On hot days with high humidity, your skin may feel sticky and very uncomfortable. So what is it about high humidity that makes the body feel so uncomfortable?
The answer is that high humidity can make us feel hotter compared to sunny days with dry air. This is because humidity—the amount of water vapor in the air—can hinder the body’s ability to shed excess heat through sweating, according to the National Oceanic and Atmospheric Administration (NOAA).
Typically, when sweat particles are on the surface of the skin, heat from the body evaporates that sweat into the air. This cools the skin. However, high humidity prevents sweat from evaporating easily into the air, as the surrounding air is already saturated with moisture and cannot absorb much more. As a result, less sweat evaporates from the skin, making us feel increasingly uncomfortable and hot.
When humidity is high, the sticky water vapor on the skin also makes people feel uncomfortable.
According to the U.S. Environmental Information Center, warm air can hold more moisture than cool air. An increase of 0.55 degrees Celsius corresponds to a 4% increase in water vapor in the atmosphere. This explains why high humidity in summer makes people feel more uncomfortable than in winter. Not only does it feel stifling, but when humidity is high, the sticky water vapor on the skin also contributes to discomfort.
Although there is no specific index indicating the humidity threshold that worsens human comfort, the U.S. Environmental Information Center generally considers relative humidity (RH) above 50% and a dew point (or frost point) above 65 degrees Fahrenheit (18 degrees Celsius) to cause high discomfort.
The good news is that the body can somewhat adapt to high temperatures and humidity. However, this adaptation is not quick enough to make a person feel comfortable immediately if transitioning from a cool air environment to a tropical one within a few days. Larry Kenney, a professor of physiology, told Live Science: ‘It takes an average of 9 to 14 days for the body to fully adapt to a new environment.’
One of the early stages of the adaptation process to heat is the dilation of blood vessels. This allows more blood to flow near the surface of the skin, where excess heat can easily escape the body. Meanwhile, this leads to a lower heart rate and more fluid available for sweating. By the 3rd or 5th day, the rate of sweating begins to increase. However, the aforementioned adaptation is only temporary.
