In this activity, students use a relative humidity sensor and a soda bottle to measure humidity near surfaces, such as over a leaf or above an ice cube.

Where does humidity come from?

There is always some water vapor in the air. We can’t see it directly, but we can measure its presence with a relative humidity sensor.

Relative humidity is a measure of how much water is in the air. At any one temperature there is a maximum amount of water the air can carry. The relative humidity is a percent of this maximum. For example, when air holds all the moisture it can, the relative humidity is 100. If more moisture is added, or if the temperature goes down, some moisture condenses out as liquid water droplets in the form of fog or mist. When air holds only half the maximum amount of moisture, the relative humidity is 50. Relative humidity is the amount of moisture in the air relative to the maximum it can hold.

Warmer air holds more moisture than colder air. So relative humidity is affected by both temperature and moisture content. For instance, you can raise relative humidity either by adding water vapor to the air or by lowering the temperature.

What are two ways to lower relative humidity?

Picture a natural landscape that you know well — your back yard, a special place you go in summer. Picture it in a certain season, at a certain time of day. You can draw it if you wish with the draw tool below.

The relative humidity won’t be the same everywhere in this picture. Where will it be higher? Where will it be lower? Why?

• relative humidity (RH) sensor, wand type
  • Other types of relative humidity sensors may be used, with adaptations.
• 1 liter soda bottle
• scissors
• modeling clay
• ice cubes
• aluminum foil
• shallow bowls
• other materials to test: green leaves, soil, sand, etc.
• temperature sensor (optional)

1. Cut the bottom off of a soda bottle, about 1/3 of the way from the bottom.
   
2. Wrap the RH sensor with a finger-thick tube of clay.
3. Install the RH sensor through the top of the bottle, using the clay to hold it in place.
   
4. You now have a relative humidity tester that can measure how much moisture is put into the air by different surfaces.

Think of several micro-environments you would like to test. Here are some examples, but you can also invent your own.

• cold water in a dish
• hot water in a dish
• ice cubes in a dish
• ice cubes covered with aluminum foil (so that the ice surface is not exposed)
• snow
• the palm of your hand
• under a bright, hot light bulb
• green leaves
• brown leaves
• soil rich in humus
• sand
• a moisture-absorbing desiccant, such as calcium chloride

Study your list. Which surface will raise the relative humidity compared to the air, and which will lower it?

Measure the relative humidity of the air, with nothing closing off the sensor. Then pick several surfaces and test them. You can make a new dataset for each material, or test them one after the other and label the graph.

Use the text box to take notes about what you tried.

Continue your experiments. You can make a new dataset for each material, or test them one after the other and label the graph.

Use the text box to take notes about what you tried.

Often the RH will change due to a change in temperature. If you have a temperature sensor, you can check to see if the temperature change may have caused the observed change in RH.

1. Did your measurements match your predictions?

2. Were there surprises in your results?

3. In each case, was the RH change due to a change in moisture, or a change in temperature?

Recall the natural environment that you pictured. Based on your observations, where the RH be high and where would it be low?

Think of some examples where organisms have adapted to and made use of the differences in micro-environments in nature. For example, where does mold grow? Why?