Measuring Relative Humidity - Science Fair Projects

You may have noticed that humidity (the amount of water vapor in the air) makes a difference in how comfortable you feel outside. When the humidity is high in the summer we say it is "muggy." When it is cool and humidity is high we may say that it feels "clammy." Humidity can make a warm day feel warmer and a cool day feel cold. You can observe the difference that humidity makes.

In this experiment, you will determine the relative humidity using wet-bulb and dry-bulb thermometers. Such a device is called a psychrometer.

What you need:

2 identical alcohol thermometers (Celsius)
Heavy cardboard rectangle, about 10 inches long
Strong tape
Fan
Cotton balls
Rubber band
Warm water

What to do:

1. Use tape to secure the thermometers to the cardboard rectangle about 5 inches apart. Make sure the bulb ends of the thermometers hang below the edge of the cardboard by at least 1 inch and the numbered sides of the thermometers can be easily read.

2. Set the thermometers in front of a small fan set to a moderate speed for 5 minutes and record the temperature readings. This is to verify that your thermometers are calibrated and show the same temperature under control conditions.

3. Saturate several cotton balls with warm water and use the rubber band to fix them around the bulb of one thermometer. Set the thermometers in front of the fan as before. After 5 minutes record the “wet bulb” temperature reading and the “dry bulb” temperature reading.

4. To determine relative humidity subtract the lower “wet bulb” temperature from the higher “dry bulb” temperature to determine the difference and compare to the chart.

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Relative Humidity (%)
Dry-Bulb Temp (°C)	Difference between Wet-Bulb and Dry-Bulb Temperatures (°C)
0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
-20	100	28	Row 3 - Cell 3	Row 3 - Cell 4	Row 3 - Cell 5	Row 3 - Cell 6	Row 3 - Cell 7	Row 3 - Cell 8	Row 3 - Cell 9	Row 3 - Cell 10	Row 3 - Cell 11	Row 3 - Cell 12	Row 3 - Cell 13	Row 3 - Cell 14	Row 3 - Cell 15	Row 3 - Cell 16
-18	100	40	Row 4 - Cell 3	Row 4 - Cell 4	Row 4 - Cell 5	Row 4 - Cell 6	Row 4 - Cell 7	Row 4 - Cell 8	Row 4 - Cell 9	Row 4 - Cell 10	Row 4 - Cell 11	Row 4 - Cell 12	Row 4 - Cell 13	Row 4 - Cell 14	Row 4 - Cell 15	Row 4 - Cell 16
-16	100	48	Row 5 - Cell 3	Row 5 - Cell 4	Row 5 - Cell 5	Row 5 - Cell 6	Row 5 - Cell 7	Row 5 - Cell 8	Row 5 - Cell 9	Row 5 - Cell 10	Row 5 - Cell 11	Row 5 - Cell 12	Row 5 - Cell 13	Row 5 - Cell 14	Row 5 - Cell 15	Row 5 - Cell 16
-14	100	55	11	Row 6 - Cell 4	Row 6 - Cell 5	Row 6 - Cell 6	Row 6 - Cell 7	Row 6 - Cell 8	Row 6 - Cell 9	Row 6 - Cell 10	Row 6 - Cell 11	Row 6 - Cell 12	Row 6 - Cell 13	Row 6 - Cell 14	Row 6 - Cell 15	Row 6 - Cell 16
-12	100	61	23	Row 7 - Cell 4	Row 7 - Cell 5	Row 7 - Cell 6	Row 7 - Cell 7	Row 7 - Cell 8	Row 7 - Cell 9	Row 7 - Cell 10	Row 7 - Cell 11	Row 7 - Cell 12	Row 7 - Cell 13	Row 7 - Cell 14	Row 7 - Cell 15	Row 7 - Cell 16
-10	100	66	33	Row 8 - Cell 4	Row 8 - Cell 5	Row 8 - Cell 6	Row 8 - Cell 7	Row 8 - Cell 8	Row 8 - Cell 9	Row 8 - Cell 10	Row 8 - Cell 11	Row 8 - Cell 12	Row 8 - Cell 13	Row 8 - Cell 14	Row 8 - Cell 15	Row 8 - Cell 16
-8	100	71	41	13	Row 9 - Cell 5	Row 9 - Cell 6	Row 9 - Cell 7	Row 9 - Cell 8	Row 9 - Cell 9	Row 9 - Cell 10	Row 9 - Cell 11	Row 9 - Cell 12	Row 9 - Cell 13	Row 9 - Cell 14	Row 9 - Cell 15	Row 9 - Cell 16
-6	100	73	48	20	Row 10 - Cell 5	Row 10 - Cell 6	Row 10 - Cell 7	Row 10 - Cell 8	Row 10 - Cell 9	Row 10 - Cell 10	Row 10 - Cell 11	Row 10 - Cell 12	Row 10 - Cell 13	Row 10 - Cell 14	Row 10 - Cell 15	Row 10 - Cell 16
-4	100	77	54	32	11	Row 11 - Cell 6	Row 11 - Cell 7	Row 11 - Cell 8	Row 11 - Cell 9	Row 11 - Cell 10	Row 11 - Cell 11	Row 11 - Cell 12	Row 11 - Cell 13	Row 11 - Cell 14	Row 11 - Cell 15	Row 11 - Cell 16
-2	100	79	58	37	20	1	Row 12 - Cell 7	Row 12 - Cell 8	Row 12 - Cell 9	Row 12 - Cell 10	Row 12 - Cell 11	Row 12 - Cell 12	Row 12 - Cell 13	Row 12 - Cell 14	Row 12 - Cell 15	Row 12 - Cell 16
0	100	81	63	45	28	11	Row 13 - Cell 7	Row 13 - Cell 8	Row 13 - Cell 9	Row 13 - Cell 10	Row 13 - Cell 11	Row 13 - Cell 12	Row 13 - Cell 13	Row 13 - Cell 14	Row 13 - Cell 15	Row 13 - Cell 16
2	100	83	67	51	36	20	6	Row 14 - Cell 8	Row 14 - Cell 9	Row 14 - Cell 10	Row 14 - Cell 11	Row 14 - Cell 12	Row 14 - Cell 13	Row 14 - Cell 14	Row 14 - Cell 15	Row 14 - Cell 16
4	100	85	70	56	42	27	14	Row 15 - Cell 8	Row 15 - Cell 9	Row 15 - Cell 10	Row 15 - Cell 11	Row 15 - Cell 12	Row 15 - Cell 13	Row 15 - Cell 14	Row 15 - Cell 15	Row 15 - Cell 16
6	100	86	72	59	46	35	22	10	Row 16 - Cell 9	Row 16 - Cell 10	Row 16 - Cell 11	Row 16 - Cell 12	Row 16 - Cell 13	Row 16 - Cell 14	Row 16 - Cell 15	Row 16 - Cell 16
8	100	87	74	62	51	39	28	17	6	Row 17 - Cell 10	Row 17 - Cell 11	Row 17 - Cell 12	Row 17 - Cell 13	Row 17 - Cell 14	Row 17 - Cell 15	Row 17 - Cell 16
10	100	88	76	65	54	43	33	24	13	4	Row 18 - Cell 11	Row 18 - Cell 12	Row 18 - Cell 13	Row 18 - Cell 14	Row 18 - Cell 15	Row 18 - Cell 16
12	100	88	78	67	57	48	38	28	19	10	2	Row 19 - Cell 12	Row 19 - Cell 13	Row 19 - Cell 14	Row 19 - Cell 15	Row 19 - Cell 16
14	100	89	79	69	60	50	41	33	25	16	8	1	Row 20 - Cell 13	Row 20 - Cell 14	Row 20 - Cell 15	Row 20 - Cell 16
16	100	90	80	71	62	54	45	37	29	21	14	7	1	Row 21 - Cell 14	Row 21 - Cell 15	Row 21 - Cell 16
18	100	91	81	72	64	56	48	40	33	26	19	12	6	Row 22 - Cell 14	Row 22 - Cell 15	Row 22 - Cell 16
20	100	92	82	74	66	58	51	44	36	30	23	17	11	5	Row 23 - Cell 15	Row 23 - Cell 16
22	100	92	83	75	68	60	53	46	40	33	27	21	15	10	4	Row 24 - Cell 16
24	100	92	84	76	69	62	55	49	42	36	30	25	20	14	9	4
26	100	92	85	77	70	64	57	51	45	39	34	28	23	18	13	9
28	100	93	86	78	71	65	59	53	47	42	36	31	26	21	17	12
30	100	93	86	79	72	66	61	55	49	44	39	34	29	25	20	16

Results:

Even though the air temperature did not change, you should have noticed a lower temperature reading from the wet-bulb thermometer. This happens because as the cotton balls were drying, some of the water evaporated (changed to water vapor.)

This change of state, from liquid water to water vapor, requires a bit of heat energy from the surrounding atmosphere. The same thing happens when ice melts into liquid water. On the other hand, a tiny bit of heat is released when water vapor condenses back into liquid form or when water freezes to form ice. This explains why you feel cold when you get out of a pool on a windy day, even though the air temperature is high, the water evaporating from your skin is pulling the evaporation heat from your body instead of the air!