Actual humidity and relative humidity
Actual humidity refers to the amount of water vapor present in the air, usually measured in grams of water vapor per cubic meter. Relative humidity, on the other hand, is the ratio of the amount of water vapor present in the air to the maximum amount of water vapor the air can hold at a specific temperature and pressure.
For example, if the actual humidity is 10 grams of water vapor per cubic meter and the air at that temperature and pressure can hold a maximum of 20 grams of water vapor per cubic meter, then the relative humidity would be 50%.
In summary, actual humidity measures the absolute amount of water vapor in the air, while relative humidity compares this amount to the maximum amount of water vapor the air can hold at a specific temperature and pressure.