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|Tag(s)|| Weather Phenomena|
Mist and Fog
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
3,280.84 ft while Fog is the term used when visibility is less than 1 km0.54 nm
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;
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 thicker 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, then 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 for radiation fog are:
- clear skies,
- moist air, and
- a light wind.
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 common in Spring over western Europe when warm moist air flows in from the Atlantic over the cold land mass.
Advection Fog can also occur when warm maritime air passes over cold water, such as a current flowing south from Arctic waters or cold water welling 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 major hazard to aviation. Visibility can change from unlimited to zero in a very short space of time because of a slight change in the wind bringing moist warmer maritime air across the colder landmass. The fog can clear and the visibility can improve just as rapidly as it declined, if the wind direction changes again to blow the air from the land towards the sea. Such conditions can be difficult to forecast.
Frontal Fog and Hill Fog
Frontal fog occurs in two ways:
- 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.
- 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 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
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°C273.15 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.
- Low visibility. Low visibility in fog clearly affects flying operations.
Airport operations in Fog
Reduced visibility because of fog may result in restrictions on movements at an airport, reduced capacity (because of procedural increased separation between aircraft take-offs and landings in order to maintain safety), and may lead to the closure of the airport.
- 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.
- 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.
- In circumstances where advection fog can quickly make an aerodrome unusable, particularly in the high Arctic, pilots are wise to carry sufficient contingency fuel to be able to keep a number of potential diversion airfields available as options in the event that the destination weather deteriorates unexpectedly. "Always keep another card up your sleeve"
- 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
- DHC6, vicinity Oksibil Indonesia, 2009 (CFIT WX HF) (On 2 August 2009 a DHC-6 being operated by Merpati Nusantara Airlines on a scheduled domestic passenger flight from Sentani to Oksibil in West Papua in daylight and on a VFR Flight Plan was in collision with terrain 6nm from destination resulting in the destruction of the aircraft and the death of all 15 occupants.)
- B742 / B741, Los Rodeos Tenerife, 1977 (RI AGC WX) (On 27 March 1977, a KLM B747-200 commenced its daylight take off at Los Rodeos airport, Tenerife in very poor visibility, recorded as 300 metres three minutes earlier, after receiving only a departure clearance and continuing the take off roll even after ATC advised "standby for take off". Collision with a Pan American Airways Boeing 747-100 which was taxiing on the runway in accordance with its ATC clearance issued on the same radio frequency. All 248 people on board the KLM aircraft died and only 61 of the 396 people on board the Pan American aircraft survived.)
- T154, vicinity Smolensk Russian Federation, 2010 (CFIT HF WX FIRE) (On 10 April 2010, a Polish Air Force Tupolev Tu-154M on a pre-arranged VIP flight into Smolensk Severny failed to adhere to landing minima during a non precision approach with thick fog reported and after ignoring a TAWS ‘PULL UP’ Warning in IMC continued descent off track and into the ground. All of the Contributory Factors to the pilot error cause found by the Investigation related to the operation of the aircraft in a range of respects including a failure by the crew to obtain adequate weather information for the intended destination prior to and during the flight.)
- A343, Nairobi Kenya, 2008 (WX RE) (On 27 April 2008 an Airbus A340-300 being operated by Virgin Atlantic on a scheduled passenger flight from London had carried out a night auto ILS approach to Runway 06, Nairobi airport, Kenya. Just prior to touchdown, The aircraft entered an area of fog and the PF lost sight of the right side of the runway and the runway lights. The landing was rejected and a diversion to the nomiated alternate was made.)
- S61, vicinity Bødo Norway, 2008 (CFIT HF WX) (On 24 February 2008, a Sikorsky S-61N being operated by British International Helicopters on a passenger flight from Værøy to Bødo attempted a visual approach at destination in day IMC and came close to unseen terrain before accepting an offer of assistance from ATC to achieve an ILS approach to runway 07 without further event. None of the 18 occupants were injured.)
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