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.
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!