This can be expressed as a simple rule of thumb:įor every 1 ☌ difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH=100% when the dew point equals the dry bulb temperature. This expression is based on the 'Magnus' (or 'Magnus-Tetens') approximation for the saturation vapor pressure of water in air as a function of temperature. Where the temperatures are in degrees Celsius and 'ln' refers to the natural logarithm. OK for most, but all perceive the humidity at upper edge Somewhat uncomfortable for most people at upper edge Most inhabitants of these areas will consider dew points above 21 ☌ (70 ☏) to be oppressive. Those accustomed to continental climates often begin to feel uncomfortable when the dew point reaches between 15 and 20 ☌ (59 to 68 ☏). Humans tend to react with discomfort to high dew points, as a high dew point corresponds with a high ambient temperature (causing the body to perspire and produce sweat) and/or a high relative humidity (which inhibits the evaporation of sweat, by which the body is cooled) as a result the body may overheat, resulting in discomfort.Ī lower dew point, meanwhile, corresponds to a lower ambient temperature or lower relative humidity, either of which allow the body to more effectively regulate its temperature to avoid overheating. Coming back to the New York - Denver example, this means that if the dew point and temperature in both cities are the same, then the mass of water vapor per cubic meter of air will also be the same in those cities. In the same way, increasing the absolute humidity after a temperature drop brings the dew point back up to its initial level. Reducing the absolute humidity will bring the dew point back down to its initial value. If the temperature rises without changing the absolute humidity, the dew point will rise accordingly, and water condenses at a higher pressure. For this reason, the same dew point in New York and Denver (which is at a much higher altitude) will imply that a higher fraction of the air in Denver, CO consists of water vapor than in New York, NY.Īt a given temperature but independent of barometric pressure, the dew point indicates the absolute humidity of the air. In the same way, increasing the mole fraction after a pressure drop brings the dew point back up to its initial level. making the air dryer, will bring the dew point back down to its initial value. If the barometric pressure rises without changing this mole fraction, the dew point will rise accordingly, and water condenses at a higher temperature. Dew point is a monotonic function of the partial pressure of water vapor, so dew point can be determined from partial pressure of water vapor alone, and vice versa.Īt a given barometric pressure, independent of temperature, the dew point indicates the mole fraction of water vapor in the air, or, put differently, determines the specific humidity of the air. The formation of dew would occur at the dew point even if the only gas present is water vapor. The behavior of water vapor does not depend on the presence of other gasses in air. With higher temperatures, the equilibrium partial pressure of water vapor increases and more water evaporates. The graph above shows the maximum percentage (by mass) of water vapor that can exist in air at sea level across a range of temperatures. The dew point is an important statistic for general aviation pilots, as it is used to calculate the likelihood of carburetor icing and fog, and estimate the height of the cloud base.

Given a constant dew point, an increase in temperature will lead to a decrease in relative humidity.Īt a given barometric pressure, independent of temperature, the dew point indicates the mole fraction of water vapor in the air, and therefore determines the specific humidity of the air. A high relative humidity indicates that the dew point is closer to the current air temperature if the relative humidity is 100%, the dew point is equal to the current temperature. The dew point is associated with relative humidity. When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.

The dew point (sometimes spelled dewpoint) is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water.

Relative humidity vs dew point