It’s All Relative!
Name: ____________________________ Hr.: ___
Date: ________________ Due Date: ___________
Have you ever heard the word “muggy” used to describe the weather?
How about the phrase, “hazy, hot, and humid”? These phrases are used to
describe times when there is considerable moisture in the air. Of course,
there is always moisture in the air, but the amount varies. If air is warm and
dry, it is easier for water to evaporate and enter the air as water vapor. If the
air is cold and already has a lot of water vapor in it, much less will evaporate
into the air.
At every temperature there is a limit to the amount of water vapor that
the air can hold. The warmer the air is, the higher this limit is. When the air is holding all the water
vapor it can, it is saturated. Usually, however, there is less water vapor present in the air than it can
hold.
Relative humidity is a measure of how much water vapor is actually in the air compared to the
amount the air could possibly hold. On a muggy day, relative humidity can be high, between 80 and
100 percent. When the relative humidity is this high, perspiration (sweat) does not easily evaporate.
This means the heat in the sweat is not removed as quickly and as a result, we feel very warm.
Saturated air has a relative humidity of 100 percent. Clouds or fog form when, and where, the
air is saturated. In desert areas there is little water to enter the air and relative humidity is low.
Relative humidity is measured with an instrument called a sling psychrometer or with a hygrometer.
A sling psychrometer is a simple device made up of two thermometers. One thermometer’s
bulb is covered with material which is dipped in distilled water and is called the “wet bulb”. The other
one stays uncovered and dry, and is called the “dry bulb”. After the wick is dipped in water, the sling
psychrometer is whirled around using the
handle. Water evaporates from the wick on the
wet-bulb thermometer, cooling the thermometer
because heat is removed when the water
evaporates away. This value is known as the
wet-bulb temperature. The drier the air the more
the thermometer cools and hence, the lower the
wet-bulb temperature.
To determine relative humidity, record the
dry bulb and wet bulb temperatures, find the
differences between the two, and then use a
table like the one on the next page. First, find
the row for the observed dry bulb temperature,
then move across the row to the column for the difference between the observed dry and wet
thermometers. The number in that square is the percent relative humidity. For example, if the dry
bulb temperature is 20C and the difference between the two temperatures is 10C, the relative
humidity is 26%.
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RELATIVE HUMIDITY TABLE (SHOWN IN %)
_______________________________________________________
DIFFERENCE BETWEEN WET AND DRY BULB READINGS IN CELSIUS
_______________________________________________________
1
2
3
4
5
6
7
8
9
10
Dry Bulb Reading
_______________________________________________________
10
88 77 66
55 44 34 24 15
6
11
89 78 67
56 46 36 27 18
9
12
89 78 68
58 48 39 29 21
12
13
89 79 69
59 50 41 32 22
15
7
14
90 79 70
60 51 42 34
2
18
10
15
90 80 71
61 53 44 36 27
20
13
16
90 81 71
63 54 46 38 30
22
15
17
90 81 72
64 55 47 40 32
25
18
18
91 82 73
65 57 49 41 34
27
20
19
91 82 74
65 58 50 43 36
29
22
20
91 83 74
67 59 53 46 39
32
26
21
91 83 75
67 60 53 46 39
32
26
22
83 76 68
61 54 47 40 34
28
23
92 84 76
69 62 55 48 42
36
30
24
92 84 77
69 62 56 49 43
37
31
25
92 84 77
70 63 57 50 44
39
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1. Use the sling psychrometer to find the relative humidity of the classroom.
A. Record the two readings in Celsius: Wet Bulb: ________ Dry Bulb: ________
B. Calculate the difference between the readings. __________
2. Use the Relative Humidity Table above to determine the relative humidity of your classroom.
Relative humidity of the classroom: ___________
3. Predict how the relative humidity will change or not change if these steps were performed outside.
_____________________________________________________________________________
4. Rewet the material around the wet bulb, and repeat steps 3-5 outside your school.
Relative humidity of the playground: _________________
5. How does the playground reading compare with inside the classroom? _____________________
_____________________________________________________________________________
6. In what rooms of your school would you expect to find high relative humidity? ________________
Why? ________________________________________________________________________
7. In what rooms would you expect to find low relative humidity? ___________________________
Why? ________________________________________________________________________
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8. What happens to the air’s capacity (ability) to become saturated (filled up with) with water vapor,
as the temperature increases? ___________________________________________________
____________________________________________________________________________
Assessment:
1. Why use the term relative humidity instead of just humidity? ___________________________
___________________________________________________________________________
2. Draw the line on a graph showing the relationship between humidity and temperature based on
what happens at 100% relative humidity. (See the graph in your textbook.)
Humidity (%)
Temperature (Cº)
3. Only the reading on the wet-bulb thermometer is changed as air passes across each
thermometer. Explain why. ______________________________________________
____________________________________________________________________
4. Write a brief paragraph explaining why it takes longer for a towel to air dry in the summer than
in the winter, or why peoples’ skin gets dry in the winter and not so much in the summer.
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
5. Describe what type of weather you could expect if relative humidity is at 100%?
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
6. Explain why it is possible to be cooler on a hot dry day than on a hot humid day.
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
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