MET 08- Visibility and Fog

Advection fog is formed when warm, moist air moves (advects) horizontally over a surface that is sufficiently colder than the air’s dew point, causing the air mass to cool to saturation.
Unlike radiation fog, which relies on nocturnal cooling and calm conditions, advection fog:
• Requires Air Movement: It is characterized by the movement of air and can deepen as wind speed increases (up to about 15 knots).
• Time of Occurrence: It can occur at any time of the day or night.
• Persistence: Advection fog is generally more extensive and persistent than radiation fog, sometimes lasting for 24 hours or more in winter.
⭐️ ⭐️ Key Data to Remember:
• Formation Mechanism: Warm, moist air moving over a cold surface.
• Timing: Not limited to night; occurs regardless of the time of day.
• Condition: Requires wind (up to 15 kt over land).

The official international definition of fog requires that visibility be less than 1,000 meters (or 1 km). Fog is fundamentally a cloud based at the surface. The reduction in visibility is caused by minute visible water droplets or ice crystals suspended in the air. Water vapor, being an invisible gas, does not restrict visibility.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
• Visibility Threshold: <1,000 meters.
• Composition: Water droplets or ice crystals.
• Relative Humidity: Near 100%.
• RVR Reporting: RVR (Runway Visual Range) assessment and reporting begins when meteorological visibility or RVR falls below 1,500 meters, or when shallow fog is reported or forecast.

Advection fog is fundamentally defined by the horizontal movement (advection) of warm, moist air over a sufficiently cold surface, which cools the air to its dew point and causes condensation.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Formation Mechanism: Cooling from below due to horizontal air movement.
• Conditions: Requires wind (up to 15 kt, potentially stronger over sea) to move the air across the cold surface.
• Persistence: Generally more persistent than radiation fog, often lasting for days or weeks.
• Location: Common along coastlines (e.g., Newfoundland, California) where warm air moves over cold ocean currents or over land areas when moist air moves over cold-soaked ground.

Fog, defined as a cloud at the surface, requires a highly stable atmospheric structure to form and persist.
1. Stability: An inversion is a temperature increase with altitude. This condition represents extreme stability, as the Environmental Lapse Rate (ELR) is negative.
2. Mechanism: This extreme stability acts as a “lid,” trapping cooling air, moisture, and condensation nuclei in a shallow layer near the surface and suppressing vertical convection that would otherwise dissipate the fog.
3. Common Fog Types:
◦ Radiation Fog forms when the ground cools rapidly at night, which cools the adjacent air to its dew point, inherently producing a surface inversion above the fog layer.
◦ Subsidence Fog (often associated with anticyclones) forms below a subsidence inversion, where sinking air (subsidence) warms the air aloft, capping the cool, moist air below.
An isothermal layer (constant temperature with height) is also stable, but the inversion (increasing temperature) represents the most extreme form of stability favorable for trapping low-level moisture and generating dense fog. Adiabatic processes (SALR/DALR) describe the cooling/warming rates of vertically moving air parcels. Fog is characterized by limited vertical movement in stable air.

Advection fog is formed by the advection (horizontal movement) of warm, moist air over a surface that is sufficiently colder. The warm air cools by contact with the cold surface to a temperature below its dew point, leading to saturation and condensation. Unlike radiation fog, which requires clear skies and light wind/calm conditions, advection fog requires wind to move the air mass over the cooling surface.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Mechanism: Warm, moist air moving over a cold surface cools from below.
• Conditions: Requires wind up to approximately 15 knots (or slightly stronger over sea).
• Persistence: Generally much more persistent than radiation fog, sometimes lasting for days or weeks.
• Location: Common along coastal areas, over land in winter/early spring, or over sea areas in late spring/early summer.

Advection Fog forms due to the horizontal movement (advection) of moist air across a cold surface. For this type of fog to occur, the air mass, which often has high relative humidity, must move or “pass” over a surface that is cooler than the air’s dew point. This conductive cooling from below lowers the air temperature to saturation, causing condensation and fog formation.
⭐️ ⭐️ Key Data to Remember (Distinguishing Fog Types):
• Advection Fog: Requires air movement (up to 15 knots, or stronger over the sea). The air is cooled by the underlying cooler surface.
• Radiation Fog: Typically requires clear skies and light wind/calm conditions (air “settles”) for cooling to occur by terrestrial radiation.

Advection fog is characterized by the horizontal movement (advection) of warm, moist air over a surface that is sufficiently colder to cool the air mass below its dew point, causing condensation. This process is possible over both terrestrial and oceanic surfaces.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Formation over Land: Common in winter and early spring when warm, moist maritime air flows over cold-soaked ground.
• Formation over Sea (Sea Fog): Common in late spring and early summer when warm air moves over relatively cooler ocean waters.
• Conditions: Requires wind up to approximately 15 knots (or up to 20 knots over the sea) to move the air.
• Persistence: Advection fog is typically more persistent than radiation fog and can last for extended periods.

Fog formation, especially radiation fog, requires atmospheric stability, light winds, and air remaining in prolonged contact with the cool surface to reach saturation.
1. Highs (Anticyclones) and Ridges: These pressure systems are characterized by descending air (subsidence), which prevents cloud from forming aloft and generates good weather. The subsidence often creates a temperature inversion (subsidence inversion) that caps the lower layers, trapping moisture, pollution, and leading to foggy conditions, particularly in winter.
2. Cols: A col is a region of very little pressure variation between two highs and two lows, leading to very light or stationary winds. This stagnation allows the air to remain in contact with the ground for an extended period, which favors the formation of fog or low stratus, especially in autumn and winter.
3. Lows (Depressions) and Troughs: These systems are characterized by convergence and rising air. Rising air and the typically strong winds associated with lows tend to dissipate stability and surface-based condensation, making them generally unfavorable for fog persistence.

Radiation fog (also known as ground fog or valley fog) is generated when the ground loses heat by terrestrial radiation at night. This process, known as radiational cooling, causes the ground to become cold. The air immediately in contact with the cold surface is subsequently cooled by conduction below its dew point, resulting in the condensation of water vapor into fog droplets.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Mechanism: Radiational cooling of the surface, creating a temperature inversion near the ground.
• Conditions: Typically requires a clear sky (to maximize radiation loss) and light wind (2 to 8 knots) to mix the cooled air layer near the surface.
• Timing: Forms almost exclusively at night or near daybreak.
• Location: Occurs over land (not over the sea due to low diurnal temperature variation).

Early morning temperature is lowest.
ODDMENOUT
– No advection fog can be formed because there is no wind.
– no hill fog because there is no hill.
– only radiation fog can be formed and better option is early morning.

Radiation fog occurs mostly over North India during the winter season. In the context of Indian Climatology, January is representative of the winter season. Extensive fog over the plains of North India is specifically observed during the months of December to March. This fog is often triggered in the rear sector of Western Disturbances, where atmospheric conditions provide low-level moisture content, stability, and light wind conducive to fog formation,.

with a wind speed up to 15 kt – no radiation fog
summer season – no radiation fog
on hill — no radiation fog

in an anticyclone in winter – or high pressure – Radiation fog will form.

This combination correctly describes Advection Fog. Advection is the horizontal transfer or movement of air. When warm, moist air moves (or “passes”) over a surface that is cooler than the air’s dew point, the air is cooled from below to saturation, causing condensation and fog formation.
⭐️ ⭐️ Key Data to Remember (Advection Fog Operational Context):
• Mechanism: Horizontal movement of air over a cold surface.
• Cooling Condition: Surface temperature must be lower than the dew point of the moving air.
• Location: Can occur over both land and sea.

The Transmissometer is the electronic instrument primarily used in aviation meteorology for the automatic and continuous measurement of horizontal visibility, particularly the Runway Visual Range (RVR). It measures the transmittance (opacity) of the atmosphere by emitting a light source to a photoelectric cell receiver. The measurement of RVR is dependent on the transmissometer system. The ceilometer, conversely, is used to measure the height of the cloud base (cloud height).
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
• Function: Measures the transmissivity of light through the atmosphere, which is then translated into visibility and/or RVR.
• Application: Essential for instrument reporting of Runway Visual Range (RVR).
• RVR Reporting Criteria: RVR assessment begins when meteorological visibility or RVR falls to less than 1,500 meters.
• Alternative Instrumentation: Forward Scatter Visibility Meters are now replacing transmissometers at some locations.

Radiation fog forms as the ground loses heat by terrestrial radiation, cooling the adjacent air to its dew point. The three critical atmospheric conditions required for this process are:
1. Sufficient Moisture: High relative humidity (near 100%) is necessary so that only a small amount of cooling is required to reach saturation.
2. Clear Sky: Clear skies maximize the loss of the earth’s heat via outgoing longwave radiation, leading to rapid cooling of the surface.
3. Light Wind: A light wind, typically 2 to 8 knots, is necessary to mix the air layer immediately above the ground. This mixing helps draw slightly warmer, moist air down to the cooling surface, deepening and thickening the fog layer. Calm conditions (nil wind) tend to produce dew or frost, while strong winds disperse the fog or lift it into stratus cloud.
⭐️ ⭐️ Key Data to Remember:
• Mechanism: Radiational cooling of the surface.
• Sky Condition: Clear (or high, scattered clouds only).
• Wind Speed: Light, usually 2 to 8 knots (or less than 5 knots).
• Location: Over land only, typically in anticyclones, ridges, and cols.

Radiation fog requires light winds (typically 2 to 8 knots) to form and persist. Light wind conditions are directly related to a slack (or gentle/weak) pressure gradient, characterized by widely spaced isobars.
Pressure systems associated with radiation fog (such as anticyclones, ridges, and cols) are all areas where the wind is light or the air is stationary. Strong pressure gradients, conversely, produce strong winds which would mix the air vertically and quickly dissipate any nascent fog layer.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Fog Requirement: Light wind is critical (2–8 knots).
• Pressure Relation: Light wind implies a slack pressure gradient.
• Associated Systems: Highs, Ridges, and Cols—all characterized by light winds and high stability.
• Forecasting Clue: A slack pressure gradient at night over land, coupled with clear skies and high relative humidity, strongly favors radiation fog formation.

Explanation (ICAO/Meteorological Context):
Fog is meteorologically defined as a cloud resting on the ground.
The cloud type that fog most closely resembles, and into which it frequently transitions upon lifting, is the Stratus (ST) cloud. Stratus is characterized by being a uniform, layered cloud of large horizontal extent and little vertical development.
• When a thick fog “lifts,” the resulting cloud is a deck of low stratus.
• An observer on a mountain in a stratus layer would call it fog.
• Stratus is usually the lowest of all cloud types.
⭐️ ⭐️ Key Data to Remember:
• Definition: Fog is a surface-based cloud where visibility is less than 1,000 meters.
• Classification: It is the surface equivalent of a Stratus cloud layer.

Radiation fog is a surface-based phenomenon caused by the radiational cooling of the Earth’s surface at night, which cools the adjacent air to its dew point. This process is restricted to land areas because the sea surface exhibits insufficient diurnal temperature variation (DV) to cool rapidly enough to produce fog. While it forms at night, it typically dissipates or “burns off” after sunrise due to increased solar insolation.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Location: Exclusively over land.
• Mechanism: Radiational cooling of the ground.
• Time of Occurrence: Night and early morning; dispersal usually occurs after sunrise.
• Favorable Conditions: Clear sky, high relative humidity, and light wind (2–8 kt).

Mist (coded as BR) is defined by the presence of very small water droplets in the air with a Relative Humidity (RH) of more than 95%. Crucially, the visibility range for mist is between 1,000 meters and 5,000 meters.
• Fog (FG): Requires visibility less than 1,000 meters. RH is very close to 100%.
• Haze (HZ): Is caused by solid particles (sand, dust, or smoke) and is not primarily linked to water droplets or an RH near 100%.
The specified visibility range (1000 m to 5000 m) coupled with near 100% RH defines Mist.

Advection fog forms through the horizontal movement (advection) of a warm, moist air mass over a sufficiently cold surface (land or sea). For condensation and fog to occur, the air layer in contact with the cold surface must be cooled below its dew point. The process of cooling air to its dew point results in saturation and the formation of water droplets, restricting visibility.
⭐️ ⭐️ Key Data to Remember (Advection Fog):
• Mechanism: Warm, moist air moving over a colder surface.
• Condition: Surface temperature must be lower than the air’s dew point.
• Dispersal: Requires a change of air mass or increase in wind speed (usually over 15 kt) which lifts the fog into stratus cloud.
• Persistence: This type is typically more persistent than radiation fog.

RVR reporting uses standard incremental scales to convey visibility data to pilots. The smallest increments are reserved for the lowest visibility values. Sources indicate that RVR is reported in increments of 25 meters for the lowest ranges.
The standard reporting scale for RVR is structured as follows:
• 25 m increments: For RVR values between 0 and 200 m. (Note: Some regional practices may extend the 25 m increment up to 400 m).
• 50 m increments: For values between 200 m (or 400 m) and 800 m.
• 100 m increments: For values above 800 m.
Therefore, 25 m up to 200 m represents the correct smallest increment for reporting RVR changes.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA Context):
RVR Range
Increment
Supporting Source
0 to 200 m
25 m
200 to 800 m (or 400 to 800 m)
50 m
Above 800 m
100 m
(RVR assessment begins when horizontal visibility or RVR falls below 1500 m, or when shallow fog is reported or forecast.)

Radiation fog is a meteorological phenomenon resulting from the cooling of the Earth’s surface by terrestrial radiation. This process, known as radiational cooling, requires the surface to cool rapidly at night to chill the adjacent air below its dew point.
This cooling process occurs exclusively over land. It does not occur over the sea because water surfaces have insufficient diurnal temperature variation (DV) to cool rapidly enough to produce radiation fog. Radiation fog is typically common in autumn and winter when there is a long night for cooling.
⭐️ ⭐️ Key Data to Remember:
• Location: Over land only.
• Reason for Sea Exclusion: Sea surfaces have insufficient diurnal temperature variation (DV).
• Mechanism: Radiational cooling of the ground.

In regions affected by the Western Disturbance (WD), such as North India, conditions become favorable for widespread fog occurrence, particularly radiation fog, in the rear sector of the disturbance. After the passage of the WD and the cessation of precipitation, the atmosphere retains sufficient low-level moisture while the skies often clear. This clearing allows for intense nighttime radiational cooling, resulting in radiation fog typically one or two days after the rainfall ceases.
⭐️ ⭐️ Key Data to Remember (Climatological/Regional Context):
• System: Western Disturbance (WD).
• Timing: Fog occurs in the rear sector of the WD, one or two days after rainfall ceases.
• Mechanism: WD supplies the necessary low-level moisture; the clearing skies post-passage allow for radiational cooling and inversion formation.
• Location: Extensive fog occurs over the plains of North India during the winter season (December to March) in association with an active WD.

Runway Visual Range (RVR) assessment and reporting are mandatory procedures under specific low-visibility conditions. RVR is reported when the meteorological optical range (MOR) or the Instrumented RVR (IRVR) falls to less than 1,500 meters, or when shallow fog is reported or forecast.
While RVR readings are often taken every 30 minutes when traffic is continuous, the criteria for reporting RVR in routine METAR/SPECI messages is based on the visibility threshold of 1,500 meters.
⭐️ ⭐️ Key Data to Remember (ICAO/FAA/Operational Context):
• RVR Trigger: RVR reporting is required when the meteorological visibility or RVR itself is ≤1,500 meters.
• Fog/Mist Distinction: Mist is reported when visibility is between 1,000 m and 5,000 m. Fog is reported when visibility is <1,000 m. Since 1,500 m is the threshold, RVR reporting covers all fog occurrences and a portion of mist occurrences where visibility is 1,000 m to 1,500 m.
• Supplementary RVR: RVR values between 1,500 m and 2,000 m may be reported in the supplementary information (RMK group) but are generally not disseminated internationally.

Runway Visual Range (RVR) assessment and reporting are specifically initiated in low-visibility conditions to provide critical information for takeoff and landing. RVR is reported when the Meteorological Optical Range (MOR) or the Instrumented RVR (IRVR) falls to less than 1,500 meters, or when shallow fog is reported or forecast.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Trigger Condition: RVR reporting begins when meteorological visibility is 1500 meters or less.
• Fog Threshold: Note that Fog (FG) itself is defined as visibility less than 1,000 meters. RVR is reported for conditions slightly better than fog, extending into the visibility range defined as Mist (1,000 m to 5,000 m).
• Purpose: RVR is the maximum distance a pilot at 15 feet above the runway threshold can see runway markings or lights, and is vital for precision approaches (CAT I, II, III).

Frontal fog (or precipitation-induced fog) is primarily associated with warm fronts and warm occlusions.
Mechanism:
1. Location: This fog develops in the shallow layer of cold air located ahead of the warm front.
2. Saturation: Precipitation (typically continuous rain or snow from Nimbostratus (NS) cloud) falls from the warmer air mass aloft, evaporating into the colder air mass beneath. This process raises the moisture content (dew point) of the cold air to saturation.
3. Extent: This forms a band of fog that can be up to 200 NM wide and travels with the front.
4. Alternative Fronts: While frontal fog is most commonly associated with warm fronts, it can also occur with slow-moving cold fronts or stationary fronts. However, fast-moving, vigorous cold fronts are typically associated with strong lifting and cumuliform clouds (Cu/CB) and showers, which do not favor widespread, persistent fog formation.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Type: Precipitation-induced fog.
• Location: In the cold air mass, ahead of a Warm Front or Warm Occlusion.
• Hazard: Can be quite dense and extensive, sometimes completely suspending air operations.
• Dispersal: Dissipates when the front passes and the air mass changes.

The movement of warm, moist air over a cooler surface is the defining mechanism of Advection Fog.
1. Stabilization: When warm air moves over a cold surface, it is cooled from below. Cooling the air near the surface while the air aloft remains warmer creates a temperature inversion or increases the stability of the air mass.
2. Condensation: This cooling below the dew point leads to saturation and condensation, forming droplets. When this saturated air is in contact with the ground, it is called fog.
3. Stratus Formation: If the wind speed is moderate (typically stronger than 8 knots, or up to 15 knots), the fog may be lifted off the surface by turbulence to form a layer of low stratus or stratocumulus cloud.
Because cooling from below increases stability, high-level convective clouds like thunder clouds (Cumulonimbus) are prevented from forming. Frontal clouds are caused by the slow, widespread ascent along a front, not specifically by advection over a cold, passive surface.
⭐️ ⭐️ Key Data to Remember (Operational Context):
• Mechanism: Advection (horizontal flow) of warm, moist air over a colder surface.
• Atmospheric Result: Stability (cooling from below).
• Weather Result: Advection fog (visibility < 1,000 m) or low stratus cloud.

Advection fog is characterized by its persistence, making it a severe hazard when it forms. Unlike radiation fog, which relies on nocturnal cooling and typically dissipates shortly after sunrise due to solar heating (insolation), advection fog:
• Persistence: Is “much more persistent than radiation fog” and “can last several weeks”. It can certainly last for 24 hours or more, particularly in winter.
• Time of Occurrence: Can form regardless of the time of day or night, as its formation depends on horizontal air movement over a cold surface, not radiational cooling.
• Dispersal: Clears primarily by a change of air mass (wind shift) or by a sufficient increase in wind speed (generally over 15 kt) which lifts the fog into low stratus cloud.
While advection fog is common when warm, moist air (often Maritime Tropical air, mTw) flows over cooler land or sea surfaces, its capability to persist for long periods defines it as an exceptionally difficult visibility restriction for aviation.

Radiation fog requires four key conditions for formation: sufficient moisture (satisfied by high relative humidity, such as 80% in the prompt), radiational cooling, clear skies, and light winds.
1. Season (Autumn/Winter): Radiation fog is most common in autumn and winter when there is a long night allowing the land sufficient time to cool.
2. Sky Condition (Clear Skies): A clear sky is essential because it maximizes the rate of terrestrial radiation, cooling the ground surface efficiently.
3. Wind Speed (2 to 8 knots): A light wind of 2 to 8 kt is necessary to mix the air layers near the surface. This mixes warmer air from above down to the cold surface for cooling, which both thickens the fog layer and keeps the droplets in suspension.
◦ Calm conditions (no wind) result in dew or frost forming, not fog.
◦ Stronger winds (like 15/20 kt) cause too much mixing, which either dissipates the fog or lifts it into stratus cloud.
The combination of autumn, clear skies, and 2−8 kt wind provides the optimal environment for widespread radiation fog formation over land.

A station equipped with Instrumented RVR (IRVR) typically uses three transmissometers to measure RVR at the touchdown, mid-point, and stop-end zones. When an RVR report only contains a single value (the touchdown reading, e.g., R 1000), it indicates that the mid-point and stop-end values have been suppressed (omitted) because they met specific ICAO/Aeronautical criteria.
Mid-point and stop-end RVR values are suppressed when:
1. They are equal to or higher than the touchdown zone value (unless less than 400 m).
2. They are 800 meters or more.
Therefore, if the reported value is 1000 m, the omission of the mid-point and stop-end values implies that those values were 800 meters or greater.
⭐️ ⭐️ Key Data to Remember (RVR Omission):
• Suppression Threshold: RVR values of 800 m or more for mid-point/stop-end are omitted, and only the touchdown value is reported.
• Report Example: An RVR sequence of “1000 900 900” (touchdown, mid-point, stop-end) would be reported simply as R 1000 (or R 900 for 900/850/950).

In ICAO meteorological standards, Fog (FG) is defined as a reduction in visibility caused by water droplets where visibility is less than 1,000 meters. The relative humidity (RH) during fog conditions is reported as very close to 100%. The condition for reporting FG without descriptors (like shallow or patches) requires the visibility to be less than 1,000 meters.
⭐️ ⭐️ Key Data to Remember (ICAO Visibility Criteria):
• Fog (FG): Visibility <1,000 m. RH near 100%.
• Mist (BR): Visibility between 1,000 m and 5,000 m. RH greater than 95%.
• RVR Trigger: Runway Visual Range (RVR) is reported when visibility falls to less than 1,500 m.

Frontal fog, also known as precipitation fog, is primarily associated with warm fronts and warm occlusions.
Mechanism: It forms in the shallow layer of cold air lying ahead of a warm front when continuous precipitation (rain or snow) falls from the warm air mass aloft (Nimbostratus cloud). This precipitation evaporates into the underlying colder air, raising its dew point to saturation and forming fog.
⭐️ ⭐️ Key Data to Remember:
• Location: Usually develops in the cold air mass, just ahead of the surface position of a warm front.
• Extent: Can form along a belt up to 200 NM wide, traveling with the front [142, 648/649].
• Persistence: Can be quite dense and continue for an extended period, sometimes completely suspending air operations.
• Alternative Occurrence: Less commonly, it may occur with slow-moving cold fronts or stationary fronts.

Radiation fog is fundamentally a nocturnal phenomenon because its formation is dependent on radiational cooling of the Earth’s surface. This cooling occurs most effectively during clear, long nights, chilling the air in contact with the ground to the dew point.
• Time: Radiation fog forms almost exclusively at night or near daybreak (early morning) after a prolonged period of cooling.
• Mechanism: Terrestrial radiation (heat loss by the ground) is required, which is maximized when solar heating (insolation) has ceased.
• Dispersal: It usually dissipates or “burns off” after sunrise due to increased insolation and thermal turbulence.
⭐️ ⭐️ Key Data to Remember:
• Cause: Radiational cooling (only occurs over land).
• Peak Time: Night and early morning (around dawn when temperature is minimum and Relative Humidity is maximum).
• Conditions: Clear sky, high relative humidity, and light wind (2 to 8 kt).