What is humidity, and why does it make heat feel worse?

Humidity is simply the amount of water vapor (water in gas form) present in the air. What weather reports usually give is relative humidity, the percentage of how much water vapor the air is holding compared to the maximum it could hold at that temperature. At 100 percent relative humidity, the air is saturated and cannot absorb any more moisture (which is when you get dew, fog, or rain). Warm air can hold much more water vapor than cold air, which is why humidity, temperature, and comfort are all tangled together, and why a muggy summer day and a crisp winter one feel so different even at similar humidity percentages.

Humidity makes heat feel worse because it sabotages your body's main cooling system: sweating. Your body cools itself by producing sweat, which draws heat from your skin as it evaporates into the air, evaporation is a cooling process. But sweat can only evaporate if the surrounding air has room to absorb more moisture. When humidity is high, the air is already crowded with water vapor, so your sweat evaporates slowly or barely at all, leaving you drenched, sticky, and unable to shed heat. Your internal temperature climbs, and you feel far hotter and more miserable than the actual air temperature would suggest, which is genuinely dangerous, because high heat plus high humidity can overwhelm the body's ability to cool itself and cause heat exhaustion or heat stroke. This is exactly what the heat index (the feels-like temperature) measures: it combines air temperature and humidity to estimate how hot it actually feels to your body, and on humid days the heat index can be many degrees above the thermometer reading. It is also why dry heat (like a desert at 100 degrees) feels more bearable than humid heat at a lower temperature, in dry air, sweat evaporates efficiently and cools you well. So humidity is water vapor in the air, and it makes heat oppressive by blocking the evaporation of sweat that your body depends on to stay cool.