The words “humidity” and “dew point” are both used to describe the moisture content of the atmosphere, but they represent two fundamentally different concepts. They are often confused in daily conversation, yet each provides distinct information about the air around us. Understanding the difference between relative humidity (a percentage) and dew point (a temperature) is key to accurately assessing both the weather and personal comfort levels.
Understanding Relative Humidity
Relative Humidity (RH) is a measure of how saturated the air is with water vapor, expressed as a percentage. This metric compares the moisture currently present in the air against the maximum amount the air can hold at its current temperature. For example, 100% relative humidity means the air is completely saturated and cannot hold any more water vapor.
Relative humidity is highly temperature-dependent. Warmer air has a greater capacity to hold water vapor than cooler air. Therefore, the RH percentage changes even if the actual amount of water vapor in the air remains constant. If the air temperature drops overnight, the relative humidity will rise, potentially reaching 100% and causing dew or fog to form. This dependency on temperature makes RH a potentially misleading indicator of true moisture content.
Understanding Dew Point Temperature
The Dew Point is an absolute measure of the actual water vapor content in the air, expressed as a temperature. It represents the temperature to which the air must be cooled, at a constant pressure, for the water vapor to begin condensing into liquid water. This condensation forms dew, fog, or clouds.
The dew point is not affected by changes in the air temperature, making it a stable indicator of the air’s moisture level. If the air temperature rises but no new moisture is added, the dew point remains the same, while the relative humidity drops. A higher dew point temperature always indicates a greater concentration of water vapor in the air.
Why the Distinction Matters for Comfort and Weather
The difference between these two measures has practical implications for both personal comfort and weather forecasting. While relative humidity is useful for determining the likelihood of precipitation, the dew point temperature is the superior metric for gauging human comfort. High moisture content, indicated by a high dew point, hinders the evaporation of sweat, which is how the human body naturally cools itself.
For most people, a dew point below 55°F is considered dry and comfortable, allowing for effective cooling. As the dew point rises to between 55°F and 65°F, the air begins to feel noticeably sticky and muggy. Once the dew point climbs above 65°F, the air becomes oppressive because the high moisture level severely limits evaporative cooling.
Meteorologists use the dew point to predict phenomena like fog or frost. When the air temperature is forecast to drop to the dew point, condensation is imminent. This condensation will form dew or fog, or if the dew point is below freezing, it will form frost.