Chapter 19 Visibility Visibility reducing phenomena Meteorological Visibility

Chapter 19 Visibility & Visibility reducing phenomena

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 visibility is variable the lowest value is reported.

Visibility & Visibility reducing phenomena FOG, MIST & HAZE

Fog/Mist/Haze • Fog: – is composed of small water droplets in suspension (or ice crystals in ice fog). – Visibility is reduced to less than 1000 m. – Relative humidity is generally 100%. – Plentiful supply of condensation nuclei.

Types of Fog • • Radiation Fog Smoke Fog (Smog) Advection Fog Thaw Fog Arctic Sea Smoke (Steam Fog) Frontal Fog Hill Fog

Mist • Mist: – is also caused by small droplets in suspension. – Visibility is 1000 m or more. – METAR and TAF codes impose an upper limit of 5000 m. – RH is at least 95% but less than 100%. – Plentiful supply of condensation nuclei.

Haze • Haze: – caused by solid particles in suspension. – visiblity in haze may reduce to fog levels but generally it is better and has an upper limit of 5000 m for reporting purposes. – airborne solid particles from industrial processes are the most frequent cause of haze as well as fires.

Visibility

Solid Visibility Reducers Smoke

Smoke • Consists of solid particles produced by combustion (e. g forest fires) • Conditions worse under stable (subsiding air) atmosphere

Smoke • Reduction dependent upon: –Rate of production –Rate of dispersal by wind –Distance from smoke source

Smoke • Smoke can also produce the necessary hygroscopic nuclei for radiation to form on

Dust • Particles less than 0. 08 mm in diameter • Can be carried high in the atmosphere, especially with wind speeds > 15 kt

Dust • Dust storm areas: – West & North Africa (Harmattan, Khamsin, etc) • Dust storms occur in daylight due to diurnal variation • Particles may remain in suspension causing poor visibility for a day or two

Sand • Consists of particles between 0. 08 & 0. 3 mm in diameter • Sand storms with winds of 20 kt or more • A daylight event due to DV of wind

Visibility • Dust or sand storms imply a visibility < 1000 m

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.

Effect of Precipitation on Visibility • Snow and blown snow are both very effective in restricting visibility to less than 1, 000 m. • Heavy snow may lead to visibilities of the order of 50 -250 metres.

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.

Flight in Rain • 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 cause refraction.

In-Flight Visibility in Poor Conditions

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

In-flight visibility above a layer of poor visibility

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

In-flight visibility on the approach nt Ra ng nge e t Ra Slan Sla

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.

Visibility 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 takeoff 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 touchdown.

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 1500 m) 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

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.

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.