128 A YEAR IN SCIENCE 



14. Under what conditions may dust be harmful? 



15. How is the atmosphere heated? 



16. What are. the sources of water vapor in the 

 atmosphere ? 



17. What is meant by the expression "air is 

 saturated"? 



18. Define humidity. Dew point. 



19. Under what conditions is dew formed? Frost? 



20. Why does covering plants with a piece of paper 

 or cloth help to keep them from freezing? 



21. What are clouds? What is their distance from 

 the surface of the earth? 



22. Under what conditions is' rain formed? Hail? 

 Snow ? 



23. How can you account for the fact that a large 

 hailstone is made of concentric layers of ice? 



