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Hair Hygrometer

In this activity, the students use a piece of their own hair to create a tool to measure humidity

Ever have a bad hair day? Don’t blame it on your comb; your bad hair day is probably a result of humidity. 

Although air is invisible, it has a lot of “stuff” in it, including water vapour. The amount of water vapour in the air at a given temperature is measured as humidity.

Humidity is measured as a percentage called relative humidity (RH). The amount of water vapour that air can hold is depends on the temperature of the air, so when the relative humidity is 100%, the air is saturated (holding as much water vapour as it possibly can at that temperature). Hot air can hold more water vapour than cold air, because cold air is dense, with the molecules packed more tightly together, leaving less space for water vapour in between them.

Humidity causes curly hair to go frizzy or straight hair to go limp because it changes hair length by 3 percent, from dry (0% RH) to very humid (100% RH) conditions. Whether your hair is dark or light, straight or curly, this ratio is stays the same. That’s why we can make relatively accurate measurements of air humidity using human hair.

The tool we use to measure humidity in this activity is called the hair hygrometer. In fact, the 1783 invention proved to be so reliable that it was not replaced by an electrical instrument until the 1960s.

Objectives

  • Create and use tools that measure relative humidity.

Materials

  • Per Student:
    1 piece cardstock (letter size or longer)
    tape
    1 long strand of hair
    1 paperclip
    1 pen
    30cm ruler
    1:4 ratio of rubbing alcohol & water mix (optional)
    cotton swab (optional)

Key Questions

  • What is humidity?
  • Why is hair affected by humidity?
  • How can a simple human hair predict relative humidity?
  • Why would a hair be longer in more humid conditions and shorter in less humid conditions?

What To Do

Demonstration

  1. Snip a long strand of hair from a willing volunteer.
  2. If it’s not already curly, curl it by pinching the hair between your thumbnail and the fleshy part of your forefinger and pulling the hair gently through.
  3. Dip the hair into some water and demonstrate that by wetting it the hair will become straight again.

Activity

  1. Roll the cardstock into a tube and tape it together. (Alternatively, use a paper towel roll.)
  2. Snip a long strand of hair from a willing volunteer.
  3. OPTIONAL: Mix 5ml of rubbing alcohol with 20ml of water and wipe the hair with the solution using a cotton swab. This removes the oil in the hair strand allowing it to absorb more moisture from the air.
  4. Attach the hair to the end of the cardboard tube using some tape. Position it so that the tip of the hair is at the top of the tube.
  5. Tie the hair around a paperclip so that it hangs straight down.
  6. Using the pen and ruler, mark graduations along the cardboard tube: cm from the top of the hair and mm near the end of the paperclip.
  7. Measure the length of the hair to the nearest mm.
  8. Place the hair hygrometer somewhere safe, away from direct sunlight and take measurements every day. Days when the hair is longer means the air is more humid. On days when the hair is shorter, the air is dryer.

Extensions

  • Build a hygrometer using a pinecone. Hint: wood absorbs water.
  • Measure air pressure by building a barometer

Other Resources

Home Science Tools | Build a Hygrometer with a Pinecone