Why is it so much more miserable in the summer when it is humid?

"It's not the heat, it's the humidity." Well, really, it's both. The heat makes it necessary for the body to cool off, while the humidity makes the process difficult. The inability to cool off is what makes it feel so uncomfortable when it's hot and humid. In a related post, I talk about how sweating is an efficient way to draw heat out and away from the body. The mechanism works best when the sweat can evaporate off of the skin quickly. The evaporation process is what takes the heat away from the body. When it is humid, the sweat has a hard time evaporating and instead sits on the skin. It no longer helps us cool, or at least not very much. All it does is make us feel gross. Meanwhile the body still wastes water producing sweat (more of it in fact, in a doomed effort to speed up the cooling process), so one has to drink a lot, even if it may feel like simply breathing should supply all the water we need.

An aggravating factor on hot an humid days is that the water vapor in the air traps the heat from the Sun. Water vapor is an often ignored, but effective green house gas. When the air is dry, the temperature drops quickly at sunset and nights are cool. Hot and humid days tend to stay hot even after the sun goes down (love those muggy nights!). The lack of relief in the evening and overnight makes it that much more miserable.

Why do we sweat when we are hot?

While it feels gross, sweating is an effective way of cooling down the body. It utilizes the difference in thermal energy contained in a liquid versus that contained in a gas. A gas contains more thermal energy (because the gas molecules move fast) than a liquid. Therefore turning a liquid into a gas requires an input of energy.

When it's hot, our body wants to expend some thermal energy so it can cool to a comfortable temperature. When sweating, our body takes some of our water and puts it out on the skin, where it comes in contact with air. In air, the sweat evaporates. The water turns into water vapor, from its liquid form to its gas form. This requires an input of thermal energy, which our body is happy to provide. The water vapor floats away, carrying that extra energy with it and helping us keep cool.

To replenish the sweat, we need water and that is why when it's hot, we feel thirsty and have to drink a lot of water.