Visibility

Wing Commander Rajagopal

Visibility

Definition

  • Meteorological Optical Range (Visibility) is the distance to which prominent objects can be seen and recognised by the naked eye.
  • Stable conditions in high-pressure areas cause poor visibility since polluted or moist air is not permitted to rise.
  • Meteorological offices report the lowest horizontal visibility around the airfield.
  • Smoke, dust, sand, precipitation, and suspended water droplets reduce visibility.

Visibility by Day and Night

  • Day visibility: greatest horizontal distance at which a dark object near the ground can be recognised against a bright background.
  • Night visibility: greatest distance at which a lighted object of 1000 candle power can be recognised against an unlit background.
  • Gold’s visibility meter or scopograph are used for measurement.
  • A graduated filter and distant light of known intensity are used to determine meteorological visibility.

Variation of Visibility

  • Slant visibility refers to visibility from an altitude; it may be poorer than vertical visibility.
  • Oblique visibility occurs when viewing through fog; it improves when climbing above haze.
  • Visibility varies along the flight path and may not always match reported meteorological visibility.
  • Looking into the sun reduces visibility; looking away improves it.

Runway Visual Range (RVR)

  • Runway Visual Range is the maximum distance a pilot can see runway markings or lights along the runway.
  • RVR is reported when it is less than 1500 m.
  • A transmissometer measures light intensity along the runway using photoelectric cells.

RVR Report

  • RVR is reported every 15–30 minutes in defined intervals.
  • Intervals:
    • 0–200 m: 25 m intervals
    • 200–800 m: 50 m intervals
    • Above 800 m: 100 m intervals
  • Example: R27L 600 400 550 (touchdown, midpoint, stop-end values).

Mist, Haze, and Fog

  • Mist: visibility 1000–5000 m, relative humidity ~95%.
  • Fog: visibility below 1000 m, relative humidity ~100%.
  • Smog: mixture of smoke, haze, and fog.
  • Fog dissipates when temperature rises above dew point.

Smoke, Sand, and Dust Haze

  • Smoke haze: caused by industrial or vehicle emissions.
  • Haze: suspended particles reducing visibility below 5000 m.
  • Dust/sand haze: common in deserts, extending up to 6–8 km.
  • Dust storms can reduce visibility below 1000 m.

Rain, Snow, Sea Spray, and Volcanic Ash

  • Heavy rain and snow can reduce visibility below 1 km.
  • Moderate rain may reduce visibility below 10 km.
  • Sea spray consists of salt particles over ocean surfaces.
  • Volcanic ash significantly reduces visibility and affects aviation.

Radiation Fog

  • Forms due to nocturnal cooling of the ground by conduction.
  • Occurs in stable atmosphere, high humidity, clear skies, and light winds.
  • Light winds (3–7 knots) allow fog formation up to 1 km or 1500 ft.
  • Fog dissipates after sunrise due to surface heating or mixing.

Location of Radiation Fog

  • Radiation fog forms over land at night in high pressure or col areas
  • Valley floors provide moisture source from rivers increasing probability of Radiation Fog
  • During winter months radiation fog can be seen after the passage of western disturbance
  • Fog dissipates when surface air gets warmer above dew point after sunrise
  • Introduction of hygroscopic nuclei to create water droplets also dissipates fog
  • Radiation fog also dissipates due to advection of dry winds into the area converting fog into low level stratus clouds

Hill and Advection Fog

  • Hill fog covers hilltops, reducing visibility below 200 m.
  • Advection fog forms when warm moist air moves over a cold surface.
  • Common in coastal regions such as Bengal and Odisha.
  • Requires high humidity, cold surface, and moderate winds.

Frontal and Steaming Fog

  • Frontal fog occurs along frontal boundaries, especially warm fronts.
  • Steaming (sea smoke) occurs when cold air moves over warm sea water.
  • Common in polar regions and can extend up to 500 ft above sea level.