Visibility and Fog

Meteorology · Chapter 7 · DGCA CPL and ATPL

What steals a pilot's view of the runway, from Delhi's winter fog to dust haze and volcanic ash, and the delicate balance that makes radiation fog form.

Remember

Fog: visibility below 1000 m, RH near 100%. Mist: 1000-5000 m, RH near 100%. Haze: 5000 m or less, dry particles. Dust storm: 5000 m or less.
Reported visibility is the lowest horizontal visibility at the airfield. RVR is reported when visibility or RVR is below 1500 m.
Radiation fog: high RH, clear sky, light wind (3-7 kt), stable air; highs, ridges and cols. Calm gives dew, strong wind gives stratus. It forms or thickens around sunrise.
In North India radiation fog is a winter hazard, especially after a Western Disturbance has brought rain and then cleared. Depth: a few hundred feet up to about 1 km.
Advection fog: warm moist air over a cold surface, over land and sea, at any time of day. Steaming fog (sea smoke): cold air over warm water. Frontal fog: with warm fronts.

What can hide a runway from a pilot?

What hides the runway

Real satellite pictures of real days over India. Swipe or tap Next.

27 December 2019: the plains vanish A sheet of fog fills the whole Indo-Gangetic plain from Punjab to Bihar, pressed against the Himalayas. Fog is a suspension of water droplets (or ice crystals) with visibility below 1000 m and humidity near 100%: cloud sitting on the ground. Delhi, Lucknow, Patna and Varanasi were all under it that morning.
Clear on top, blind at the bottom Radiation fog is usually shallow, a few hundred feet, with a sharp top and clear air above. From overhead a pilot may see the field straight down through it; on the approach, looking through a long slant of fog, the runway can disappear.
4 November 2019: smoke over Punjab and Delhi Smoke from crop-stubble fires and cities spreads as a grey haze when the wind is light and a ground inversion traps it. Haze is a suspension of tiny particles; visibility is 5000 m or less and the air looks milky. Mixed with fog it becomes smog: very poor visibility and a health hazard.
14 June 2018: a wall of dust Strong summer winds lift dust from the deserts of Pakistan and Rajasthan. In a dust storm visibility is 5000 m or less, often far less; it clears fast if rain follows. Dust haze can stretch from Rajasthan across Punjab, Haryana, UP and Bihar, and reach 6-8 km up, so diversion airfields are often just as bad. It is thicker by day, when the wind is strong, than at night.
The most dangerous obscurity Ash from an eruption is carried far by the wind and stays aloft for a long time. Visibility drops to almost nothing, and the ash can stop jet engines and sandblast the windscreen. India has an active volcano on Barren Island in the Andamans (12.16° N 93.51° E), erupting more than ten times since 1787, most recently in 2020; the Baratang mud volcano, 150 km north of Port Blair, last erupted in 2005.
Salt and water Salt from breaking waves helps water condense into a haze of drops; flying low, salt can even crust the windscreen. Rain lowers visibility by its intensity and drop size: moderate rain 1-3 km, heavy rain or showers below 1000 m, drizzle 3 km or less, snow below 1000 m.

Takeaway: Water drops (fog, mist), dry particles (haze, dust, smoke), volcanic ash and falling rain all cut visibility. Which word is used depends on what is in the air and how far you can see.

What exactly is "visibility"?

What visibility means

The definition, by day and by night.

By day The greatest distance at which a black object of suitable size, near the ground, can be seen and recognised against a bright background.
At night The greatest distance at which lights of about 1000 candela can be seen and identified against an unlit background. Visibility is the greater of the two.
How it is judged By day observers use runway markings and objects at measured distances; at night, known lights. These are visibility landmarks. The report gives the lowest horizontal visibility at the airfield (it can differ by direction), which can also differ from what a pilot sees vertically or on a slant.
Towards the sun Looking towards the sun, visibility is worse; looking towards the moon it is better, thanks to the moonlight it reflects. Higher up the air is clearer, except in cloud and precipitation.

Takeaway: Visibility is the greater of: the farthest distance a dark object near the ground can be seen and recognised against a bright background, or the farthest a light of about 1000 candela can be seen against a dark one. The met office reports the lowest horizontal visibility round the field.