Dictionary Definition

The amount of water vapor in the air.

Formation

The atmosphere always has moisture in the form of water vapor, but the amount depends on the temperature. A rise in temperature increases the amount of vapor. When the atmosphere is saturated with water, the level of discomfort is high because the evaporation of perspiration, with its accompanying cooling effect, isn't possible.

Measuring

Explanation

There are methods of measuring the amount of water in the air so you can know what to expect that day. Weathermen on television and radio announce the humidity so that people know how to prepare themselves for lots of moisture or dry air. You can measure the humidity yourself, though, using instruments called psychrometers. Use the experiments below to learn more about humidity and how to measure it.

Experiment #1: Using Hair

What You'll Need:

• glue
• marker
• large glass jar
• pencil
• cellophane tape
• flat toothpick
• straight strand of hair about five inches (12 cm) long

Directions:

1. Secure one end of the strand of hair to the center of the toothpick using a small piece of tape.
2. Color the pointed end of the toothpick with a marker.
3. Tape the free end of the piece of hair to the center of the pencil.
4. Put the pencil across the moth of the jar with the toothpick hanging down inside the jar horizontally. If the toothpick isn't horizontal, add a drop of glue to the light end to balance the toothpick.
5. Leave the jar alone and watch for a week which way the toothpick points.

What's Happening?

The toothpick changes direction because this hair hygrometer measures the amount of water in the air (humidity) using the stretching of the hair. The hair stretches when the humidity increases, and the hair shrinks when the humidity decreases. This stretching and shrinking pulls the toothpick and makes it move.

Experiment #2: Using Salt

What You'll Need:

• 5 ml teaspoon
• table salt
• black construction paper
• scissors
• two saucers
• pencil

Directions:

1. Cut two pieces of construction paper to fit the bottom of each saucer.
2. Sprinkle 1/2 teaspoon (1.23 ml) of salt on the black paper in each saucer.
3. Blow lightly toward the salt in one saucer for about two minutes from about six inches (15 cm) away.
4. Stir the salt in both saucers with the pencil.

What's Happening?

The salt in the saucer that was blown on clumps together when stirred, while the salt crystals of the other saucer stay separate. Since exhaled breath has water vapor, the crystals stick together. Air that has a large amount of water also causes salt to become soggy. There is a high humidity when the salt is difficult to shake from saltshakers.

Experiment #3: Using Static

What You'll Need:

• clean, dry, oil-free hair
• plastic comb

Directions:

1. Briskly comb your hair and record the results.
2. Repeat the quick brushing over a period of several days to see the differences.

What's Happening?

On some days, a crackling sound is heard as you comb your hair, but on other days there is no sound. This is because electrons are rubbed from the hair onto the comb. Sound waves are produced when this happens, and the sound is heard best when the air is cool and dry and not heard at all when the air is warm and wet. Wet air has many molecules of water that serve as stepping stones for the electrons to use, but as the air becomes dryer there is less water so the electrons have to jump farther. The electrons clump together until they have enough energy to get across the gap. The movement of this group produces the crackling sound. So, you can tell how humid it is by the noise you hear. When there is a high humidity level, you won't hear anything, but when the level is low you will hear a crackling sound.

Relative

Explanation

Relative humidity is the ratio between the actual vapor content in the atmosphere and the vapor content of air at the same temperature saturated with water vapor. Look at the experiment below to see how you can determine the humidity level using relative humidity.

Experiment #4: Relative Humidity

What You'll Need:

• two thermometers
• one cotton ball
• fan

Directions:

1. Place the thermometers on a table.
2. Record the temperature of both thermometers.
3. Wet the cotton ball with water. Place it over the bulb of one of the thermometers.
4. Position the fan so it blows across both of the thermometer's bulbs.
5. Record the temperature of the two bulbs after five minutes.

What's Happening?

The thermometer with the covered bulb has a lower temperature because it cooled as the water evaporated from the cotton. The faster the water evaporates, the lower the temperature of this thermometer. The dry bulb thermometer records the air temperature. A low humidity shows a large difference between the wet and dry bulb reading. When you compare the temperatures to determine the humidity in this way, you are using another psychrometer.

Evaporation

Evaporation is the gradual change of a liquid into a gas without boiling. The molecules are moving; the speed depends on the temperature. Individual molecules approach the surface fast that they can escape the surface and pass into the space above as gas molecules. When the molecules escape, the average speed and temperature of the remaining molecules is lowered.

Evaporation works as a cooling process, and the evaporating drops are extremely cold. Water can evaporate by absorbing energy in the surrounding air. When the water converts to gas it adds water vapor into the atmosphere. Humidity is the level of water vapor in the air, and the water gets there because of evaporation.