Humidity is simply the amount of water vapor present in the air. It can be expressed as an absolute quantity, such as grams of water per cubic meter, or as relative humidity, the percentage of the maximum the air could hold at that temperature.

Warm air can hold more water vapor than cold air. That is why relative humidity often rises at night even if the actual moisture content stays constant. High humidity makes warm days feel hotter and can limit the body’s ability to cool through sweating.

Humidity influences cloud formation, precipitation, and human comfort. Instruments called hygrometers measure it. Dew point provides a more direct indication of moisture content than relative humidity alone, which is why many forecasts emphasize dew point during muggy weather.

Indoor humidity also affects health, materials, and energy use.