The dew point is the temperature at which air must be cooled, at a constant pressure, to become completely saturated with water vapor, reaching 100% relative humidity. This precise atmospheric measurement indicates the absolute moisture content in the air. When the air temperature drops to this level, water vapor condenses into liquid water, forming dew, fog, or frost. Finding this temperature is fundamental in meteorology and provides a reliable gauge of humidity, independent of the current air temperature.
Measuring Dew Point with Physical Instruments
A traditional and highly accurate method for determining the dew point involves using a sling psychrometer, which is a pair of thermometers mounted together. The dry-bulb measures the ambient air temperature, while the wet-bulb has its bulb covered in a water-soaked cotton wick.
The instrument is whirled rapidly to promote evaporation, which draws heat away and causes the wet-bulb temperature to drop. The amount of cooling is directly related to the moisture content of the air; drier air allows for faster evaporation and a greater temperature drop.
The difference between the readings, known as the wet-bulb depression, is then used with a specialized psychrometric chart. This chart cross-references the dry-bulb temperature and the wet-bulb depression to find the corresponding dew point temperature.
Alternatively, electronic dew point meters, or chilled-mirror hygrometers, offer a more automated approach by actively cooling a mirrored surface until condensation forms, and then measuring the temperature of that surface.
Calculating Dew Point from Existing Data
When physical instruments are unavailable, the dew point can be accurately derived using existing air temperature and relative humidity data, commonly provided by weather stations. This mathematical derivation relies on the relationship between these two variables and the saturation vapor pressure of water.
The air temperature indicates the maximum amount of moisture the air can potentially hold, while the relative humidity expresses the percentage of that potential that is currently filled. Meteorological and online calculators frequently employ approximation formulas, such as the Magnus-Tetens equation, to solve for the dew point.
These formulas translate the relationship between the measured temperature and the relative humidity into a single temperature value. To use an online calculator, one only needs to input the current air temperature and the relative humidity percentage. The closer the relative humidity is to 100%, the closer the dew point temperature is to the actual air temperature.
Why Dew Point is a Key Indicator
The dew point is a more reliable indicator of atmospheric moisture and human comfort than relative humidity because it represents the actual water vapor content of the air. When the dew point is \(55^\circ \text{F}\) or lower, the air feels dry and comfortable, as sweat can evaporate easily from the skin.
As the value rises above \(55^\circ \text{F}\), the air begins to feel sticky or muggy. A dew point of \(65^\circ \text{F}\) or higher is considered oppressive, making the air feel heavy and reducing the body’s ability to cool itself through sweating.
In weather forecasting, this value is used to predict the formation of low-level moisture phenomena. If the overnight low temperature is forecast to drop to the evening’s dew point, forecasters predict the formation of dew or fog. Furthermore, a rising dew point can signal the potential for substantial precipitation and severe weather, as it indicates an influx of atmospheric moisture to fuel storms.
In agriculture, the dew point is important for scheduling irrigation and anticipating frost formation. When the dew point is below \(32^\circ \text{F}\), it is referred to as the frost point, and if the temperature falls to this level, damaging frost or ice is likely to form on surfaces.