather Channel, 16 July 2024, weather.com/news/weather/news/2024-07-16-weather-words-dew-point-temperature. Accessed 26 Feb. 2025.

"Dew Point." Britannica, 25 Feb. 2025, www.britannica.com/science/dew-point-temperature. Accessed 26 Feb. 2025.

Eliasen, Terry. "What Is Dew Point? How It's Different Than Humidity and a Better Measure of "Sticky" Weather." CBS News, 18 Sept. 2024, www.cbsnews.com/boston/news/what-is-dew-point-humidity-sticky-weather/. Accessed 26 Feb. 2025.