Humidity is the concentration of water vapor present in the air, which is usually invisible to the human eye. Humidity affects the likelihood of precipitation, dew, or fog to be present, as well as the comfort and health of living beings. Humidity is often measured by relative humidity, which is the ratio of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature, expressed as a percentage. High relative humidity means that the air is close to saturation, which can make it feel hot and sticky, and prevent sweat from evaporating. Low relative humidity means that the air is dry, which can cause dehydration and irritation. Humidity also influences the formation of weather phenomena such as clouds, rain, and hurricanes.