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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.               
 
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.               
  
[[File:Evaporationfog.jpg|none|thumb|400px|alt=Evaporation fog|Evaporation Fog formed by cool air flowing over a lake; NOAA's National Weather Service (NWS) Collection]]
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[[File:Evaporationfog.jpg|thumb|400px|alt=Evaporation fog|Evaporation Fog formed by cool air flowing over a lake; NOAA's National Weather Service (NWS) Collection]]
  
 
==Effects==
 
==Effects==

Revision as of 12:39, 16 December 2013

Article Information
Category: Weather Weather
Content source: SKYbrary Logo SKYbrary.gif
Content control: EUROCONTROL Logo EUROCONTROL.gif
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

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.

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

Anticipating Radiation Fog

The three conditions 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 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:

  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

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.

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

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 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.

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 Controlled Flight Into Terrain (CFIT) if forward visibility and Situational Awareness is lost.

Advection Fog

  • 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"

Information on improved forecasts of clearing time of advection clouds over the approach 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.

Related Articles

Accident & Incident Reports Involving Operations in Fog

  • C500, vicinity Santiago Spain, 2012 (On 2 August 2012, a Cessna 500 positioning back to base after completing an emergency medical team transfer operation earlier in the night crashed one mile short of the runway at Santiago in landing configuration after being cleared to make an ILS approach. The Investigation concluded that the approach was unstabilised, had been flown without following the ILS GS and that the crew had used DME distance from the VOR near the crash position rather than the ILS DME. Fog was present in and around the airport.)
  • AN26, vicinity Cox’s Bazar Bangladesh, 2016 (On 29 March 2016, an Antonov AN-26B which had just taken off from Cox’s Bazar reported failure of the left engine and requested an immediate return. After twice attempting to position for a landing, first in the reciprocal runway direction then in the takeoff direction with both attempts being discontinued, control was subsequently lost during further manoeuvring and the aircraft crashed. The Investigation found that the engine malfunction occurred before the aircraft became airborne so that the takeoff could have been rejected and also that loss of control was attributable to insufficient airspeed during a low height left turn.)
  • 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.)
  • 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.)
  • T154, vicinity Smolensk Russian Federation, 2010 (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.)

... further results