Visibility and Visibility Reducing Phenomena

1. Visibility and Visibility Reducing Phenomena

2. Meteorological Visibility

3. Meteorological Visibility • Is a measure of the clarity of the atmosphere. • It is more properly known as the Meteorological Optical Range (MOR). • Visibility (Met Vis) • is the greatest horizontal distance at which suitable objects can be recognised for what they are in daylight or • at which lights of specified intensity can be seen at night by a person with normal sight. • Where the visiblity is variable the lowest value is reported.

4. Visiblity Reporting • Lowest value is normally reported for Met Vis. • In METARS the lowest value is always given and the highest values may be reported under certain conditions. • Runway Visual Range (RVR) reporting begins when the Met Vis or the RVR fall to below 1500 m. • RVR is the maximum distance in direction of take-off or landing at which designated runway markers or lights can be seen from a centreline height corresponding to the eye-level of the pilot on touch-down.

5. Visibility Reporting (cont.) • RVR reporting begins when the horizontal visibility or the RVR is less than 1500 m. • At a/d’s with Instrumented RVR (IRVR) systems, RVR may also be reported when • the observed value is at or below the maximum reportable value (usually 1500m) or • when shallow fog is forecast or reported. • RVR is passed to a/c before take-off and during approach to land. • Changes in RVR are passed to a/c during approach or if pilot reports or ATC observation indicate the RVR is worse than that indicted by equipment

6. Visibility Reporting (cont) • RVR is reported by human observer (OBS) or by IRVR systems. • Multi site IRVR systems report touchdown (TDz), midpoint (MID) and stop-end (END) values. • Mid-point and/or stop-end values are suppressed when: • they are 800 m or more or • they are equal to or higher than the touch-down zone unless they are less than 400 m.

7. Visibility Reporting Problems • Lack of reference objects e.g. at sea or level snow areas. • Depends on size, shape and colour of the object. • Object illumination and background. • Transparency of wind shield. • Visibility varies in layers and in direction. • Slant visual range in flight. • Met vis and RVR may vary widely.

8. In-Flight Visibility in Poor Conditions

9. Summary • Inside an obscuring layer • Increasing height decreases circle of visibility. • Decreasing height increases circle of visibility.

10. In-flight visibility above a layer of poor visibility

11. Summary • Above an obscuring layer • decreasing height decreases circle of visibility • increasing height increases circle of visibility

12. In-flight visibility on the approach Slant Range Slant Range

13. Summary • On the approach • Airfield clearly visible from directly above. • On glide path forward visibility lost due to slant range through obscuring layer. • Contact with airfield will be lost. • Instrument approach will be necessary.

14. Effect of Precipitation on Visibility • Drizzle commonly restricts visibility to between 500 metres and 3,000 metres. • Light rain has virtually no effect. • Moderate rain can reduce visibility to 3-10 km. • Heavy rain showers of temperate latitudes rarely cause a reduction below 1,000 metres. • Snow and blown snow are both very effective in restricting visibility to less than 1,000 metres. • Heavy snow may lead to visibilities of the order of 50-250 metres.

15. Flight in Rain • Rain on the windscreen can have two opposite effects. • It can scatter light and so reduce its intensity. • the runway lighting may appear further away than it is or, • It may cause the runway lights to “bloom” thus • they appear larger and closer than they really are. • Care must be taken to avoid an inadvertent descent well below a safe approach path, when subject to such illusion. • In heavy rain a film of water on the windscreen can cause refraction.