In atmospheric air there is always a certain dissolved amount of water vapor. This is what we call air humidity.
If the humidity increases and reaches the saturation point (dew point), mist (in the air), clouds (in the upper atmosphere) and dew (on plants, for example) begin to form.
Relevant air humidity is disclosed in the methodology bulletins. This value is defined as the quotient between the partial pressure of the water vapor present in the air and the maximum pressure of the water value at the same temperature.
This ratio can be expressed as a percentage, with relative humidity between 50% and 70% being considered comfortable for most people.