Humidity refers to the amount of water contained in the air. As the air’s temperature changes, so does the amount of water the air can hold.
One of the ways your body cools itself is by sweating. The sweat then evaporates from your skin, and it carries heat away from the body as it leaves.
However, humidity throws a wrench in that system of evaporative cooling. As relative humidity increases, the evaporation of sweat from your skin slows down. Instead, the sweat just drips, which leaves you with a damp T-shirt and none of the cooling effect. When the humidity spikes, we effectively lose a key tool that would normally cool us down.
When we think of wind on a hot day, we think of a nice, cooling breeze. That’s the normal state of affairs, but when the weather is really, really hot—think high-90s hot—a dry wind actually heats us up. When it’s that hot out, wind actually draws sweat away from our bodies before it can evaporate to help cool us down. Thanks to this effect, what might have been a cool breeze acts more like a convection oven.
However, being in the shade can make quite a big difference. Direct sunlight can add up to 15 degrees to the calculated heat index.