on other ships in the area, as well as coastal formations and prominent objects ashore. Coastal formations and prominent objects ashore may be used as visibility markers. Direct coordination between the observer and CIC is necessary to obtain timely and accurate distances to observable objects. This may be done through one of the  Lookouts  equipped  with  a  sound-powered  phone, on the "JL" circuit. All ships and most large coastal objects may also be used as distance markers at night; their navigation lights should be clearly seen. CIC will also be able to inform you of the probable light patterns and  colors  that  various  objects  may  be  showing. Radar returns from landmasses or isolated rain showers may also provide a valuable indication of visibility range. The distance to the horizon also plays an important part in visibility observations at sea. (The distance to the horizon in nautical miles is 1.15 times the square root of the height, in feet, of your eyes above the water.) From the deck of a small boat, the horizon is only about 3 nautical miles away. From ships with weather  decks  about  30  feet  above  the  water,  an observer sees the horizon at about 7 nautical miles. And from the flight deck of an aircraft carrier (average 65 feet above the water), a standing observer’s eyes are about 70 to 71 feet above the water, and the horizon is seen at just under 10 nautical miles. These distances usually limit what an observer may be able to see. A table of the distance an object may be seen based on the height of the observer’s eyes above sea versus the height of the object is provided in NAVMETOCCOMINST 3144.1. In  certain  situations,  prevailing  visibility  may fluctuate up and down during the observation period. In those cases, average visibility is used and is called variable visibility. The observer must note the lowest and highest visibility for entry on the observation record. SECTOR  VISIBILITY A sector is any portion of the area surrounding the station out as far as the horizon. When the visibility surrounding  the  station  is  not  uniformly  equal  in  all directions   and   the   difference   is   operationally significant, then each area with a different visibility is designated a sector. The size of the sector, extending in a pie-slice out from the observation point, is as large or as small as is required to describe the area affected by the different visibility, but must be limited to 1/8 (45°) of the horizon circle. Sector visibility is commonly used at air stations that have lakes, rivers, or swamps nearby, which favor fog development. Visibility in fog over a swamp area may, for example, be 3 miles, while the remainder of the area has 7 miles visibility in haze. Smoke and localized rain showers are other phenomena that commonly cause poorer visibility in a sector. Once  an  area  of  lower  or  higher  visibility  is identified, the directions of the S-point compass (N, NE, E, SE, S, SW, W, and NW) are usually used to identify the sector. Each sector, using the 8-point compass, covers 45° of azimuth centered on the compass point identified. Sector visibility is reported in the observation only when it differs from the prevailing visibility, and either the prevailing or sector visibility is less than 3 miles. DIFFERING  LEVEL  VISIBILITY Differing level visibility is any prevailing visibility observed from an elevation or location other than the official  observation  site.  Differing  level  visibility  is commonly evaluated from the aircraft control tower by certified tower visibility observers. In this case the prevailing  visibility  is  usually  called  tower visibility. Tower visibility may differ from the airfield-level prevailing visibility based on the type of obstruction-to- vision present. Differing level visibility is only reported when  the  prevailing  visibility  is  4  miles  or  less. RUNWAY VISUAL RANGE The runway visual range, abbreviated RVR, is an instrument measurement of the distance the pilot can see down the runway as an aircraft touches down during landing. RVR is observed at shore stations using the AN/GMQ-32  transmissometer  when  the  prevailing visibility or sector visibility falls below 2 miles, but is only reportable when the prevailing visibility is 1 mile or less or the RVR value for the runway is 6,000 feet or less. NAVMETOCCOMINST 3 141.2 further outlines procedures for reporting RVR. When the prevailing visibility falls to less than 7 miles, the reason that the visibility is restricted must be noted in the observation. Any phenomenon that reduces visibility is called an "obstruction to vision." The occurrence  of  "weather,"  such  as  precipitation,  also may reduce visibility. In the next section, we will cover weather and obstructions to vision. REVIEW QUESTIONS Q31. What are the four types of visibility that may be observed? Q32. Define prevailing visibility. 1-29