Date: May 1994

These lesson plans are the result of the work of the teachers who have attended the Columbia Education Center's Summer Workshop. CEC is a consortium of teacher from 14 western states dedicated to improving the quality of education in the rural, western, United States, and particularly the quality of math and science Education. CEC uses Big Sky Telegraph as the hub of their telecommunications network that allows the participating teachers to stay in contact with their trainers and peers that they have met at the Workshops.

Grade Level(s): 3, 4, 5, 6, 7, 8, 9, 10, 11, 12

Subject(s):

• Science/Meteorology

OVERVIEW: Humidity is moisture in the air. When air feels damp, the humidity is high. When air feels dry, the humidity is low. The amount of water the air can hold is determined by the temperature. Warm air can hold much more moisture than cold air. (See Relative Humidity Table for specific examples.)

When air can hold more moisture at a given temperature it is said to be saturated. Relative humidity is the percentage of saturation. A psychrometer is one tool for measuring relative humidity. The bulb, or rounded end of one thermometer is covered with a wet cloth. After the thermometers are fanned the wet bulb temperature will usually be lower that the dry bulb. The water evaporating from the cloths lowers the temperature of the wet bulb because evaporation is a cooling process. The relative humidity can then be read from the chart (provided at the end of this lesson).

OBJECTIVES:

Students will demonstrate their knowledge of the atmosphere and humidity by building and using their own psychrometer.

MATERIALS:

• tape
• water
• 2 identical thermometers
• gauze (2" X 2")
• rubberband
• piece of cardboard (8" X 11")
• Relative Humidity Chart

Relative Humidity Chart in .pdf format; requires free Adobe Acrobat Reader.

Click the icon to obtain the free Reader.

1. Wrap the gauze around one of the bulbs and tie it firmly in place with a rubber band.

2. Wet the gauze.

3. Place the thermometers side by side with the two bulbs just hanging over the edge of a desk or table.

4. Use the cardboard to fan the thermometers. (Be careful not to bump or hit the thermometer while they are taped down, or they could break).

5. Fan vigorously until the temperature of the thermometer with the wet bulb stops going down.

6. Record the temperature readings on both thermometer.

7. Subtract the wet bulb temperature from the dry bulb temperature and record the difference. Note: With the more advanced classes use Fahrenheit thermometers and require them to convert from Fahrenheit to Celsius and then find the difference!!

8. Look at the Relative Humidity Table. The numbers in the left hand column represent the dry bulb reading. The numbers across the top represent the difference between the wet and dry bulb readings. This is the answer you obtained from step #7. Follow your dry bulb temperature across and follow your difference column down until they intersect. Record the number at this point. The number you have recorded is the relative humidity of your room.

Extension:

1. Find the Relative Humidity of another room (i.e. an empty classroom). Any difference? Why?
2. Find the Relative Humidity outdoors