The air around us contains water vapor, and understanding its quantity is important for both weather forecasting and personal comfort. The dew point provides insight into this moisture, helping us interpret how “muggy” or “dry” the air feels.
What Dew Point Means
The dew point is the temperature at which air, at a constant pressure, must be cooled for its water vapor to begin condensing into liquid water, forming dew. When the air temperature cools to the dew point, the air becomes completely saturated with water vapor, reaching 100% relative humidity.
At this saturation point, the air can no longer hold all its water vapor, leading to the formation of liquid water droplets. If cooling continues below the dew point, more water vapor will condense out of the air. This process explains why a cold glass of water collects condensation on a warm day; the air immediately surrounding the glass cools to its dew point, causing condensation to form on the surface.
How Dew Point Influences Comfort and Conditions
Dew point plays a role in how comfortable we feel and in the formation of various weather phenomena. A higher dew point indicates more moisture in the air, which can make the environment feel humid and sticky. This is because the human body cools itself by evaporating perspiration, but high moisture content in the air hinders this evaporation, making cooling less effective.
For comfort, dew points below 50°F (10°C) feel dry and pleasant. As the dew point rises to between 55°F and 65°F (13-18°C), the air begins to feel sticky or muggy. When the dew point exceeds 65°F (18°C), the air becomes oppressive and uncomfortable.
Beyond comfort, the dew point helps understand weather conditions. When the air temperature drops to or below the dew point, condensation occurs, which can lead to the formation of dew on surfaces, frost if temperatures are below freezing, or fog. In the atmosphere, as air rises and cools to its dew point, water vapor condenses around tiny particles to form clouds. This process can eventually lead to precipitation when the condensed water droplets become heavy enough to fall.
Dew Point Compared to Relative Humidity
While both dew point and relative humidity measure moisture, they do so in different ways, and the dew point is often considered a more reliable indicator of comfort. The dew point is an absolute measure of the moisture content; it remains constant as long as the amount of water vapor does not change, regardless of temperature fluctuations. This means a dew point of 60°F always signifies the same amount of moisture, whether the air temperature is 70°F or 90°F.
In contrast, relative humidity expresses the percentage of moisture the air currently holds compared to the maximum it could hold at its current temperature. Because warmer air can hold more moisture than cooler air, relative humidity changes with temperature even if the actual amount of water vapor stays the same. For example, 50% relative humidity on a hot day can feel more humid than 100% relative humidity on a cold day, because the higher temperature allows for a much greater absolute amount of moisture at that 50% mark. Therefore, checking the dew point provides a clearer understanding of how “wet” or “dry” the air is and how it will impact your comfort.