Visibility — Page 274, Lesson 263

I want to walk you through visibility measurement, starting with how different precipitation types affect visibility, then moving into how we actually measure it, and finally into Runway Visual Range and the transmissometer. Let's begin with the visibility reductions caused by precipitation. For drizzle, visibility is reduced to between 500 and 3000 metres. For rain, it depends on intensity: moderate rain gives visibility between 3000 metres and 10 kilometres, while heavy rain brings visibility down to less than 1000 metres. Snow is much more restrictive: moderate snow gives 1000 metres visibility, heavy snow drops it to between 50 and 200 metres. Now, there's an important distinction between drifting snow and blowing snow. Drifting snow is defined as snow less than 2 metres above the surface, and it will reduce the visibility values I just gave you. Blowing snow is defined as snow at or above 2 metres above the surface, and it will greatly reduce those visibility values. So the key difference is the height of the snow particles above the ground, and blowing snow has a much more severe effect. Now let's look at how visibility is actually measured. By day, measurements are made by reference to suitable objects at known distances from an observing position. You can see this arrangement in figure 15.9. By night, we use instruments: transmissometers or forward scatter meters are used to measure visibility at night. This brings us to Runway Visual Range, or RVR. RVR is defined as the maximum distance that a pilot, positioned 15 feet above the runway in the touchdown area, can see marker boards by day or runway lights by night when looking in the direction of take-off or landing. So it's a very specific measurement: the pilot's eye height is 15 feet, the location is the touchdown zone, and the direction is along the runway. RVR can be assessed manually by positioning an observer 76 metres from the centre line of the runway in the touchdown area, who then sights the number of marker boards or lights in the appropriate direction. RVR is reported when meteorological optical range, or MOR, or instrumented RVR, sometimes written as (I)RVR, falls to less than 1500 metres, or when shallow fog is reported or forecast. The United Kingdom standard RVR reporting incremental scale is very specific: 25 metre increments between 0 and 400 metres, 50 metre increments between 400 and 800 metres, and 100 metre increments above 800 metres. So the finer the visibility, the smaller the reporting step. As for how often readings are taken: if traffic is more or less continuous, readings are taken every 30 minutes, or when a significant change in the normal visibility occurs. If traffic is light, readings are taken 15 minutes before a take-off or landing. Importantly, RVR is never forecast — it's a real-time measurement only. There is no direct connection between RVR and MOR, but there are factors which may be applied for regulatory purposes, which you'll see in Air Law Notes. Generally speaking, RVR is greater than MOR. Finally, let's look at the transmissometer. This is an electronic device where the intensity of a light at a distance from a photo-electric cell gives an indication of the equivalent daytime visibility. The advantage is that it provides constant measurement of visibility. The disadvantage is that only a small portion of the atmosphere is being sampled — so it's a very localised measurement. You can see this setup in figure 15.11.