MOISTURE IN THE AIR

By Cookie Davis

Water is present as a liquid and as a gas in the atmosphere. When water is a gas, it is called water vapor. You can’t see water vapor in the air, because water molecules are very small. When water is a liquid, you can see it in the form of clouds. It can also fall from the sky as precipitation.

Humidity

The amount of water vapor in the air is called humidity. The amount of water vapor the air can hold depends on the temperature of the air. Warm air can hold more water vapor, while cold air cannot hold as much.

Relative Humidity

We describe how much water vapor is in the air by using the term relative humidity. Relative humidity tells us how much water vapor is in the air compared to how much it could hold at that temperature. It is shown as a percent. For example, a relative humidity of 50% means the air is holding one half of the water vapor it can hold. Again, this is all dependent on the temperature.

Dew Point

The water droplets that you find on blades of grass and spider webs in the early morning are called dew. Dew happens when there are warm days and cool nights. During the warmth of the day, water evaporates from the warm ground into the air. At night, as the air cools, the water vapor within it condenses back into a liquid form—dew—on objects near the ground like blades of grass and spider webs. This happens because the air holds less water when it is cooled. The temperature when condensation begins is called the dew point. If the dew point temperature becomes lower than 32 °F (0 °C), the dew turns into frost.

Dew forms best when the atmosphere is clean, the air is calm, and there is not much wind. It is not just at night and outdoors, when it forms though. When eyeglasses get steamy in warm, wet rooms, dew can form, though people use the word condensation more often than dew.

Wikimedia Commons

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Darius, Anton. “Portrait of Adolescence.” Wikimedia Commons, Unsplash, 31 Oct. 2016, commons.wikimedia.org/wik/File:Portrait_of_Adolescence_(Unsplash).jpg.

“Dew Facts for Kids.” Kiddle, Kiddle Encyclopedia, 18 Apr. 2020, kids.kiddle.co/Dew.

“Moisture in the Air – Humidity.” Tree House Weather Kids, University of Illinois Extension, web.extension.illinois.edu/treehouse/clouds.cfm?Slide=1.

“Why Is the Grass Wet In the Morning?” Wonderopolis, National Center for Families Learning, www.wonderopolis.org/wonder/why-is-the-grass-wet-in-the-morning.