HOW MOISTURE GETS INTO' THE AIR 



TOPIC 1. HOW MOISTURE GETS INTO THE AIR 



SUGGESTED PROBLEMS AND QUESTIONS 



1. Where does moisture in the air come from? 



2. What is evaporation and what are the factors 

 which affect it? 



3. What is the importance of relative humidity 

 and how is it measured? 



4. How does the electric refrigerator work? 



SUGGESTIONS AND HELPS FOR STUDY 



1. Carefully review the study plan suggested on 

 page 1. The answers to the following questions will 

 aid in the solution of problem 2. 



a. What examples of evaporation have you noticed? 



b. What happens when evaporation takes place? 



c. What sort of evaporation is boiling? 



2. Your solution of problem 2 will be aided by 

 learning the factors which affect evaporation and then 

 experimenting with them. Several experiments are 

 suggested below. 



3. In solving problems 3 and 4, try to find answers to 

 the following questions either in your reference study 

 or by carrying on experiments. 



a. When evaporation takes place, what is the effect 

 upon the temperature of the surroundings? 



b. How does evaporation affect our body tempera- 

 ture? 



c. Why does an electric fan make one feel more 

 comfortable on a hot day? 



d. How does relative humidity affect our health? 



e. How may relative humidity be determined? 



4. You may find the following new words and 

 phrases in this study: 



dew point the temperature at which dew begins to form 

 because of the condensation of the moisture in the air. 



evaporation changing from the liquid to the vapor state. 



humid moist, filled with water vapor. 



humidity the moisture in the air. 



precipitation water which comes out of the air in the 

 form of rain, snow, or sleet. 



relative humidity the amount of moisture actually pres- 

 ent in the atmosphere at any temperature compared 

 with the amount which could be held without precipi- 

 tation at the same temperature. It is usually ex- 

 pressed in percentage. 



saturated completely filled. 



vaporize to change from a liquid to a vapor. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTION'S 



Experiment 19. What factors affect evaporation? 



a. With a cloth or sponge moisten two spots of equal size 

 on a blackboard or a piece of slate. Fan one spot and leave 

 the other undisturbed. 



b. Warm a place on a blackboard or a piece of slate by 

 shining an electric heater on it, or by a gas flame. Moisten 

 a spot on this warm place and another of equal size on a 

 colder portion. Leave them for a few moments and ob- 

 serve closely. 



c. From wooden strips about two inches by one inch 

 and eighteen inches long, make a square frame. Secure au 

 old towel or piece of cheesecloth and tack over this frame. 

 Wet the cloth thoroughly. Moisten two spots of equal size- 

 on the blackboard or a piece of slate. Leave one open to 

 the air and place the frame over the other. Observe them 

 after several minutes. 



' In your notebook 1 record the notes of this experiment and 

 complete the statements given below. 



a. The moisture from the . spot evaporated more 



rapidly. This shows that wind (aids, retards) evapora- 

 tion. T& 



b. The water on the spot which was warmed the 



water vapor rapidly than did the one which was not 



warmed. 



c. Water will evaporate more rapidly if the air near it 



is This was proved when the water evaporated from 



the (open, covered) spot more rapidly than from the 



one which was . 



Experiment 20. Does evaporation cool? 



Place a few drops of alcohol on the back of your hand 

 and notice the result. Write your observations and infer- 

 ences from this experiment in your notebook. 2 



Experiment 21. How may the relative humidity of 

 the air be measured? 



Secure two thermometers which read 

 exactly alike. About the bulb of one 

 place a piece of cheesecloth which has 

 been moistened in distilled water. Fan 

 the wet bulb and set the dry-bulb 

 thermometer aside. When the mercury 

 of the wet bulb has gone down as low 

 as it will go, read and record the tem- 

 perature of each thermometer. Record 

 your results in your notebook. 3 Study 

 carefully the table on page 30 and find 

 the row of figures across the top marked 

 as the "difference between the dry-bulb 

 and wet-bulb reading." Find the dif- 

 ference which your experiment gave. 

 Read down the column of figures under 

 that difference until you come to the 

 one opposite the reading of the dry-bulb 

 in the column at the extreme left. This 

 is the relative humidity for the time of 

 your experiment. For example, if your 

 dry-bulb reading is 71 degrees and the FIG. 48 



1 See accompanying workbook, p. 12. 

 1 See accompanying workbook, p. 12. 

 ' See accompanying workbook, p. 13. 



