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Materials perTeam

• two thermometers

• pieces of cloth orpaper towels

• water

• string (optional)

• wet and dry bulb

Relative HumidityHow Much Water Is in the Air?

Spill a small amount of water onto a hard surface (spreadingsome with a sponge onto the chalkboard works well) or pour it

into a shallow container. Notice how rapidly it evaporates. Discussthe fate of the water with the students—where did it go?

Water commonly exists in three forms—solid (ice), liquid, andgas (vapor). When it transforms from one stage to another, itundergoes a physical change. The water molecules in our examplechanged to a gaseous state by evaporating. The opposite changefrom gas to liquid is called condensation.

At different times, atmospheric conditions can cause quicker orslower evaporation of water. Meteorologists can use a device calleda hygrometer to measure evaporation rates and the resulting valueis called relative humidity. Relative humidity is expressed as apercentage ratio—what is the current evaporation rate comparedto the maximum, or saturation, rate for the current temperature?Stated more simply, how much water is in the air?

At 100% relative humidity, an equal amount of water condensesas evaporates. The air is in effect full of water, so when more waterevaporates, an equal amount condenses. Think about a night whenfog or mist is in the air.

Conversely, as relative humidity values become lower, there isa greater difference between evaporation and condensation. Theair has a high capacity to accept evaporated water without havingany condense. Think of a very dry day in the desert when waterdries up quickly. By recording and comparing the differencebetween the temperature of a thermometer with a dry bulb andone with a wet bulb, students can measure relative humidity.

To begin, have each team set up two thermometers. Theyshould wrap the bulb of one with a cloth or paper towel dipped inroom temperature water. This one is the wet bulb thermometer.The other will simply measure air temperature, so it will be the drybulb thermometer. Have students record the temperatures for atwenty-minute period or until the wet bulb temperature becomes

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steady. They should graph both temperatures on the same padusing different colors.

The wet bulb temperature will be lower because the evaporatingwater creates a cooling mechanism. Technically, the water’s changein state requires energy, which it removes from the air in the formof heat energy.

A large difference between temperatures means lots ofevaporation is occurring around the wet bulb, lowering itstemperature. More water is evaporating than condensing. Therelative humidity is low so the air has lots of capacity to acceptwater vapor.

If the difference between the wet bulb and dry bulbtemperatures is small, little evaporation is occurring and relativehumidity is high. The air’s capacity for water vapor is small. Thereis no room for newly evaporating water unless some vapor that isalready in the air condenses. Use the chart on the next page to helpstudents convert the temperature data to relative humidity.

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All temperatures are shown in degrees Celsius.

Wet and Dry Bulb TableTo determine relative humidity, find the dry bulb temperature on the left and the difference betweenthat and the wet bulb temperature on the top. Where the row and column meet, read the relativehumidity as a percentage.

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

5 72 58 45 33

6 73 60 48 35

7 74 62 50 38 26

8 75 63 51 40 29 19

9 76 64 53 42 32 22 12

10 77 66 55 44 34 24 15 6

11 78 67 56 46 36 27 18 9

12 78 68 58 48 39 29 21 12 7

13 79 69 59 50 41 32 23 15 10

14 79 70 60 51 42 34 26 18 13

15 80 71 61 53 44 36 27 20 13 6

16 81 71 63 54 46 38 30 23 15 8

17 81 72 64 55 47 40 32 25 18 11

18 82 73 65 57 49 41 34 27 20 14 7

19 82 74 65 58 50 43 36 29 22 16 10

20 83 74 66 59 51 44 37 31 24 18 12 6

21 83 75 67 60 53 46 39 32 26 20 14 9 6

22 83 76 68 61 54 47 40 34 28 22 17 11 8

23 84 76 69 62 55 48 42 36 30 24 19 13 10

24 84 77 69 62 56 49 43 37 31 26 20 15 12 5

25 84 77 70 63 57 50 44 39 33 28 22 17 14 8

26 85 78 71 64 58 51 46 40 34 29 24 19 16 10 5

27 85 78 71 65 58 52 47 41 36 31 26 21 18 12 7

28 85 78 72 65 59 53 48 42 37 32 27 22 19 13 9

29 86 79 72 66 60 54 49 43 38 33 28 24 21 15 11

30 86 79 73 67 61 55 50 46 39 35 30 25 22 17 13

Difference between Dry Bulb and Wet Bulb Temperature

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R E A D I N G:

It’s Not the Heat,It’s the Humidity

Have you ever heard an uncomfortable person say, “It’s notthe heat, it’s the humidity?” They are suffering not only

because the temperature is high but also because the air is damp.Maybe you’ve heard another weather cliché—”It’s hot, but it’s adry heat.” That saying downplays the discomfort one feels in hightemperatures in dry climates. When the air is dry, heat is not sooppressive. What do these expressions mean, and are they true?

Water is present in air as a vapor. We see water condense fromthe air when drops appear on cooling surfaces. For example, aglass holding a cold drink will get wet on the outside. Dew formson low-growing plants and other surfaces when the temperaturedrops at night and reaches the dew point. These examples also tellus that warm air can hold more water than cool air.

In 1783, Horace de Saussure (1740–1799) used a human hair totell if the amount of water in the air was high or low. As manypeople notice when they complain about a “bad hair day,” thelength of a hair varies according to humidity. Saussure’s inventionand other devices that measure water vapor content are calledhygrometers. Some hygrometers compare temperature data fromwet and dry thermometers. With that information, you could thenread a chart telling relative humidity for each difference.

If the air were saturated, or full of water vapor, it would beholding as much vapor as possible at that particular temperature.Any new water evaporating makes vapor already in the aircondense, or become liquid. Think about a foggy night when theair feels wet. Relative humidity measures the percentage of themaximum water vapor the air is currently holding.

If you can safely get near a heater that is blowing warmed air(a heat vent in a room or the outlet of a clothes dryer will work),try this experiment. Hold your hand in the path of the heated airand notice how warm it feels. Then wet your hand with roomtemperature or warm water. Place your hand in front of the heateragain. How does the air feel now?

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Even though the water on your hand is warm and the airhitting your hand is also warm, you feel a cooling from the heater’sair flow. That’s because the water on your hand is evaporating andthat requires energy. Your hand is cooled.

You can measure relative humidity using two thermometers. Ifyou keep the bulb of one thermometer wet and one dry, theirtemperatures will differ. Some of the water around the bulb of thewet thermometer will evaporate. Measure the two temperaturesand plot them on a chart. The greater the difference between the twotemperatures, the lower the relative humidity. Can you explain why?

The wet bulb temperature will be lower because the evaporatingwater creates a cooling mechanism. The water’s change in statefrom liquid to gas requires energy, which it removes from the air inthe form of heat energy. As the heat is absorbed by the evaporatingwater, the temperature around the wet bulb drops.

A large difference between temperatures means lots ofevaporation is occurring around the wet bulb. The relativehumidity is low so the air has lots of capacity to accept watervapor.

A small difference between the wet bulb and dry bulbtemperatures means little evaporation is occurring and relativehumidity is high. The air’s capacity for water vapor is small. Thereis no room for newly evaporating water unless some vapor alreadyin the air condenses.

Let’s return to our two weather sayings. When someone says,“It’s not the heat, it’s the humidity,” they are probably sweaty andsticky. Perspiration is the body’s way of cooling itself. Waterevaporates from our skin, using heat energy to change from liquidto vapor. The process of cooling is happening in the same way theblowing heater cools a hand in the example described earlier. Buton a humid day, the air is closer to saturation, and perspirationevaporates slowly. Cooling does not occur. In a dry climate, waterevaporates quickly, so if we perspire, the water evaporates quicklyand we feel more comfortable. That means a dry heat is morecomfortable than a humid heat. However, some climates are drybut so hot that people still feel uncomfortable. After all, an oven isa dry heat, too!

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Vocabulary Words

condense ................................. to become liquid from vapor

dew ............................................water that condenses on cooler surfaces

perspiration ............................. salty liquid released through the skin; “sweat”

relative humidity .................... amount of water vapor in the air, described as apercentage of the maximum amount of vapor the aircan hold

saturated .................................. unable to hold any more; completely full

vapor ......................................... gas

TOPICS FORFURTHER STUDY

Horace deSaussure

dew point

hygrometersrelativehumidity for avariety of cities