31 



CHAPTER IV. VISIBILITY 



4000. GENERAL 



4001. Visibility is a term that denotes the greatest distance from an ob- 

 server that an object of known characteristics can be seen and identified. 

 For purposes of weather reports, the term expresses the maximum visibility 

 common to one-half or more of the horizon circle. This is called the 

 prevailing visibility. 



4100. DETERMINATION OF VISIBILITY 



4101. Whenever possible, 

 objects whose distance from 

 other ships). Estimations of 

 apparent size and the portion 

 determining the distance of a 

 is a guide to determining the 

 a familiar type of ship wh 

 viewed from a height of 40 

 7.6 nautical miles. 



estimations of visibility will be based upon 

 the observer is known (i.e., the horizon or 



the distance to a ship may be based on its 

 visible. Visibility may also be estimated by 

 passing vessel by means of radar. Table 4-1 

 distance to the horizon and to objects such as 

 ose height is known, e.g., the horizon, when 

 feet above sea level, appears at a distance of 



4102. Estimations of visibility will be based on the sharpness with 

 which the object stands out. Sharp outlines, with little or no blurring of 

 color, indicate that the visibility is much greater than the distance to the 

 object. On the other hand, blurred or indistinct objects indicate the pres- 

 ence of haze or other phenomena that has reduced the visibility to about 

 the distance to the objects. 



Table 4-1. — Distance to objects on the horizon at sea (nautical miles) 



Height of observer's 



eyes above sea 



level (feet) 



10 



15 



20 



25 



30 



35 



40 



45 



50 



60 



70 



80 



90 



100 



Height of object above sea level (feet) 



3.8 

 4.6 

 5. 4 

 6.0 

 6.6 



7. 1 

 7.6 

 8.0 



8. 5 



9. 3 

 10.0 



10. 7 



11. 4 

 12.0 



10 



7.2 



8.0 



8.7 



9.3 



9.9 



10. 4 



10.8 



11.3 



11. 

 12. 

 13. 

 13. 

 14. 

 15. 



20 



8. 7 



9. 5 

 10. 2 

 10. 8 

 11.4 

 11.9 

 12. 3 

 12. 8 

 13.2 

 14. 



14. 7 



15. 4 



16. 

 16.6 



30 



9.9 

 10.7 



11. 4 



12. 



12. 6 



13. 1 



13. 5 



14. 

 14. 4 

 15 2 

 15 9 



16. 6 



17. 2 

 17. 8 



40 



10. 8 

 11.6 

 12. 3 



12. 9 



13. 5 



14. 

 14. 4 

 14. 9 

 15 3 

 16. 1 



16. 8 



17. 5 



18. 1 

 18.7 



60 



12. 5 



13. 3 



14. 

 14. 6 



15 

 15 

 16. 

 16. 

 17. 



17. 8 



18. 5 



19. 2 



19. 8 



20. 4 



80 



13. 

 14. 

 15. 

 16. 

 16. 

 17. 

 17. 

 18. 

 18. 

 19. 

 19. 

 20. 

 21. 

 21. 



100 



15 1 



15. 9 



16. 6 

 17.2 

 17.8 

 18.3 



18. 7 



19. 2 



19. 6 



20. 4 



21. 1 



21. 8 



22. 4 

 23.0 



150 



17 

 18. 

 19. 2 



19. 8 



20. 4 



20. 9 



21. 3 



21. 8 



22. 2 

 23.0 



23. 7 



24. 4 

 25.0 



25. 6 



200 



19. 8 



20. 6 



21. 3 



21. 9 



22. 5 

 23.0 



23. 4 



23. 9 



24. 3 



25. 1 

 25. 8 

 26 5 

 27. 1 

 27.7 



300 



23. 5 



24. 3 

 25.0 



25. 6 

 26 

 26 

 27 

 27 



28. 

 28.8 



29. 5 



30. 2 



30. 8 



31. 4 



400 



26. 5 



27. 3 



28. 



28. 6 



29. 2 

 29.7 



30. I 



30. 6 



31. 



31. 8 



32. 5 



33. 2 



33. 8 



34. 4 



600 



31.6 



32. 4 



33. 1 



33. 7 

 34.3 



34. 8 



35. 2 



35. 7 

 36 1 

 36 9 



37. 6 



38. 3 



38. 9 



39. 5 



800 



36 



36. 8 



37.5 



38 



38 



39. 



39. 



40. 



40. 



41. 



42. 



42. 7 



43. 3 

 43. 9 



1,000 



39.8 

 40.6 

 41.3 



41. 9 



42. 5 

 43.0 



43. 4 



43. 9 



44. 3 



45. 1 



45. 8 



46. 5 



47. 1 

 47.7 



