In paragraph 3, air that is humid is air that —

On a summer afternoon in Houston, the thermometer might read 94°F while the weather report says it "feels like" 108°F. In El Paso on a dry day, 94°F feels much closer to 94°F. The difference comes from water. Not the water you drink, but the water vapor in the air.

The human body cools itself mainly by sweating. Sweat is mostly water. When it evaporates from the skin, it changes from a liquid to a gas, and that change carries heat energy away from the body. This is also why a breeze feels so good on a hot day: moving air helps sweat evaporate faster.

How fast sweat evaporates depends on how much water vapor is already in the air. Humidity is a measure of that water vapor. When the air is dry, as it often is in West Texas, sweat evaporates quickly and the body cools well. When the air is humid, as it often is near the Gulf Coast, the air already holds so much water vapor that sweat evaporates slowly. It drips off or stays on the skin, and much less heat is carried away.

The National Weather Service combines air temperature and humidity into a single number called the heat index. The heat index is an estimate of how hot it feels to the human body in the shade. When the heat index is high, the body has a harder time cooling down, and the risk of heat exhaustion and heat stroke goes up.

Knowing the heat index can help people make safer choices. Coaches may move practice to early morning, and outdoor workers may take more breaks in the shade. On the most humid days, drinking water and resting out of the sun matter even more, because the body's usual cooling system is not working as well.