In simple terms, the dew point is the temperature at which the air can no longer hold all of its water vapor so that some of that vapor condenses into liquid water, forming dew or frost. More technically, it is a meteorological term that refers to the temperature to which air must be cooled to become saturated with water vapor, assuming constant moisture content and air pressure. When the air temperature drops to the dew point temperature, relative humidity becomes 100%.
- The dew point is the temperature where air is saturated with water vapor. Below this temperature, dew (or frost) forms.
- At the dew point, the relative humidity is 100%.
- Increasing humidity (amount of water vapor) or increasing pressure raises the temperature of the dew point.
- Dew point figures into the heat index. When humidity is high, perspiration doesn’t evaporate, it feels sticky or muggy, and the risk of heat exhaustion and heat stroke increase.
Dew Point and Relative Humidity
The dew point closely relates to relative humidity, which is a measure of the amount of water vapor present in the air compared to the maximum amount the air can hold at that temperature. As the air temperature approaches the dew point, the relative humidity increases. When the air temperature equals the dew point, the air is saturated, and the relative humidity is 100%. Conversely, a high difference between the air temperature and the dew point indicates low relative humidity.
Dew Point and Pressure
At first glance, it might seem that dew point and pressure are unrelated, but they are indirectly linked through the ideal gas law. Pressure affects air temperature, and air temperature affects dew point. However, for a given temperature and humidity, changes in pressure have a minimal direct effect on the dew point. Still, as pressure decreases with altitude, air tends to cool, which can lower the dew point.
For example, Denver is at a higher elevation than New York City and typically has a lower barometric pressure. If the temperature and the dew point are the same in both cities, the air in Denver contains more water vapor. Or, if the temperature and amount of water vapor are the same, then New York has a higher dew point.
Dew Point and Human Comfort
Dew point plays a significant role in human comfort. The higher the dew point, the muggier it feels, because perspiration doesn’t evaporate as easily. When the dew point is below 60°F (16°C), most people find the air comfortable. Between 60°F and 70°F (16°C and 21°C), it starts feeling more humid. Above 70°F (21°C), it feels uncomfortable or oppressive. Low dew points, below 40°F (4°C), feel noticeably dry.
This ties into the heat index, which factors in the dew point (relative humidity) and temperature. The heat index rises when it’s not only hot outside, but has a high dew point. So, a “dry heat” really does feel better than when it’s hot and humid.
Applications of Dew Point
Understanding dew point is critical for many fields, including meteorology, HVAC (heating, ventilation, and air conditioning), and industrial processes.
- Meteorologists use dew point to predict weather patterns. A high dew point indicates high moisture content in the air, which can lead to cloud formation and precipitation.
- Pilots use dew point for predicting the height of the cloud base and the risk of fog and carburetor icing.
- In HVAC systems, knowing the dew point helps control indoor humidity levels. This provides comfortable environments and prevents condensation, which can damage the infrastructure.
- Various industrial processes (e.g., painting, coating, or drying) are sensitive to humidity and dew point to ensure quality and efficiency.
Frost Point and Cloud Point
Two other important concepts related to the dew point are the frost point and the cloud point. Understanding these terms is crucial for predicting various weather phenomena, like frost occurrence and cloud formation.
The frost point is the temperature at which water vapor in the air freezing into frost. This occurs when the dew point is below the freezing point of water.
The cloud point is the temperature below which solids condense out of liquids, creating a cloudy appearance. In meteorology, it means something different. Here, the cloud point refers to the altitude at which water vapor starts to condense and form clouds, which depends on the temperature and dew point gradient with altitude.
Calculating and Approximating the Dew Point
There are various formulas for calculating the dew point, such as the Magnus-Tetens formula. This formula requires temperature, relative humidity, and a variety of constant sets. However, there is a simple approximation that relates dew point, temperature, and relative humidity that is accurate within about 1°C, providing the relative humidity is above 50%:
Tdew point ≈ T – (100 – RH)/5
RH ≈ 100 – 5(T – Tdew point)
Here, RH is relative humidity and T is the dry bulb temperature.
In other words, for every 1°C difference between the dew point and dry bulb temperature, relative humidity decreases by 5%. The relative humidity is 100% when the dew point equals the dry bulb temperature.
The Hygrometer
The hygrometer is a device that measures the dew point. It consists of a polished metal mirror. Condensation forms on the mirror when the temperature cools to the dew point.
References
- Lawrence, Mark G. (2005). “The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications”. Bulletin of the American Meteorological Society. 86 (2): 225–233. doi:10.1175/BAMS-86-2-225
- Lin, Tzu-Ping (2009). “Thermal perception, adaptation and attendance in a public square in hot and humid regions”. Building and Environment. 44 (10): 2017–2026. doi:10.1016/j.buildenv.2009.02.004
- Wallace, John M.; Hobbs, Peter V. (2006). Atmospheric Science: An Introductory Survey. Academic Press. ISBN 978-0-08-049953-6.