continuous indication of the visibility (reported in statute  miles or fractions of miles) for the runway. RVV is used in lieu  of prevailing visibility in determining minimums for a particular  runway.  Tower visibility is the prevailing visibility determined from  the airport traffic control tower at locations that also report  the surface visibility.  Runway visual range is an instrumentally derived value, based on standard calibrations, that represents the horizontal distance a pilot sees down the runway from the approach end. It is based on the sighting of either high intensity runway lights or on the visual contrast of other targets, whichever yields the greater visual range. RVR, in contrast to prevailing or runway visibility, is based on what a pilot in a moving aircraft should see looking down the runway. RVR is horizontal visual range, not slant visual range. RVR is reported in hundreds of feet, so the values must be converted to SM if the visibility in SM is not reported. [Figure 1-10] It is based on the measurement of a transmissometer made near the touchdown point of the instrument runway and is reported in hundreds of feet. RVR is used in lieu of RVV and/or prevailing visibility in determining minimums for a particular runway. The following are types of RVR that may be used: Another set of lower-than-standard takeoff minimums is  available to Part 121 and Part 135 operations as outlined in  their respective OpSpecs document. When certain types of  visibility reports are unavailable or specific equipment is out  of service, the flight can still depart the airport if the pilot  can maintain adequate visual reference. An appropriate  visual aid must be available to ensure the takeoff surface  can be continuously identified, and directional control can  be maintained throughout the takeoff run. Appropriate  visual aids include high intensity runway lights, runway  centerline lights, runway centerline markings, or other  runway lighting and markings. With adequate visual  references and appropriate OpSpec approval, commercial  operators may take off with a visibility of 1600 RVR or ¼  SM.  RVR is the primary visibility measurement used by Part  121 and Part 135 operators with specific visibility reports  and controlling values outlined in their respective OpSpecs.  Under their OpSpecs agreements, the operator must have  specific, current RVR reports, if available, to proceed with an  instrument departure. OpSpecs also outline which visibility  report is controlling in various departure scenarios.  Ceiling is the height above the earth’s surface of the lowest  layer of clouds or obscuring phenomena that is reported as  broken, overcast, or obscuration and not classified as thin  or partial.  1-11  Ceilings  Adequate Visual Reference  Types of RVR Tower Visibility  Runway Visual Range (RVR)  Figure 1-11. AWSS installation at Driggs-Reed, Idaho.  • Touchdown RVR—the RVR visibility readout values  obtained from RVR equipment serving the runway  touchdown zone.  • Mid-RVR—the RVR readout values obtained from  RVR equipment located near the runway midpoint .  • Rollout RVR—the RVR readout values obtained from  RVR equipment located nearest the rollout end of  the runway.  • Far End RVR—when four RVR visibility sensors (VS)  are installed, the far end RVR VS is the touchdown  RVR VS on the reciprocal runway. The far end sensor  will serve as additional information. 