SUNLIGHT 17 



heat waves from the sun set in motion air currents. 13. How 

 • !•• ■ i iids cool the surface of the earth? 14. How are rains dis- 

 tiil.iit.d over the earth? 15. How is the earth kept warm at 

 night? 16. What is radiation? 17. Why do farm crops grow 

 more rapidly in northern than in southern countries? 





