Glossary of Obscuration in Meteorology and Aviation

Obscuration, in meteorology, signifies any non-precipitation phenomenon that substantially reduces horizontal visibility. For aviation, accurate understanding and reporting of obscuration is critical—affecting flight planning, safe navigation, and airport operations worldwide.

Table of Meteorological Obscuration Types

Mist (BR)

Definition and Abbreviation

Mist (BR) is a visible suspension of microscopic water droplets reducing horizontal visibility to between 1 km and 7 km. It is less dense than fog, but still impairs visual clarity—especially for pilots during approach, landing, or low-level flight.

Physical Characteristics and Formation

Mist typically forms at night or early morning, especially in valleys or low-lying areas, when air is nearly saturated and wind is light. It forms through radiational cooling and condensation on hygroscopic nuclei such as dust or pollen.

Effects on Aviation

Mist can render distant runways, terrain, or obstacles indistinct, increasing approach and landing risks. It is commonly reported in METARs and may develop into fog if cooling continues. Pilots should be alert to rapid visibility changes in the presence of mist, especially during dawn or dusk.

Fog (FG)

Definition and Abbreviation

Fog (FG) is a dense suspension of water droplets that reduces surface horizontal visibility below 1 km. Fog is one of aviation’s most disruptive obscurations, often forcing airports to close or restrict operations to only the most capable aircraft and crews.

Types and Formation

• Radiation fog: Cools overnight on clear, calm nights.
• Advection fog: Moist air flows over a cooler surface.
• Upslope fog: Air is forced up terrain and cools.
• Evaporation/steam fog: Cold air over warmer water.
• Ice fog: Minute ice crystals in very cold air.

Effects on Aviation

Fog can reduce visibility to near zero, making visual navigation, taxi, takeoff, and landing hazardous. It frequently triggers instrument-only procedures (IFR), delays, or cancellations. Airports with advanced ILS may operate in fog, but only with specialized training and equipment.

Smoke (FU)

Definition and Abbreviation

Smoke (FU) consists of fine, suspended combustion particles—carbon, ash, and other organic materials. It is generated by wildfires, industrial activity, and urban pollution.

Formation and Hazards

Wildfires, industrial emissions, and agricultural burning are common sources. Smoke can extend over hundreds of kilometers and reduce visibility below 1 km. Inhalation is a health hazard, and dense smoke can obscure runways, other aircraft, and terrain—leading to potential CFIT (Controlled Flight Into Terrain) incidents.

Haze (HZ)

Definition and Abbreviation

Haze (HZ) is a suspension of extremely fine, dry particles (aerosols) that scatter light and reduce visibility, often imparting a bluish or yellowish tint to the sky.

Causes and Effects

Haze is associated with pollution, industrial activity, or biomass burning. It builds gradually, sometimes going unnoticed until significant visibility loss occurs. It impairs visual approaches and can cause misjudgment of distances—especially over water or featureless terrain.

Volcanic Ash (VA)

Definition and Abbreviation

Volcanic Ash (VA) consists of fine, abrasive rock, mineral, and glass particles from volcanic eruptions. Ash clouds can reach high altitudes and travel thousands of kilometers.

Aviation Hazards

Ash clouds are extremely dangerous: ingestion can cause engine failure, sandblast cockpit windows, block sensors, and damage critical systems. Encounters have led to near-loss of aircraft; avoidance is the only safe course. Airports affected by ashfall may close or require extensive cleaning before operations resume.

Dust (DU)

Definition and Abbreviation

Dust (DU) involves fine soil, silt, or clay particles suspended by wind. Common in arid and drought-affected regions, dust storms can reduce visibility to less than 400 meters.

Aviation Hazards

Dust impairs visibility, can infiltrate engines and avionics, and poses respiratory risks. Severe dust storms may force flight delays or diversions and can contribute to widespread operational disruptions.

Sand (SA)

Definition and Abbreviation

Sand (SA) is similar to dust but with larger, heavier particles—typically from desert regions. Sandstorms can rapidly reduce visibility and damage exposed aircraft components.

Effects on Aviation

Sand can clog engines, scratch surfaces and windows, and obscure runways or taxiways. Operations in sand-prone regions require special maintenance and precautions.

Spray (PY)

Definition and Abbreviation

Spray (PY) consists of sea water droplets lofted by strong winds along coastlines or over large bodies of water. It can reduce visibility, especially during storms or high surf.

Aviation Impact

Spray is of particular concern for coastal airports and low-level overwater flight. It may combine with fog or haze for added visibility reduction.

Blowing Snow (BLSN)

Definition and Abbreviation

Blowing Snow (BLSN) is surface snow lifted by strong winds, creating a whiteout with visibility sometimes below 400 meters.

Impact on Aviation

Blowing snow can obscure runways and taxiways, hide obstacles, and make visual navigation impossible. It frequently results in airport closures, especially in blizzard conditions.

Obscuration Reporting in Aviation

Obscuration phenomena are reported in METARs, TAFs, and SIGMETs using standardized ICAO codes. Pilots, controllers, and dispatchers use these reports for quick assessment and decision-making. For example:

• FG: Fog
• BR: Mist
• FU: Smoke
• HZ: Haze
• VA: Volcanic Ash
• DU: Dust
• SA: Sand
• PY: Spray
• BLSN: Blowing Snow

Thresholds for reporting are set by ICAO and national authorities, often requiring observation when visibility drops below defined levels, or when the phenomenon is observed to affect operational safety.

Operational Considerations and Safety Advice

• Flight Planning: Always review visibility forecasts and recent observations. Consider alternate airports and minimums for your aircraft type and crew.
• In-Flight: Monitor ATIS, AWOS/ASOS, and pilot reports (PIREPs) for changing obscuration conditions.
• On the Ground: Exercise caution during taxi, takeoff, and landing when obscuration is present. Use all available lighting and follow low-visibility procedures.
• After Exposure: Inspect aircraft for contamination—especially after volcanic ash, dust, sand, or smoke exposure.

Conclusion

Obscuration is a critical meteorological factor for aviation, with the potential to disrupt all phases of flight and ground operations. Understanding each type—its causes, characteristics, and hazards—is essential for safe, efficient, and compliant flight operations worldwide.

For a comprehensive guide to aviation weather codes and detailed operational procedures, consult ICAO Annex 3, the WMO Manual on Codes, and your national aviation authority.

Further Reading

• ICAO Annex 3: Meteorological Service for International Air Navigation
• WMO Manual on Codes (FM 15–XII, FM 16–XII)
• FAA Advisory Circulars on Weather and Obscuration
• National Weather Service Aviation Weather Center

Obscuration reporting and mitigation are fundamental to modern aviation safety—know the codes, know the risks, and always plan ahead.