RVR is the primary minimum for takeoff and landing when it is available. And it is unique to a partricular runway. Some large, busy airports may have RVR for two or more runways, and they may not necessarily be the same. If RVR is available, it is the only requirmemt that must be met; ceiling and prevailing visibility are not considered. But what exactly is RVR. It is not the same as the prevailing reported visibility, it is always higher. It is reported in feet or meters rather than miles of kiliometers. But what is RVR telling is exactly? Two great big ata-boys to the winner.Bob P

RVR is the primary minimum for takeoff and landing when it is available. And it is unique to a partricular runway. Some large, busy airports may have RVR for two or more runways, and they may not necessarily be the same. If RVR is available, it is the only requirmemt that must be met; ceiling and prevailing visibility are not considered. But what exactly is RVR. It is not the same as the prevailing reported visibility, it is always higher. It is reported in feet or meters rather than miles of kiliometers. But what is RVR telling is exactly? Two great big ata-boys to the winner.Bob P

RVR is calculated by determining how far down an aircraft can be sitting on the center line and still see the runway markings. Essentially it is telling you how far you must be able to see down the runway in order to make a safe landing.

RVR is reported as a replacement for the generic patchy (ground) fog when it is available. It has the most effect for CAT III landings but can apply to any landing where air and above ground limits are OK and only ground fog is the limitation. It is finer ground visibility report than the rounded "normal" report in METARS.Slant or vertical visibility might be good enough to get site of the runway threshold but RVR insures that a successful touchdown and roll-out can be accommodated in heavy precipitation.If long enough runways may have several detectors, one at each end and one at mid-point.http://en.wikipedia.org/wiki/Runway_visual_rangehttp://rvr.fly.faa.gov/rvr/help.htmlhttp://rvr.fly.faa.gov/rvr-cgi-bin/rvr-status.plhttp://rvr.fly.faa.gov/rvr-cgi-bin/rvr-det...large&fs=lg

Along each instrument runway with RVR is an instrument called a transmissometer. It is a light source of known intensity at one end and a photoelectric detector about 600 to 1000 feet away. Knowing how much light leaves the light source and how much arrives at the detector gives a measurement of what the restrictions to visisibility are along that runway. From this raw data a prevailing visibility can be determined. RVR goes one step further. Along each side of the instrument runway are high-intensity runway lights defining the boundaries of the runway. There are five settings for these runway lights, setting one the being the lowest and setting five being the highest. RVR is determined only for the highest 3 settings: three through five. The setting in use is dermined by the control tower operator. RVR is how far away you will be able to see those lights with the previling visibilty at that light setting.The raw data from the transmissometer is fed into a computer which samples it for an for an appropriate time period and updates the display in the tower cab, approach control, and the weather, office. There are sets of tables available to find an instantaneous RVR reading from the chart displaying the raw data. In this way, the observer may determine a potential RVR if the light intensity were changed to a higher setting. There are actually six different tables, three for daylight, and three for nighttime. At night lights are much more easily observed, so nighttime RVR values will be much higher than daytime values with the same prevailing visibility conditions. In other words it's easier to get landing minimums at night.Bob P

Along each instrument runway with RVR is an instrument called a transmissometer. It is a light source of known intensity at one end and a photoelectric detector about 600 to 1000 feet away. Knowing how much light leaves the light source and how much arrives at the detector gives a measurement of what the restrictions to visisibility are along that runway. From this raw data a prevailing visibility can be determined. RVR goes one step further. Along each side of the instrument runway are high-intensity runway lights defining the boundaries of the runway. There are five settings for these runway lights, setting one the being the lowest and setting five being the highest. RVR is determined only for the highest 3 settings: three through five. The setting in use is dermined by the control tower operator. RVR is how far away you will be able to see those lights with the previling visibilty at that light setting.The raw data from the transmissometer is fed into a computer which samples it for an for an appropriate time period and updates the display in the tower cab, approach control, and the weather, office. There are sets of tables available to find an instantaneous RVR reading from the chart displaying the raw data. In this way, the observer may determine a potential RVR if the light intensity were changed to a higher setting. There are actually six different tables, three for daylight, and three for nighttime. At night lights are much more easily observed, so nighttime RVR values will be much higher than daytime values with the same prevailing visibility conditions. In other words it's easier to get landing minimums at night.Bob P

Thanks for the explaination.