Visibility & fog

B. Advection fog

Advection means horizontal movement. Warm moist air carried over colder land or sea is cooled from below to its dew point - advection fog. Radiation fog needs no movement, just a clear night.

B. Below 1000 m

FG is reported when water droplets reduce the visibility to less than 1000 m. From 1000 m up to 5000 m with humid air it is mist (BR).

B. 1000 m to 5000 m

BR (mist) is reported for visibility from 1000 m to 5000 m with high humidity. Below 1000 m it becomes fog; in dry air the same visibility would be haze.

B. Moisture, a clear sky and a light wind

A clear sky lets the ground radiate its heat away; moist air reaches its dew point; a light wind (about 2-8 kt) mixes the cooling through a shallow layer. Calm gives only dew, and a strong wind mixes too deep or lifts it into stratus.

B. 10 km or more

CAVOK needs all three: visibility 10 km or more; no cloud below 1500 m (5000 ft) or the highest minimum sector altitude, whichever is greater, and no CB or TCU; and no significant weather.

B. After a western disturbance

After a WD passes, the skies clear, the winds fall light and the ground is still moist from its rain - perfect for radiation fog over north India on the winter nights that follow.

C. Around midnight to early morning

It builds through the night as the ground keeps cooling, and is usually thickest around dawn, just after the minimum temperature, before the sun burns it off.

B. Rime

Supercooled fog droplets freeze on contact with the cold airframe as rime - white, opaque and rough.

B. Ahead & during

Ahead of a warm front the steady rain from As and Ns lowers the visibility, and frontal fog forms as the rain saturates the cold air underneath. At the front itself the cloud base is at its lowest, the rain heaviest, and the visibility at its worst. It improves once the front has passed.

C. At any time of day or night

Advection fog depends on warm moist air moving over a cold surface, not on night-time cooling, so it can form - and persist - at any time, day or night, and with stronger winds than radiation fog.

A. Rain from the warm air falls into and saturates the cold air under the frontal surface

Frontal (pre-frontal) fog: rain falling from the warm air evaporates into the cold air below, raising its dew point until it saturates. It forms ahead of the warm front and can cover a wide area.

A. Nocturnal

Radiation fog forms when the ground cools by radiation on a clear night, so it is essentially a night-time phenomenon, thickest around dawn and burning off after sunrise.

C. Both

Sunshine warms the ground and air above the dew point, and an increase in wind mixes the fog with drier air above (often lifting it into low stratus). Either one clears it.

B. Moist air being cooled as it is forced up the hills

Hill (orographic) fog is really low cloud: moist air forced up a slope cools adiabatically to its dew point and covers the high ground.

B. December–February

Radiation fog over north India is a winter feature, mainly December to February, when long clear nights, light winds and moisture left by western disturbances combine.

B. Transmissometer

Visibility at aerodromes is measured with a transmissometer, which measures how much of a light beam survives a known path length. A ceilometer measures cloud base.

A. Lift to form low stratus

Radiation fog needs a light wind. A stronger wind mixes the cold surface layer with warmer air above, so the fog lifts off the ground and becomes a sheet of low stratus rather than simply clearing.

B. Stratus

Fog is simply stratus whose base is on the ground: a layer of tiny water droplets formed by cooling of moist air near the surface.

C. Dust haze

In the hot weather season gusty, dust-laden winds blow over the Gangetic plains during the day and the dust hangs in the air as haze over a large area. Fog belongs to winter.

A. A dark object of suitable size near the ground can be seen and recognised against the horizon sky by day

Met visibility is defined on a black object against the sky by day (and on lights of known strength by night). The pilot's view of runway lights is RVR, a different measurement.

C. 5000 m to 9999 m

Steps are 50 m below 800 m, 100 m from 800 to 5000 m, and 1000 m from 5000 m up to 9999 (which means 10 km or more).

A. Mist

BR comes from the French "brume" - mist. Haze is HZ, smoke FU, fog FG.

C. Both over land and sea

Advection fog needs only warm, moist air moving over a colder surface - cold sea or cold land. It forms over both, and at any time of day or night.

C. Highs and cols

Fog needs light winds and, for radiation fog, clear skies - the conditions of a high or a col. In a low the winds and cloud usually keep fog away.

A. The temperature and dew point are close together

Fog forms when the air cools to its dew point. If the evening spread between temperature and dew point is already small, a night of clear-sky cooling is enough to close it by dawn.

A. Greater than about 15 kt

Advection fog needs a wind to carry the moist air over the cold surface, but a wind stronger than about 15 kt mixes the layer enough to lift the fog off the surface as a sheet of stratus.

A. A strengthening wind

Fog needs the cooling to stay in a shallow layer near the surface. As the wind strengthens - beyond roughly 15 kt - turbulence mixes that cooling through a deeper layer, so the condensation level rises off the ground and the fog becomes a sheet of low stratus instead.

C. Mist

Mist is a suspension of water droplets giving a visibility of 1000 m or more but not more than 5000 m, with the relative humidity close to 100%. Below 1000 m the same droplets are called fog. Haze is dry - dust and smoke particles, not water.

C. Radiational cooling of the earth at night

On a clear night the ground radiates its heat away to space, cools, and chills the shallow layer of air touching it until that air reaches its dew point. It is a night-time land phenomenon, which is exactly why it burns off through the morning as the sun warms the surface again.