Key concepts

Does your hair go crazy during "April showers," when the weather turns damp? If so, have you wondered why this is? Strands of hair can relax and lengthen when the humidity increases, and then contract again when the humidity decreases. In fact, the rate of change in the length of hair strands is so dependable that they can actually be used as the basis for a hygrometer, a device that measures the humidity level in the air.

To understand how a hygrometer made with hair strands works, it is important to understand the structure of a shaft of human hair. (The shaft is the part of the hair that sticks out above the surface of the skin, whereas the hair follicle is the part that is below the skin's surface.) In human hair shafts, several outer layers of flattened cuticle cells surround a layer of cortical cells (making up the cortex), which surrounds the central core of cells, called the medulla. The flattened cuticle cells are relatively tough and have a scalelike appearance when magnified, along with a coating of specialized molecules (called lipids) that repels water, helping to protect the hair strand. The cortical cells are fibrous and contain pigment granules of varying darkness that give hair strands their natural color.

The predominant proteins in hair are from a group called keratins, from the same protein family that forms your fingernails. The chemical bonds link the keratin molecules are what give a hair strand its strength and flexibility as well as its predictable response to humidity.

•    Rubbing alcohol (70 percent isopropyl alcohol)
•    Measuring spoons
•    Small bowl
•    Three long strands of human hair (about 20 centimeters long)
•    Cotton swabs
•    Measuring tape or ruler
•    Thin, flat piece of plastic that can easily be cut (about 8.0 cm long and 8.0 cm wide), such as from the lid of a disposable deli container
•    Scissors (strong enough to cut the plastic piece)
•    Dime
•    Tape
•    Two small nails
•    Glue
•    Scrap piece of wood or flat Styrofoam (about 25 cm long and 10 cm wide)
•    Hammer
•    Pencil or pen
•    Hair dryer
•    Plastic box that can be sealed and is large enough to fit the wood or Styrofoam piece inside of it
•    Wet sponge or small towel (either paper or cloth)

•    Make a solution of 25 percent rubbing alcohol and 75 percent water by mixing one tablespoon of water with one teaspoon of rubbing alcohol in a small bowl.
•    Dab a cotton swab in the diluted rubbing alcohol solution and wipe down each hair strand. This will remove any residual oils and allow water to permeate the hair strands more easily. Set the hair strands aside somewhere where you won't lose them.
•    Cut the piece of plastic into a triangular pointer shape, specifically an isosceles triangle that is 7.0 cm on two sides and 3.0 cm on the third side.
•    Position the plastic pointer you made so that it is pointing to your right, then tape a dime onto the pointer, centered 4.5 cm away from the pointer's short (left) side. (Some of the dime will stick out a little above and a little below the other two sides of the pointer.)
•    Centered about 0.5 cm from the left side of the pointer, poke one of the nails through it. Be careful when using the hammer on the nails. Be sure to only use the hammer on a surface that can safely withstand the force and is resistant to damage.
•    Wiggle the nail until the pointer moves freely and loosely around the nail. Remove the nail for now.
•    About 1.0 to 1.5 cm to the right of the nail hole, glue the hair strands to the pointer. Make sure the glue has dried before starting the procedure.

•    Position the pointer on the wood or Styrofoam base about three quarters of the way down (about 19 cm down) with the left side of the pointer about 1.0 cm from the base's left side. Hammer a nail through the pointer's nail hole and securely into the base. Make sure the pointer can still turn easily.
•    About 2.5 cm from the top of the base, in line with where the hair was glued to the pointer, hammer the other nail into the base.
•    Gently pull the hair strands taut so that the pointer continues to point right, making it parallel to the ground (and perpendicular to the hairs). The strands should be perfectly vertical and the pointer should point perfectly horizontal.
•    Glue the free ends of the hair strands to the top nail. What do you think would happen if you used much shorter strands of hair? How would this affect your humidity readings? You may need to hold the strands taut while the glue starts to set so that they retain their position. If the hairs are too long, trim the ends. Make sure the glue has dried before moving on to the next step.
•    You'll next want to calibrate your hydrometer for the two humidity extremes on your scale: 0 and 100 percent. For 0 percent humidity, dry the hairs gently with a hair dryer until the strands no longer decrease in length (the pointer no longer moves). Make this location 0 percent.
•    To gauge 100 percent humidity, enclose the hygrometer in the plastic box with the wet sponge or small towel. Wait at least 10 minutes. Mark the pointer location lightly, reseal the box and wait another 10 minutes. Keep doing this until the pointer location stops changing. Mark the final pointer location as 100 percent.
•    For intermediate values, you could just divide the space between 0 percent and 100 percent into five equal increments and mark the locations for 20 percent, 40 percent, 60 percent and 80 percent, then (if there is space) divide each of these in two to mark locations for 10 percent, 30 percent, 50 percent, 70 percent and 90 percent. Your hair hygrometer may not actually be this accurate, but it can give you a rough approximation of changes in humidity.
•    Use your hygrometer to measure different humidity levels around your home. How humid does the hygrometer say it is in a room in your home? How does this compare to the humidity in the bathroom after a shower? How humid does the hygrometer say it is outside, and how does this change depending on what the weather is like?
•    Extra: Calibrate your hair hygrometer by comparing it with another instrument, such as a psychrometer or an electric hygrometer. How similar are the readings between your hair hygrometer and these precise instruments? If they differ, why do you think this is?
•    Extra: Put your hygrometer outdoors in an area that is sheltered from rain. Monitor it regularly throughout the day over the course of several days. How much does the humidity change during the course of a "normal" day? How does a storm system moving through your area affect the humidity?

Observations and results
Did your hygrometer show that it is more humid in the bathroom after a shower than in a normal room in your home? When it was used outside, did the hygrometer give a higher reading when it was more humid, such as when it rained, compared with when it was dryer?

In the bathroom after a shower, if the room is small and the door was closed the entire time, the humidity can reach around 100 percent. If calibrated correctly, the hygrometer should give a reading of about 100 percent. In other rooms the humidity will probably be much lower than this, depending on how humid it is where you live and other factors (such as whether a humidifier or aquarium is being run). If you compared the readings of your hygrometer with another instrument that measures humidity, such as a psychrometer or an electric hygrometer, you may find that your hygrometer is 20 percent to 30 percent inaccurate, but you can recalibrate it using these devices to increase its accuracy.

As mentioned earlier, hair strands are made of protein molecules called keratins. Protein molecules are built from amino acids. In a hair strand, the keratin molecules are mainly held together by two forces: chemical cross-links between cysteines (a type of amino acid) and weaker hydrogen bonds. The keratins in hair have many such bonds, making a strand strong and flexible. The hydrogen bonds can be disrupted by water vapor in the air, with each keratin binding to water in the air instead of each other. In fact, this is what causes hair strands to regularly expand and contract in moist and dry air, respectively.

More to explore
Hair Keratin from Swiss Institute of Bioinformatics, Protein Spotlight
Hair Structure from Harley Street Hair Clinic
Understanding Relative Humidity and the Hygrometer by The Science Company
Make a Hygrometer with Strands of Hair from Science Buddies

This activity brought to you in partnership with Science Buddies