# Relative Humidity of Your School

By Betty Watt

Grade level: Middle School (5 - 8)

Summary:

In this activity the students will predict the location where the relative humidity will the highest and lowest in and around their school. The students will then use a sling psychrometer to measure the relative humidity of the various locations. The students will need to be able to read a thermometer accurately and they will be required to read a relative humidity conversion table. Each student will have a psychrometer to use. Each lab group will be assigned a different location to determine the relative humidity. The individual results will be averaged and reported to the other lab groups. A graph will be constructed of the results. Class discussion will focus on the reasoning behind the predictions and conclusions drawn from the results.

Teacher Background Information

This activity should be done after the water cycle has been introduced. The students should know the definition of weather and also the characteristics of the troposphere that cause it to be the only layer that has weather.

Student Background Information

Air's ability to hold water vapor depends on the temperature of the air. As the temperature increases, the air's capacity for water vapor increases. An increase in temperature causes molecules in air to spread apart which creates more space for water molecules. The amount of water vapor in the air compared with the maximum amount of water vapor that the air is able to hold at a given temperature  is called the relative humidity. Relative humidity is given as a percent and can calculated using the following formula:  Relative humidity =  amount of water vapor in the air  x 100.

One instrument that measures relative humidity is a psychrometer. A psychrometer contains two thermometers. The bulb of one thermometer is covered with a wet piece of cloth. It is called a wet-bulb thermometer, When water evaporates from the cloth, the wet-bulb thermometer shows a decrease in temperature because the process of evaporation requires heat. The other thermometer is called a dry-bulb thermometer. Since no evaporation takes place on the bulb of this thermometer, the dry-bulb thermometer's temperature does not change.

If the air is dry, water evaporates rates quickly from the wet-bulb thermometer and there is a large decrease in the wet-bulb temperature. This makes the difference in the temperatures of the two thermometers large. When the air is holding a large amount of water vapor, little water evaporates from the wet-bulb and the difference between the temperatures of the two thermometers is small. The difference in temperatures is an indication of the amount of water vapor in the air. A relative humidity conversion table allows you to use these two temperatures to determine relative humidity.

Objectives

Predict where the highest relative humidity will be in your school.  Predict where the lowest relative humidity will be in your school.  Use a psychrometer to measure relative humidity in various locations

Materials (one per student)

• sling psychrometer
• relative humidity conversion table
• pencil
• data table

Procedures

1. Predict where you think the highest relative humidity will be in or around your school. Write down this prediction and tell why.
2. Predict where you think the lowest relative humidity will be in or around your school. Write down this prediction and tell why.
4. Share your answers with the class.
5. Fill in the blank spaces on the data table with any places given by classmates that were not already listed.
6. Your teacher will assign you the locations your lab group is to test. At each location, follow steps 7 - 14.
7. Wet the cotton cloth with tap water. Squeeze out the excess.
8. Be sure you are far enough away from other people and objects that you can sling your psychrometer safely,
9. Slowly sling the psychrometer for 20 seconds. Record the wet- bulb temperature, sling the psychrometer again for 20 seconds. If the reading has not changed on the wet-bulb thermometer, record the dry-bulb temperature. If it has changed, record the new wet-bulb temperature. Repeat this process until the wet-bulb temperature does not change.
10. Calculate the difference between the wet-bulb and dry-bulb temperatures.
11. Find this number in the top row of the relative humidity table.
12. Find the dry-bulb reading temperature in the left-hand column of the relative humidity table.
13. Find where the column for the difference numbers and the row for the dry-bulb temperature meet. Record this as your relative humidity.
14. Calculate the average relative humidity for each location for your lab group.

Relative Humidity Data Table

Each time you use the psychrometer

1. Use your thumb to warm up the wet-bulb thermometer the cotton sock on the wet-bulb thermometer.
2. Re-wet the cotton sock on the wet-bulb thermometer.
 Place Dry Bulb Reading (oC) Wet Bulb Reading (oC) Temp Difference (oC) Relative Humidity (%) Your Lab Table Area Outside Office Counselor's Office Library Gym Cafeteria 1st Floor Halfway 2nd Floor Hallway Girl's Locker Room Boy's Locker Room

Data Table This table gives the average for each place measured.

 Place Relative Humidity % Your Lab Table Area Outside Office Counselor's Office Library Gym Caferteria 1st Floor Hallway 2nd Floor Hallway Girl's Locker Room Boy's Locker Room

Results and Conclusions

1. Construct a graph of the results on the data table of averages.
2. Where was the relative humidity the highest? Why? Did you predict this location?
3. Where was the relative humidity the lowest? Why? Did you predict this location?
4. Did any of the results surprise you? Explain your answer.

Teaching Notes

Have all of the students find the relative humidity of the area around their table and find the average for each lab group. Share those with the whole class. If there are large differences, you should have a discussion of possible reasons for this before moving on to the other locations.

Talk with the principal and other teachers involved asking permission to have students in each area. It might be a good idea to announce this activity to the entire faculty, so that everyone knows what will happen. It would still be a good idea for each group to ask permission to use the area; it would be good practice for the students of in important social skill. It also might require that they explain to another person exactly what they will be doing and why.

If the temperature outside is below freezing, the psychrometer will note give accurate results in that environment.

Sling psychrometers can be purchased through any science supply catalogue at a cost of under \$2.00 per instrument. It is also possible to make your own by taping two thermometers onto a piece of cardboard side by side. The wet-bulb cloth can be made by cutting an athletic shoe shoestring into short pieces that can be slipped over the bulb. A small rubber band can be used to hold it in place. Instead of 'slinging', you should fan the wet-bulb with an index card following the same time procedures as described in the lab.

Activity Extension

Have the students construct a daily weather chart. Have them decide individually or in groups what characteristics should be measured, what units each is measured in, and how to best set up the chart. Share ideas with the whole class. After construction of the chart, record daily weather conditions. These results can then be used to make basic forecasts, to compare with forecasts made by newspapers and TV stations, to construct graphs, etc.