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13.1.4 A signal data converter and related items.
13.1.5 A remote digital or remote display program-mer.
13.2 The transmissometer projector and receiver are mounted on towers 250 feet apart. A known intensity of light is emitted from the projector and is measured by the receiver. Any obscuring matter, such as rain, snow, dust, fog, haze, or smoke, reduces the light intensity arriving at the receiver. The resultant intensity measurement is then converted to an RVR value by the signal data converter. These values are displayed by readout equipment in the associated air traffic facility and updated approximately once every minute for controller issuance to pilots.
13.3 The signal data converter receives information on the high.intensity runway edge light setting in use (step 3, 4, or 5), transmission values from the transmissometer, and the sensing of day or night conditions. From the three data sources, the system will compute appropriate RVR values.
13.4 An RVR transmissometer established on a 250.foot baseline provides digital readouts to a minimum of 600 feet, which are displayed in 200.foot increments to 3,000 feet, and in 500.foot increments from 3,000 feet to a maximum value of 6,000 feet.
13.5 RVR values for Category IIIa operations extend down to 700.foot RVR; however, only 600 and 800 feet are reportable RVR increments. The 800 RVR reportable value covers a range of 701 feet to 900 feet and is therefore a valid minimum indication of Category IIIa operations.

13.6 Approach categories with the corresponding minimum RVR values are listed in TBL GEN 3.5.6.
TBL GEN 3.5.6
Approach Category/Minimum RVR Table
Category  Visibility (RVR) 
Nonprecision  2,400 feet 
Category I  1,800 feet* 
Category II  1,000 feet 
Category IIIa   700 feet 
Category IIIb  150 feet 
Category IIIc  0 feet 

* 1,400 feet with special equipment and authorization
13.7 Ten.minute maximum and minimum RVR values for the designated RVR runway are reported in the body of the aviation weather report when the prevailing visibility is less than 1 mile and/or the RVR is 6,000 feet or less. ATCTs report RVR when the prevailing visibility is 1 mile or less and/or the RVR is 6,000 feet or less.
13.8 Details on the requirements for the operational use of RVR are contained in FAA Advisory Circular 97.1, “Runway Visual Range (RVR).” Pilots are responsible for compliance with minimums prescribed for their class of operations in appropriate Federal Aviation Regulations and/or operations specifications.
13.8.1 RVR values are also measured by forward scatter meters mounted on 14.foot frangible fiberglass poles. A full RVR system consists of:
13.8.1.1 Forward scatter meter with a transmitter, receiver and associated items.
13.8.1.2 A runway light intensity monitor (RLIM).
13.8.1.3 An ambient light sensor (ALS).
13.8.1.4 A data processor unit (DPU).
13.8.1.5 A controller display (CD).
13.8.2 The forward scatter meter is mounted on a 14.foot frangible pole. Infrared light is emitted from the transmitter and received by the receiver. Any obscuring matter such as rain, snow, dust, fog, haze, or smoke increases the amount of scattered light reaching the receiver. The resulting measurement along with inputs from the runway light intensity monitor and the ambient light sensor are forwarded to the DPU which calculates the proper RVR value. The RVR values are displayed locally and remotely on controller displays.
13.8.3 The runway light intensity monitors both the runway edge and centerline light step settings (steps 1 through 5). Centerline light step settings are used for CAT IIIb operations.  Edge light step settings are used for CAT I, II, and IIIa operations.
13.8.4 New Generation RVRs can measure and display RVR values down to the lowest limits of Category IIIb operations (150 foot RVR). RVR values are displayed in 100.foot increments and are reported as follows:
13.8.4.1 100.foot increments for products below 800 feet.
13.8.4.2 200.foot increments for products between 800 feet and 3,000 feet.
13.8.4.3 500.foot increments for products between 3,000 feet and 6,500 feet.
13.8.4.4 25.meter increments for products below 150 meters.
13.8.4.5 50.meter increments for products between 150 meters and 800 meters.
13.8.4.6 100.meter increments for products be-tween 800 meters and 1,200 meters.
13.8.4.7
200.meter increments for products be-tween 1,200 meters and 2,000 meters.

14.
Reporting of Cloud Heights

14.1 Ceiling, by definition in Federal Aviation Regulations, and as used in Aviation Weather Reports and Forecasts, is the height above ground (or water) level of the lowest layer of clouds or obscuring phenomenon that is reported as “broken,” “overcast,” or “the vertical visibility into an obscuration.” For example, an aerodrome forecast which reads “BKN030” refers to heights above ground level (AGL). An area forecast which reads “BKN030” states that the height is above mean sea level (MSL). See FIG GEN 3.5.24 for the Key to Routine Aviation Weather Reports and Forecasts for the definition of “broken,” “overcast,” and “obscuration.”
　
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