Make a Hygrometer to Measure Humidity
What do a crazy hair day, a wooden door stuck in its frame, and the weather have in common? Humidity! In this fun hands-on weather lesson students explore surprising information about human hair, the air around them, and the weather by building a hygrometer from a strand of hair, a straw, a wooden panel, and two nails. A great way to make humidity visible!
- Explain what humidity and evaporation of water are
- Understand and give examples of processes that increase and decrease humidity
- Record qualitative humidity data taken with a hygrometer
NGSS AlignmentThis lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
- 3-ESS2-1. Represent data in tables and graphical displays to describe typical weather conditions expected during a particular season.
- 3-ESS2-2. Obtain and combine information to describe climates in different regions of the world.
Science & Engineering Practices
Constructing Explanations and Designing Solutions. Use evidence (e.g., measurements, observations, patterns) to construct or support an explanation or design a solution to a problem.
Disciplinary Core Ideas
ESS2.D: Weather and Climate. Scientists record patterns of the weather across different times and areas so that they can make predictions about what kind of weather might happen next.
Cause and Effect. Cause and effect relationships are routinely identified, tested, and used to explain change. Events that occur together with regularity might or might not be a cause and effect relationship.
Per group of 2 to 3 students:
- 1 tsp rubbing alcohol (70% isopropyl alcohol works well)
- Measuring spoons
- Small bowl
- Two long strands of human hair (about 20 centimeters each), one to use and another as backup (strands from straight, wavy or curly hair are fine, frizzy hair makes it a little more difficult).
- Cotton swab
- Drinking straw (preferably a wide, non-bending straw)
- Metric ruler
- Glue (fast drying glue or a glue gun work best)
- Scrap piece of wood (about 25 cm long and 15 cm wide)
- Two short nails (must be longer than the diameter of the straw)
- Marker, pencil, or pen
One for every other group:
- Resealable bag (large enough to fit the wood inside and have a little room to spare)
- Wet sponge or small towel (either paper or cloth)
To be shared among several groups:
- Hair dryer
- Optional: windows that face a colder area (e.g. outside on a not too hot day), or drinking glass cooled by ice-cubes.
- Optional: waterproof paint, paint brushes and other materials to decorate their hygrometer.
For the class:
- 1/4 cup measuring cup
Background Information for TeachersThis section contains a quick review for teachers of the science and concepts covered in this lesson.
Humidity is a measure of how much water vapor is in the air. Evaporation from large bodies of water like oceans, streams and lakes, as shown in Figure 1, is a large contributor of water vapor in the air. Living organisms are another large contributor. Plants and animals breathe out water vapor. As reference, a human breathes out about 300 to 50 milliliters of water in the form of water vapor per day.
Figure 1. Evaporation contributes to water vapor in the air.
The specific humidity is defined as the mass of water vapor in one kilogram of air. Because warm air can hold more water vapor than cold air, meteorologists —scientists who study weather and climate— prefer to use the relative humidity, or the amount of water vapor in the air compared to the maximum amount the air can hold at that temperature. Relative humidity is expressed in percentages; a relative humidity of 100% means the air cannot hold any more water vapor, and any additional water vapor condenses.
Warm air can hold more water vapor. One kg of air at 0°C can hold about five grams of water, the same kg of air at 38°C can hold almost 50 grams of water. That is ten times more! You can demonstrate this with the help of one small cup and one big cup (see Figure 2). The small cup represents the amount of water (in the form of vapor) cold air can hold; the larger cup represents the amount of water the air can hold after it has been heated. The second cup is much larger because warmer air can hold much more water vapor.
Figure 2. Glasses of different sizes filled with the same quantity of water can help explain how the relative humidity drastically changes with changing temperature.
Because warmer air can hold much more water vapor than cold air, the relative humidity decreases with rising temperature. You can demonstrate this by filling the small cup to the rim with water (see Figure 2). Because the cup is 100% full, it cannot hold more water. This illustrates that air with a relative humidity of 100% cannot hold more water vapor. Pour the water from the small cup in the big cup and observe how only a small percentage of the cup is filled. There is space to add water in the big cup. This demonstrates how a quantity of water that saturates cold air (relative humidity = 100%) leads to a much lower relative humidity if the air is warmer.
A hygrometer is a device that measures the humidity of the air surrounding it. Some hygrometers use a strand of hair to measure humidity. This works because a hair shows a small (a few percent at maximum) but reliable change in length with changing humidity. Hair is made of proteins called keratins, and chemical bonds hold the keratin particles together. The keratins in strands of hair have many such bonds, making a strand strong and flexible. Some of these bonds can be disrupted by water vapor in the air, which causes hair strands to expand and contract in humid and dry air, respectively. The change is small but hygrometers magnify the change so it becomes observable to the human eye. A hair hygrometer measures relative humidity. A more detailed explanation of the effect of humidity on hair can be found in the introduction of the project Make a Hygrometer with Strands of Hair.
Humidity is an important variable of the weather. High humidity can lead to cloud formation and precipitation. Continued low humidity can lead to drought. This becomes very clear in the weather cycle. This lesson introduces the concept of humidity by letting your students build a hair hygrometer and use it to measure humidity in the classroom and outside. Precipitation and/or the water cycle can be studied in a different lesson.