Rain Forests and High Humidity: How to Measure Relative Humidity? (page 2)

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Author: Janice VanCleave


You made a wet-bulb and dry-bulb thermometer and used them to measure relative humidity.


Relative humidity is the amount of water vapor in the air compared to the total amount of vapor that the air could hold at that temperature, expressed as a percentage. An instrument, like the one you made in this experiment, which contains a wet-bulb and a dry-bulb thermometer and is used to measure relative humidity, is called a psychrometer.

When the relative humidity reaches 100 percent, the air is saturated, meaning it cannot hold any more water vapor. If air with 100 percent humidity cools, some of the water vapor in the air will condense (change from a gas to a liquid). If the air is next to the ground, the extra moisture will condense as dew. Dew is water from water vapor in the air that condenses on cool surfaces. Above the ground, the extra moisture will condense into cloud droplets (tiny drops of water with diameters between 0.00004 to 0.002 inches [0.0001 and 0.005 cm] that form clouds). Clouds are visible masses of water droplets that float in the air, usually high above the earth. Fog is a cloud that is close to the ground. Raindrops can form in clouds by accretion, which is the merging of water drops that bump into one another. When the drops get large enough they fall. Raindrops also form if tiny ice crystals (solid materials whose particles are arranged in a repeating pattern) and water drops are mixed together in a cloud. The water sticks to the ice and the ice crystals grow large and become heavy enough to fall. As they fall they melt and hit the ground as rain.

More Fun With Humidity!

Animals, as well as plants, add water to the air. See how the breath of animals increases air humidity. Do this by placing the end of a drinking straw inside the opening of a quart-size resealable plastic bag. Seal the bag as much as possible around the straw. Exhale through the straw five or more times, then quickly pull the straw out of the bag and completely seal the bag. Observe how cloudy the inside of the bag becomes. If the bag is not cloudy, repeat the procedure of blowing through the straw then removing the straw and closing the bag. Rub the outside of the bag with your fingers. The cloudiness will disappear and tiny drops of water will form. Rubbing the bag causes the tiny invisible drops of water clouding the bag to combine into larger visible drops. If the drops of water are not visible, open the bag and feel the inside with your fingers.

Rain Forest Humidity

Book List

  • Allaby, Michael. How the Weather Works. Pleasantville, New York: Reader's Digest, 1995. Information and experiments that let you discover more about the weather, including humidity.
  • Christian, Spencer. Can It Really Rain Frogs? New York: Wiley, 1997. Information and experiments about humidity and other weather events.
  • VanCleave, Janice. Weather. New York: Wiley, 1995. Experiments about humidity and other weather-related topics. Each chapter contains ideas that can be turned into award-winning science fair projects.
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