If you put a pitcher of ice water on a table and let it stand a while, what happens? Moisture gathers on the outside of the pitcher. Where does this moisture come from? It comes from the air.
The fact is there is always moisture in the air in the form of water vapor. In the case of the ice pitcher, the vapor condensed on the cold surface of the pitcher and thus became visible.
But water vapor in the air is invisible. And the word “humidity” simply means the presence of water vapor in the air. It is found everywhere, even over great deserts.
This means, of course, that we always have humidity, but the humidity is not always the same. We have several ways of expressing the humidity, and two of them are absolute humidity and relative humidity.
Let’s see what each means. Absolute humidity is the quantity of water vapor in each unit volume of air. There are so many grains per cubic foot of air. But for most practical purposes, this doesn’t tell us very much.
If you want to know whether you’ll feel comfortable or not, the answer “four grains per cubic foot” won’t tell you whether the air will feel dry or humid. The more easily moisture from your body can evaporate into the air, the more comfortable you’ll be.
The evaporative power of the air depends on the temperature, and absolute humidity doesn’t indicate anything about the evaporative power of the air. Relative humidity is expressed as a percentage.
“One hundred percent” stands for air which is saturated or completely filled with water vapor. The higher the temperature, the greater the quantity of water vapor air can hold. Thus, on a hot day, a “90 percent relative humidity” means an awful lot of moisture in the air-a day that will make you mighty uncomfortable.