Why does Earth have deserts?

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Across
  1. 4. polite behavior
  2. 5. become aware of (something) for the first time
  3. 6. an imaginary circle around the middle of the Earth
  4. 9. having very few plants
  5. 11. forest a tropical forest
  6. 13. feeling strong excitement about something
Down
  1. 1. an idea or set of ideas
  2. 2. a small amount of water
  3. 3. a science that deals with weather
  4. 7. green
  5. 8. not likely
  6. 10. to feel slightly angry
  7. 12. up with
  8. 14. to travel on water