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Fog

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Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary
WX
Tag(s) Weather Phenomena,
Cloud Formations

Mist and Fog

Description

Mist and Fog are the terms used to describe low visibility caused by water droplets suspended in the air. Mist is a term used to describe visibility of greater than 1 km0.54 nm
1,000 m
3,280.84 ft
while Fog is the term used when visibility is less than 1 km0.54 nm
1,000 m
3,280.84 ft
.

Fog is effectively surface cloud and has a significant impact on the conduct of flying operations, particularly landing and take-off.

There are many different types of fog defined according to how they are formed.

Radiation Fog

Early morning radiation fog
Early morning radiation fog over central Brussels, October 2007. Photo: John Barrass

On a cloudless night, especially within a high pressure system, the land surface loses heat to the atmosphere by radiation and cools. Moist air in contact with cooling surface also cools and when the temperature falls below the dew point for that air, fog forms. This type of fog is known as radiation fog.

Formation of Radiation Fog

Initially it may be mist that forms and then thickens into fog as the temperature drops and more water vapour condenses into water droplets in the air. Air does not conduct heat very well so in still air conditions fog may not form at all and a layer of dew or frost will form on the surface instead. However, if there is a light wind of around 5 kts9.26 km/h
2.57 m/s
, then this will mix the air in contact with the surface and the layer of fog will be thicker. With stronger winds, the fog may lift to form layers of Stratus.

Dispersal of Radiation Fog

As the sun rises, and the surface temperature increases, the air in contact with the surface will warm and the fog will gradually disperse. The fog may rise to form a low layer of stratus.

If the fog is particularly thick, then it may prevent the sun from heating the surface and the fog will not clear. This situation is common in the autumn in northern Europe when some airfields may be affected by fog for many days.

Anticipating Radiation Fog

The three conditions required for radiation fog are:

  1. clear skies,
  2. moist air, and
  3. a light wind.

Advection Fog

Advection fog occurs when a warm, moist air mass flows across a colder surface. The air mass is cooled from below by the colder surface and, if the temperature of the air mass is reduced to the dew point, then fog forms.

Formation of Advection Fog

Advection Fog is a regular springtime occurrence in coastal areas of north western Europe when relatively warm moist air moves towards land from offshore - the North Atlantic and waters around the British Isles - over colder land and shallow water surfaces.

Advection Fog can also occur over deep sea areas when warm maritime air passes over colder water such as that found in water flowing south from Arctic waters or where cold water wells up from the deep ocean.

In the Arctic, particularly in northern Canada, where there are wide expanses of water and numerous islands with small airstrips, advection fog is a significant impediment to aviation. Visibility can change from unlimited to zero in a very short space of time because just a slight change in wind direction can increase the relative humidity of air over the sea by increasing the fetch or sea distance travelled prior to it reaching a colder landmass. This type of fog can clear and the visibility can improve just as rapidly as it declined and such conditions can be difficult to forecast reliably.

It should be noted that unlike radiation fog, advection fog can be accompanied by winds of any strength up to gale force.

Frontal Fog and Hill Fog

Frontal fog occurs in two ways:

  1. When, during the passage of a front, cloud extends down to the surface. This is especially the case over higher ground and may also be termed Hill Fog.
  2. When the air in contact with the surface becomes saturated by evaporation from the rain that has fallen. These conditions may occur in the cold air ahead of a warm front.

Steam Fog

Steam Fog occurs when very cold air flows across relatively warm water. Water vapour evaporating from the surface of the water rapidly cools below its dew point, as it is mixed with and cooled by the cold air, and condenses to form fog.

Formation of Steam Fog

Evaporation fog
Evaporation Fog formed by cool air flowing over a lake; NOAA's National Weather Service (NWS) Collection

Steam Fog, also known as Steaming Fog, Evaporation Fog, Frost Smoke or Arctic Sea Smoke, occurs when evaporation takes place into cold air lying over warmer water. It is usually quite shallow. This phenomenon is mainly a feature of higher latitudes especially in winter.

It is named by analogy with the condensed vapour or steam which appears above water which is heated. Invisible vapour is given off from the water but is almost immediately recondensed as it comes into contact with the colder air. The air has to be much colder than the water so that convection currents develop. Formation also requires that there is:

  • A marked surface temperature inversion in the air before it moves over the sea or inland water bodies so as to preclude the lapse rate becoming unstable through a deep layer.
  • A low air temperature, typically 0°C32 °F
    273.15 K
    491.67 °R
    or below, so that a comparatively small amount of moisture can produce supersaturation, otherwise the heating process will outweigh the tendency towards saturation.

Because of these requirements, this type of fog is usually only formed over water surfaces near to a source of cold air, such as frozen ground or ice sheets in polar regions. One classic occurrence is following the sudden break up of sea ice to expose relatively warm water.

In the steep sided fjords along parts of the Icelandic and Norwegian Coasts and similar environments elsewhere, steam fog may reach a depth of 500 feet or more and drift over adjacent land areas. Whilst relatively rare in temperate latitudes, cold air which collects in and then drifts down large river valleys and out over a relatively warm sea surface, in very light wind conditions, can occasionally lead to the formation of smaller and much shallower areas of this type of fog in winter.

In a more ephemeral context but by exactly the same process, many people will have seen the steaming of tarred road surfaces or bitumen roofs in sunshine after rain.

Effects

  • Low visibility. Low visibility in fog clearly affects flying operations.

Airport operations in Fog

Reduced visibility because of fog may result in restrictions on both ground and airborne movements at an airport and both can have the effect of reducing capacity because of the safety-predicated consequences of Low Visibility Operations (LVO). Nowadays, with many more aircraft being able to land and take off in very low surface visibility, the ultimate capacity constraint can sometimes be maintaining the safety of aircraft ground movement.

Solutions

Radiation Fog

  • When conditions for radiation fog exist, it is a good idea to examine the pattern of weather over the preceding days to see if fog has occurred and at what time of the day and at what temperature.
  • Monitoring the temperature and dewpoint at an airport can help controllers and pilots alike to predict the onset of radiation fog and plan operations accordingly.
  • If conducting local flying operations, such as flying training, beware getting airborne when there is early afternoon lifting of fog while conditions for radiation fog still exist - you could find yourself spending the night somewhere else!
  • If planning to fly to an aerodrome where conditions for radiation fog exist,
    • time your planned arrival for about an hour after local noon when maximum solar heating takes place.
    • expect delays and carry additional contingency fuel.

Frontal Fog

  • Flying at low level, i.e. below safety altitude, in conditions of frontal fog and low cloud can quickly become extremely hazardous if visual flight rules cannot be maintained. Attempting to fly between layers of Stratus, so-called "letter boxing", can result in impact with terrain CFIT if forward visibility and Situational Awareness is lost.

Advection Fog

  • In circumstances where advection fog can quickly make an aerodrome unusable, the same risk may apply to potential diversions as well and pilots should ensure that an appropriate fuel endurance is available and that alternates unlikely to be affected by advection fog remain within range in the event that the destination weather deteriorates unexpectedly.

Information on improved forecasts of clearing time of advection fog over the approaches to San Francisco International Airport is available at the following the case-study article: Forecast-base decision support for San Francisco International airport, NextGen Prototype System That Improves Operations during Summer Stratus Season (David W. Reynolds, David A. Clark, F. Wesley Wilson, and Lara Cook).

Airport Operations

  • Crews and controllers should exercise additional caution during low visibility operations - loss of situational awareness is a major contributory factor in Runway Incursion events.
  • Airports should ensure that collaborative decision making arrangements to maximise airport capacity involve met service providers.
  • Flight crews should anticipate longer taxiing times in low visibility operations and carry additional fuel accordingly.

Accident & Incident Reports Involving Operations in Fog

  • DC91 / B722, Detroit MI USA, 1990 (On 3 December 1990 a Douglas DC9-10 flight crew taxiing for departure at Detroit in thick fog got lost and ended up stopped to one side of an active runway where, shortly after reporting their position, their aircraft was hit by a departing Boeing 727-200 and destroyed by the impact and subsequent fire. The Investigation concluded that the DC9 crew had failed to communicate positional uncertainty quickly enough but that their difficulties had been compounded by deficiencies in both the standard of air traffic service and airport surface markings, signage and lighting undetected by safety regulator oversight.)
  • C501, vicinity Trier-Fohren Germany, 2014 (On 12 January 2014, the crew of a Cessna 501 on a private business flight with a two-pilot crew attempted to make an unofficial GPS-based VNAV approach in IMC to the fog-bound VFR-only uncontrolled aerodrome at Trier-Fohren. However, after apparently mis-programming the 'descend-to' altitude and deviating from the extended centre, the aircraft emerged from the fog very close to the ground and after pulling up collided with obstructions, caught fire and crashed killing all occupants. The Investigation noted an apparent absence of pre-flight weather awareness beyond the intended destination and that there was a suitable fog-free diversion.)
  • B738, Mildura VIC Australia, 2013 (On 18 June 2013, a Boeing 737-800 crew en route to Adelaide encountered un-forecast below-minima weather conditions on arrival there and decided to divert to their designated alternate, Mildura, approximately 220nm away where both the weather report and forecast were much better. However, on arrival there, an un-forecast rapid deterioration to thick fog had occurred with insufficient fuel to go anywhere else. The only available approach was flown, but despite exceeding the minimum altitude by 260 feet, no visual reference was obtained. A further approach with the reported overcast 100 feet agl and visibility 200 metres was continued to a landing.)
  • A320, Jaipur India, 2014 (On 5 January 2014, an Airbus A320 was unable to land at Delhi due to visibility below crew minima and during subsequent diversion to Jaipur, visibility there began to deteriorate rapidly. A Cat I ILS approach was continued below minima without any visual reference because there were no other alternates within the then-prevailing fuel endurance. The landing which followed was made in almost zero visibility and the aircraft sustained substantial damage after touching down to the left of the runway. The Investigation found that the other possible alternate on departure from Delhi had materially better weather but had been ignored.)
  • A319, Rio de Janeiro Galeão Brazil, 2017 (On 19 July 2017, an Airbus A319 crew ignored the prescribed non-precision approach procedure for which they were cleared at Rio de Janeiro Galeão in favour of an unstabilised “dive and drive” technique in which descent was then continued for almost 200 feet below the applicable MDA and led to an EGPWS terrain proximity warning as a go around was finally commenced in IMC with a minimum recorded terrain clearance of 162 feet. The Investigation noted the comprehensive fight crew non-compliance with a series of applicable SOPs and an operational context which was conducive to this although not explicitly causal.)

Airports near areas where Fog occurs

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