High humidity makes the air feel hotter than the actual temperature. This sensation arises because moisture in the air impacts the body’s natural cooling processes. Understanding this phenomenon involves examining how the body regulates its temperature and how atmospheric conditions interfere with that regulation.
The Body’s Natural Cooling System
The human body maintains a stable internal temperature, a process known as thermoregulation. When body temperature begins to rise, primarily due to external heat or physical activity, the brain signals sweat glands to release liquid onto the skin’s surface. This sweat is mostly water, with small amounts of salts and other substances.
The primary way this sweat cools the body is through evaporation. As the liquid sweat transforms into water vapor, it absorbs heat from the skin. This heat, necessary for the change of state from liquid to gas, is drawn directly from the body, leading to a cooling effect. This evaporative cooling mechanism efficiently removes excess heat, lowering the body’s temperature.
How Humidity Impedes Cooling
Humidity hinders the body’s evaporative cooling system. When the air already contains a large amount of water vapor, it has a reduced capacity to absorb additional moisture from sweat. This condition is described in terms of a vapor pressure gradient. Evaporation occurs most efficiently when there is a large difference between the water vapor pressure on the skin’s surface and the water vapor pressure in the surrounding air.
In a high-humidity environment, the air’s vapor pressure is already elevated, diminishing this gradient. As a result, sweat evaporates more slowly, or hardly at all. The sweat then tends to accumulate on the skin rather than evaporating, which reduces its cooling effectiveness. This inefficient cooling forces the body to work harder to shed heat, leading to a feeling of being hotter and more uncomfortable.
Quantifying the Sensation: “Feels Like” Temperature
To represent how hot it feels, meteorologists use metrics that combine air temperature with atmospheric moisture. The “Heat Index” is one example, which integrates air temperature and relative humidity into a single value indicating the perceived equivalent temperature. This index accounts for the reduced cooling efficiency caused by humidity, providing a more accurate representation of thermal stress.
Another indicator of atmospheric moisture and its impact on comfort is the dew point. The dew point is the temperature to which air must be cooled for it to become saturated with water vapor. A higher dew point signifies more moisture in the air, making it feel stickier and more uncomfortable because it further impedes sweat evaporation. Dew points above 60°F (16°C) can start to feel muggy, while those above 70°F (21°C) are considered oppressive.