What is Humidity?

Room humidity can have a major impact on the quality of the living environment.

• What is humidity?

• How do we measure humidity?

• Why is relative humidity, and not absolute humidity, cited in weather reports?

• Relation between relative humidity and temperature

What is humidity?

There's water in the air all around you. “But I'm not wet!" you might say. That's true. It is because the water in the air is in the form of gas, which is invisible to the naked eyes. The liquid water evaporates into gas, called water vapor.

So what is humidity? Humidity is the amount of water vapor in the air.

How do we measure humidity?

There are two different terms to describe humidity: absolute humidity and relative humidity.

Absolute humidity is the weight of water vapor per unit volume of air. It tells the actual amount of water in the air and is expressed in g/m(3).

Relative humidity is the ratio of the current absolute humidity (the current amount of water vapor) in the air to the maximum amount of moisture the air can hold at a given temperature.

It is considered as "relative" because it doesn't measure the actual amount of water vapor. Instead, it represents the percentage of water vapor air can possibly hold (expressed in percent). The closer relative humidity gets to 100%, the wetter the air.

Why is relative humidity, and not absolute humidity, cited in weather reports?

Absolute humidity measures the total mass of water vapor in a given air volume. It is expressed in grams of moisture per cubic meter of air. "5 grams per cubic meter of air" may not give you a good idea of how humid it "feels" outside. That’s why relative humidity comes to play.

Sweating is our body's cooling mechanism. When relative humidity is low, sweat evaporates easily and therefore cools our body. When relative humidity is high, the air is nearly full of water vapor. So sweat is unable to evaporate easily and rests on our skin. That's where the hot, sticky feeling comes from.

Relative humidity really gives us great idea of the relationship between humidity and human comfort. As a result, the term "relative humidity" is usually cited in the weather report.

Relation between relative humidity and temperature

The relation between relative humidity and temperature is inversely proportional. When temperature rises, relative humidity decreases. Whereas temperature drops, relative humidity increases.

Water molecules move with higher speed at higher temperature. It is more likely to change from liquid to gas when temperature goes up. So warm air holds more water vapor than cold air. When air is warm, it holds more water vapor, which makes relative humidity falls. Thus, air becomes drier. Whereas, air is cold, it holds less water vapor, relative humidity rises. Therefore, the air will become wetter.