148 A YEAR IN SCIENCE 



21. Of what importance is rainfall to man? 



22. What is meant by irrigation? 



23. What proportion of the land in the United 

 States is too arid to grow crops successfully? 



24. How can you account for the wet and dry sea- 

 sons of Southern California? 



25. Why is there a scanty rainfall just east of the 

 Eocky Mountains? 



26. What is the average rainfall in the region in 

 which you live? 



27. What is the cause of most of the rain in the 

 Middle West? 



28. In what parts of the United States is the amount 

 of rainfall greatest? 



29. What parts have the least rain? 



