Fog and Low Visibility Procedures (LVP)

Introduction: When the World Disappears

Fog is one of the most disruptive weather phenomena in aviation. A dense fog layer can reduce visibility to near zero, halting departures, complicating approaches, and causing cascading delays across entire airport networks. Understanding how fog forms, the different types that affect aviation, and the procedures that enable operations in low visibility is essential for pilots, controllers, and airport operators alike.

Types of Fog

Radiation fog is the most common type affecting aviation operations. It forms on clear, calm nights when the ground cools rapidly through longwave radiation, cooling the adjacent air to its dewpoint. Radiation fog is typically shallow (less than 100 feet deep) but can become dense, reducing visibility to near zero. It is most common in autumn and winter and typically burns off after sunrise as solar heating warms the ground. Radiation fog is particularly problematic at airports in valleys or low-lying areas where cool air pools.

Advection fog forms when warm, moist air moves over a cold surface. Unlike radiation fog, advection fog can persist for extended periods, even during daylight hours, and can cover large areas. It is common along coastal areas where warm marine air moves over cold land surfaces, and near large bodies of water. Advection fog can be deep and persistent, making it a more serious operational challenge than radiation fog.

Frontal (precipitation-induced) fog forms when rain falls through a cool air mass near the surface. The evaporating rain saturates the air, creating fog. This type of fog is often associated with warm fronts and can persist for extended periods. It is particularly common in temperate climates during autumn and winter.

Ice fog forms in extremely cold temperatures (below -30°C) when water vapor sublimates directly into ice crystals. It is primarily a concern at high-latitude airports and can significantly reduce visibility.

Low Visibility Procedures (LVP)

When visibility drops below specified thresholds at an airport, Low Visibility Procedures are activated. LVP is an internationally standardized set of procedures defined by ICAO that governs airport operations during low-visibility conditions. The activation of LVP affects aircraft operations, ground vehicle movements, and ATC procedures.

LVP is typically activated when:

• Runway visual range (RVR) at the touchdown zone falls below 1,200 meters (or the value specified in the airport's operations manual).
• The ILS critical area must be protected to ensure the integrity of the ILS signal.
• Visibility is below the takeoff minimum for the active runway.

During LVP, ground vehicles are restricted from entering ILS critical and sensitive areas. Aircraft are spaced more widely on approach to prevent interference with the ILS signal. ATC may limit the number of aircraft on the maneuvering area and implement specific taxi routes to minimize the risk of runway incursions.

ILS Category Differences in Low Visibility

The ability to operate in low visibility depends on the ILS category available at the airport and the certification of the aircraft and crew. The key differences lie in the decision height (DH) and minimum RVR values:

• CAT I: DH 200 feet, minimum RVR 550 meters. The pilot must have visual reference to the runway environment at the DH to continue the landing. Available at most airports with precision approach capability.
• CAT II: DH 100-200 feet, minimum RVR 300 meters. Requires special crew training, dual autopilot capability, and enhanced runway/approach lighting. Available at major international airports.
• CAT III: DH below 100 feet (or no DH), minimum RVR as low as 50 meters for CAT IIIb. Requires autoland capability, extensive crew training, and the highest levels of airport infrastructure. Only a limited number of airports worldwide support CAT III operations.

The progression from CAT I to CAT III represents a significant increase in complexity, cost, and training requirements. Airports invest heavily in ILS equipment, runway lighting, and surface movement systems to support low-visibility operations, as the economic impact of fog-related delays can be substantial.

Practical Considerations for Pilots

When fog is forecast, pilots and dispatchers should plan for potential delays and diversions. Review TAFs for fog trends: is the fog expected to lift, persist, or worsen? Check RVR values and monitor updates closely during the approach phase. If conducting a CAT II or CAT III approach, ensure all crew members are briefed on the specific procedures, callouts, and automation modes that will be used.

During taxi in low visibility, follow the progressive taxi instructions from ATC carefully. Use the aircraft's runway awareness and advisory system (RAAS) or equivalent technology to maintain situational awareness on the airport surface. Runway incursions are a heightened risk during low-visibility operations.

Real-time monitoring tools like AeroSentinel provide instant visibility and RVR data, trend analysis, and alerts when conditions approach critical thresholds, helping pilots make timely decisions about approaches, diversions, and holding.

Conclusion

Fog and low visibility represent some of the most operationally challenging conditions in aviation. Understanding fog formation, knowing the different types and their characteristics, and being proficient in low-visibility procedures enables pilots to operate safely and efficiently even when the world outside the cockpit is nearly invisible. With proper planning, training, and real-time weather monitoring, fog need not ground the aviation industry. It just requires respect, preparation, and the right tools.