What Is Humidity?
Humidity is the amount of moisture, or water vapor (water in the form of a gas) in the air around us. There is always some water vapor in the air. It evaporates from ponds, rivers, lakes and oceans.
It is the moisture from which clouds, fog, rain and snow are made. If the air contains only a little bit of moisture, the humidity is low.
When the air is very moist — when it contains a good deal of water vapor — the humidity is high. When the humidity is high, perspiration cannot evaporate easily. The perspiration stays on your skin and you feel “sticky.”
Humidity plays an important role in our daily weather. When you hear weather forecasters talk about humidity, you may hear them talk about two different terms: absolute humidity and relative humidity.
Absolute humidity is the amount of water vapor divided by the amount of dry air in a certain volume of air at a particular temperature. The hotter the air is, the more water vapor it can hold.
Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity, which will depend upon the current air temperature. Relative humidity is the term weather forecasters use most often.
Too much or too little humidity can be dangerous. For example, high humidity combined with hot temperatures is a combination that can be a health risk, especially for the very young and the very old.
Humans are sensitive to changes in humidity, because our skin uses the air around us to get rid of moisture in the form of sweat. If the relative humidity is very high, the air is already saturated with water vapor and our sweat won’t evaporate.
When this happens, we feel hotter than the actual temperature. Likewise, very low humidity can make us feel cooler than the actual temperature. This happens because the dry air helps sweat evaporate more quickly than usual.