The dew point is a measure of atmospheric moisture, defined as the temperature to which air must be cooled at a constant pressure to become saturated with water vapor. This temperature provides a direct, absolute reading of the water vapor content in the air. For human comfort, the dew point is the most reliable indicator of how humid or muggy the air will feel. The body’s ability to cool itself is directly correlated with this value, and specific thresholds exist where the moisture content begins to cause noticeable discomfort.
Understanding the Comfort Scale
The dew point scale categorizes the perception of moisture in the air, ranging from dry to physically oppressive conditions. Air with a dew point at or below 55 degrees Fahrenheit (13 degrees Celsius) is generally considered dry and comfortable for most people. This range allows for easy evaporation and minimal sensation of humidity, making it ideal for outdoor activity.
As the dew point rises into the range of 55 to 60 degrees Fahrenheit (13 to 16 degrees Celsius), the moisture content becomes slightly noticeable, though the air still feels pleasant. This is often described as the upper limit of comfortable air, where the atmosphere begins to transition toward a muggier feel. Conditions start to feel “sticky” when the dew point reaches between 60 and 65 degrees Fahrenheit (16 to 18 degrees Celsius).
Once the dew point pushes past 65 degrees Fahrenheit (18 degrees Celsius), the air contains significantly greater water vapor, and most people feel genuinely uncomfortable. This range, extending up to 70 degrees Fahrenheit (21 degrees Celsius), is commonly described as oppressive and very humid. A dew point at or above 70 degrees Fahrenheit (21 degrees Celsius) is an extreme condition, where the air is heavily saturated and can be considered dangerous, potentially inducing thermal stress.
Why High Dew Points Feel Uncomfortable
The primary reason high dew points feel uncomfortable relates to the body’s method of thermal regulation, specifically evaporative cooling. When the body’s internal temperature rises, it produces sweat, which is intended to evaporate from the skin’s surface and carry heat away. This process is highly effective because the phase change of water from liquid to gas requires a substantial amount of energy, which is drawn directly from the body.
High dew points indicate that the air is already holding a large volume of water vapor, meaning it is close to its saturation point. When the air is highly saturated, the driving force for additional moisture, like sweat, to evaporate is significantly reduced. The humid air cannot readily absorb more water vapor, causing sweat to linger on the skin instead of evaporating.
This inhibition of the body’s natural cooling mechanism results in a rapid increase in perceived heat and physical discomfort. This thermal strain can lead to symptoms of heat illness, such as dizziness, nausea, and cramping, demonstrating that high moisture content directly compromises physiological function. The body must work harder to maintain a stable internal temperature, leading to a sensation of stickiness and being overheated.
Dew Point Versus Relative Humidity
The dew point is often mistakenly confused with relative humidity, but the two metrics measure moisture content in fundamentally different ways. Relative humidity is a ratio, expressed as a percentage, that indicates how much water vapor is in the air compared to the maximum amount the air can hold at a specific temperature. Because warmer air has a greater capacity to hold moisture than cooler air, relative humidity is highly dependent on the air temperature.
For example, 50 percent relative humidity on a cool 50°F day contains far less actual moisture than 50 percent relative humidity on a warm 80°F day. This temperature dependency makes relative humidity a poor standalone indicator of human comfort. It can fluctuate dramatically throughout the day, dropping rapidly as air temperature increases, even if the absolute amount of water vapor remains unchanged.
In contrast, the dew point is an absolute measure, representing the actual quantity of water vapor present in the air as a temperature value. If the air temperature changes, the dew point typically remains constant unless new moisture is added or removed. This stability makes the dew point a more accurate and consistent gauge of how muggy the air will feel, which is why meteorologists rely on it to communicate the true level of atmospheric moisture.