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The long thin hair.

You'll need these materials:
a scrap piece of wood or flat styrofoam (about 9 inches long and 4 inches wide)
a flat piece of plastic (about 3 inches long and 3 inches wide) thin enough that you can cut
2 small nails
3 long strands of human hair (about 8 inches long)
a dime
glue
tape
hammer
scissors (strong enough to cut plastic)

The pointer.

First, cut the piece of plastic into a triangular shape (refer to pictures). Then, tape the dime onto the plastic, near the point. Poke one of the nails through the plastic pointer, near the base of the triangle. Wiggle the nail until the pointer moves freely and loosely around the nail. On the plastic pointer, between the dime and the nail hole, glue the hair strands to the plastic.

The diagram

Position the pointer on the wood or styrofoam base about three quarters of the way down the side. (Refer to picture.) Attach the nail to the base. The pointer must be able to turn easily around the nail. Attach the other nail to the base about one inch from the top of the base, in line with the pointer. Pull the hair strands straight and tight so that the pointer points parallel to the ground. That is, make sure the point of the pointer is perpendicular to the hair. The hair should hang perfectly vertical and the pointer should point perfectly horizontal. Glue the ends of the hair to the nail. If the hair is too long, trim the ends.

The human hair cells will indicate the level of moisture in the air by expanding and contracting. When their air is moist, the hair will expand and lengthen, making the pointer point down. When the air is dry, the hair will contract and shorten, making the pointer point up. When you make your hygrometer observations each day, you should make a mark to indicate where the pointer points. Over time, you'll be able to see the humidity patterns that will help you forecast the weather.

 
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