Chapter 19 Visibility & Visibility reducing phenomena

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. Meteorological Visibility

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 1000m 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 5000m 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

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 0.08mm in diameter Can be carried high in the atmosphere, especially with wind speeds > 15kt

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 & 0.3mm in diameter Sand storms with winds of 20kt or more A daylight event due to DV of wind

Visibility Dust or sand storms imply a visibility < 1000m

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 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 can 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 Slant Range

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 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.

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

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.