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Humidity is the amount of evaporated water in the air. As humidity gets higher, the air feels wetter and stickier. A hygrometer measures the humidity of the air around it.

Topics and Connections:

• Humidity
• Weather instruments
• Making measurements

Materials:

• strand of clean hair about 4-6 inches long
• 2 index cards (3" x 5")
• hole-punch
• brass paper fastener
• scissors
• tape
• ruler

Directions:

Teachers:

• Ask class if they have ever noticed that their hair behaves differently in different weather. What have they observed?

Students:

• Hold a card so it's the shape of a TV screen. Punch a hole in it about half an inch from the right side, halfway up the card.
• Cut a thin strip four inches long from the second card. It should be a little wider than the hole you punched in the first card.
• Punch a hole close to one end of this strip, and cut the other end to form a point.
• With the brass fastener, attach the two cards together at the holes.
• Tape one end of the clean hair to the middle of the strip, and tape the other end of the hair to the top of the card (or the back, if it reaches over). The pointed strip should point to the middle of the card.
• Mark the card where the pointer indicates.
• Watch what happens to the pointed part of the strip as days go by. Check it daily and mark its new location on the card.

Questions To Ask:

• What's making the strip move?
• Does everyone's hair work equally well? Is curly, straight, thick, fine hair best? Suppose you tried some material other than hair (a thread, spiderweb strand, synthetic fabric).
• This weather instrument works because hair soaks up water and expands on humid days. What other changes happen when the humidity goes up or down? How can you feel, see, or hear it? Can you think of some way to adapt any of these humidity effects into a measuring tool?

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