In this activity you will calculate the actual amount of water in the air in your classroom.

How much water is in the air?

Much of the water cycle is visible, both in the form of liquid water (oceans, rivers, lakes, precipitation such as rain) and in the form of solid water (ice or snow, precipitation such as falling snow or hail). However, a fraction of the water in the water cycle is invisible because it is held as a gas in the air. This invisible amount of water accounts for about 0.001% of all water on earth, but it is very important.

What do you think would happen to the water cycle if this “invisible” water did not exist? Get together with your group members and discuss this question. Write your thoughts below.

One key to the formation of dew and other types of condensation such as rain, sleet, and snow is that air of different temperatures can hold different amounts of water. Warmer air can hold more water and colder air can hold less water.

If you had a cubic meter of air at 10 degrees C and a cubic meter of air at 20 degrees C, and both volumes of air were holding as much water as they could hold, which would be holding more water?

Take a wild guess. How many grams (or milliliters) of water vapor do you think a cubic meter of air can hold?

• 1 temperature sensor
• 1 relative humidity sensor
  Note: if a relative humidity sensor is not available, refer to the Relative Humidity Measurement activity, which explains how to use the temperature sensor to measure relative humidity.

This activity addresses NSES standards for earth and space science and inquiry at grades 5-8 for structure of the earth system
(http://books.nap.edu/readingroom/books/nses/6d.html#es).

There are no special safety concerns in this investigation.

How much water is in the air in your classroom? To find out, you are going to use a temperature sensor to measure the relative humidity and temperature in your classroom.

A relative humidity measurement gives a measure of the amount of water in the air as compared with the maximum amount of water the air could hold without condensing out as fog. For example, a relative humidity reading of 35% means that the air is holding 35% of the water it could hold for that air temperature.

1. Scientists have measured the amount of water that air can hold at different temperatures. The interactive graph below shows the maximum amount of water per cubic meter that air can hold at any given temperature.
   
2. Measure the air temperature in your classroom. Refer to Technical Hints to connect the temperature sensor.
3. Air temperature =______.
4. Knowing your classroom temperature, use the graph to figure out the maximum amount of water that one cubic meter of your classroom air can hold (maximum water density). You can use the ruler tool to read values on the graph. See Technical Hints for using the Ruler Tool.
5. Maximum water density =_____g/m³
6. How close was this to your prediction?
7. This is the maximum amount of water a cubic meter of your classroom air can hold. This would be the water density if the relative humidity was 100. If the relative humidity was 50, the air would hold half as much water.
8. Now measure the relative humidity of your classroom air. Refer to the Relative Humidity Measurement activity to take this measurement. Record the relative humidity.
9. Relative humidity =______%.
10. Using the chart below, how much water is in a cubic meter of air in your classroom?
    
11. Classroom water density =_____g/m ³
12. Figure out a way to calculate the approximate volume of air in your entire classroom. Describe your method and record the result below.
13. Classroom air volume =____m ³
14. Knowing the volume of air and the water density for the air in your classroom, figure out the amount of water in your classroom. Be prepared to discuss your calculations and results with the class.
15. Water in the classroom air =____g

1. How many liters of water are in your classroom air?
2. Do you think the humidity is the same at night?
3. What classroom activities might change the humidity?

• Whenever the humidity is 80% or more, mold will grow on a surface. Think of places in buildings where this might be true long enough for mold to grow. Make a list of places you have seen mold growing. Was the humidity high? What caused the humidity?
• Is the humidity outdoors the same as the humidity indoors? Why or why not?

Compare the relative humidity in different places: classroom, kitchen, locker room, shower room, indoor swimming pool, walk-in cooler.