Why Does Moisture Make Us Feel Hotter?
With record high temperatures in the US and Europe that keep up raising in the 2022, it’s a good time to
answer some questions about why humidity makes us feel hotter than it really is and also some
questions about the heat index.
Why does humidity make us feel hotter?
Let’s talk about sweat. As you may remember from high school biology class, one of the ways our body
cools itself is by sweating. The sweat then evaporates from our skin and draws the heat away from our
body as it exits.
So, humidity is the key to the evaporative cooling system. As the relative humidity increases, the
evaporation of sweat from our skin slows down, the sweat just drips off, leaving us with a damp shirt
and no cooling effect. When humidity rises, we effectively lose a key tool that would otherwise keep us
What is relative to relative humidity?
Humidity refers to the amount of water in the air. However, as the air temperature changes, so does the
amount of water the air can hold. (Air can hold more water vapor as the temperature rises.) Relative
humidity compares actual humidity with the maximum amount of water vapor the air can hold at any
What about the heat index?
While the idea of humidity making days warmer is painfully obvious to anyone who has ever been
outside on a muggy day, our current system owes a great debt to textile researcher Robert G. Steadman.
In a paper from 1979 titled “An Assessment of Sultriness, Parts I and II,” Steadman laid out the basic
factors that affect how hot a person feels under given conditions. Soon after meteorologists used his
work to derive a simplified formula for calculating the heat index.
The formula is long and unwieldy, but fortunately it can be transformed into easy-to-read graphs
available online. You just need to know the air temperature and relative humidity, and the table will tell
you the rest.
Is heat index calculation same for everyone?
Not quite, but it’s close. Steadman’s research is based on a “typical” person who is outside under very
precise conditions. Specifically, Steadman’s common man is 1.70cm tall, weighs 66kg, wares long pants
and a short-sleeved shirt, and walks at a speed of just over 4km per hour in a light breeze in the shade.
Any deviation from these conditions will affect how the heat/humidity combination feels to the
What is the difference in shade?
The difference is pretty big. All weather service charts for heat index calculations, around the world
make the reasonable assumption that people will seek shade when it’s very hot and humid outside.
Direct sunlight can add up to 15 degrees to the calculated heat index.
How does wind affect how dangerous the heat is?
Usually, when we think of wind on a hot day, we think of a nice, cool breeze. That’s the normal situation,
but when the weather is really, really hot, the dry wind actually warms us even more. When it’s that hot
outside, the wind actually pulls the sweat away from our bodies before it can evaporate and cool us
down. Thanks to this effect, what could have been a cool breeze becomes more like an industrial “blast