The Weather 55 



The temperature at which condensation takes place is the dew 

 point. The dew point varies with the absolute humidity of the atmos- 

 phere. 



When water vapor condenses on cold objects, it forms dew ; when 

 it condenses on minute particles in the atmosphere, it forms fog, 

 cloud, or rain, depending on the position and size of the drops. 



Sleet is frozen rain. Hail is generally made up of alternate layers 

 of ice and snow. 



When water vapor condenses below the freezing point, it forms 

 frost on objects, and clouds or snow in the air. 



REVIEW QUESTIONS 



1. Why does moist, warm air rise? 2. How can you demonstrate 

 that warm air is less dense than cold ? 3. What effect upon the 

 density of the air has water vapor? 4. What change in the water 

 vapor of the air takes place when air rises to regions of less pressure? 

 5. What is the source of the water vapor which is in the air ? 6. What 

 evidence have you that water exists in the air as an invisible vapor? 

 7. (a) For what is the hygrometer used? (6) Describe the construc- 

 tion of the wet- and dry-bulb hygrometer, (c) Explain the principle 

 on which it works. 8. Which thermometer indicates the " sensible " 

 temperature? 9. What is meant by saturated air? 10. About 

 what is the maximum amount of water vapor which may exist in the 

 air at a temperature of 95 F. ? At normal room temperature? 

 11. In what two ways can we express absolute humidity? 



12. Explain the effect of temperature upon saturation. 13. Give 

 from the table, page 45, figures not given from the text to show that 

 you understand the meaning of relative humidity. 14. Define rela- 

 tive humidity. 15. Absolute humidity remaining constant, under 

 what change of temperature does the relative humidity rise? Fall? 



16. What is any temperature called at which condensation begins? 



17. Given an absolute humidity of .63 ounce per cubic yard and a 

 temperature of 86 F., how many degrees must the temperature fall 

 to produce saturation? If it falls to 59 F., how much moisture per 

 cubic yard will be condensed? 18. (a) Describe an experiment in 

 which the dew point was found directly. (6) How may it be com- 

 puted from the reading of wet- and dry-bulb thermometers? 



19. Explain the formation of clouds. 20. Why does it rain? 

 21. Low-pressure areas in crossing the continent give conditions for 

 precipitation usually in what region? 22. Where in the western states 

 is the annual rainfall exceptionally heavy? Why? 23. Where in the 



