Final answer:
The ratio of water vapor in air to air's water vapor capacity at the same temperature is known as the percent relative humidity, and it measures how saturated the air is with water vapor.
Step-by-step explanation:
The ratio of water vapor in air to air's water vapor capacity at the same temperature is called the percent relative humidity. Understanding relative humidity involves knowing the vapor density, which represents the amount of water vapor actually present in the air, and the saturation vapor density, which is the maximum amount of water vapor air can hold at a given temperature.
When air contains all the water vapor it can hold at a specific temperature, we say it is at saturation, and the relative humidity is 100%. The concept of dew point is related; it is the temperature at which the air must be cooled for it to reach saturation with its current water vapor content.
For example, if the air contains 9.40 g/m³ of water vapor and has a saturation vapor density of 23.0 g/m³, the percent relative humidity can be calculated as (9.40 / 23.0) × 100, which will give us the current relative humidity of that air mass.