Why Humidity Makes It Feel Hotter (The Physics in 200 Words)
It is not about the air being hotter. It is about your sweat not being able to do its job. Here is the actual mechanism.
Sweat does not cool you by being wet. Sweat cools you by evaporating. Each gram of water that evaporates from your skin removes about 580 calories of heat from your body. Multiply by the gram per minute you produce in mild exercise and that is the engine that holds your body at 98.6 F when the air around you is at 95 F.
Evaporation needs somewhere for the water vapor to go. Dry air is mostly empty space, it has plenty of room for water molecules, so sweat evaporates fast. Humid air is already loaded with water vapor, there is much less room for more, so evaporation slows.
At 95 F and 40% humidity, sweat evaporates fast enough that your skin stays close to body temperature. The heat index reads about 99 F. Manageable.
At 95 F and 80% humidity, sweat just sits on your skin. Evaporation is slow, cooling is slow, your core temperature climbs. Heat index reads 133 F, heat stroke territory in under an hour for sustained activity.
At 100% humidity, evaporation stops entirely. Sweat just runs down. Your body has lost its primary cooling mechanism. The wet-bulb temperature equals the dry-bulb temperature, and above wet-bulb 95 F, no healthy adult can survive prolonged exposure regardless of fitness or hydration.
This is why humid cities feel worse than desert cities at the same temperature.