Overview

Fog is a surface-based reduction in visibility caused by suspended water droplets that often produces low ceilings and reduced runway visual range (RVR). It is a frequent cause of delays, diversions, and missed approaches because it can change airport operating minima quickly. Pilots should treat fog as an operational planning hazard — confirm current and forecasted METAR/TAF, check NOTAMs and RVR, plan suitable alternates and fuel, and ensure required approach equipment and crew qualifications are available.

• Visibility & ceilings: Expect instrument approaches or diversions when visibility and ceilings fall below published minima.
• Timing & trends: Short-term TAF groups (FM/BECMG/TEMPO/PROB) often drive go/no‑go decisions; monitor trend indicators closely during preflight and en route.
• Alternates & fuel: Select alternates that meet approach minima and carry fuel for diversion, holding, and contingencies.
• Surface reports: Use RVR and runway lighting status to refine landing minima and approach technique decisions.

Radiation Fog

Radiation fog forms overnight as the ground cools by radiating heat to space, chilling the air in contact with the surface to its dew point. It is most common in clear, calm conditions and typically forms in low-lying areas and sheltered valleys.

• When & where: Clear nights with light winds, often in rural valleys and over moist ground; typically forms after sunset and can dissipate after sunrise.
• Characteristics: Shallow depth (often under 300–500 ft), patchy coverage, tends to burn off with sunlight or a strengthening breeze.
• Pilot actions: Expect localized reductions in visibility; rely on METARs, NOTAMs, and ASOS/AWOS for surface reports; plan alternates for low-visibility operations.

Advection Fog

Advection fog occurs when moist air is transported horizontally over a colder surface, cooling the air to its dew point. It can be widespread and persistent, particularly over coastal regions where warm moist air moves over chilly water or land.

• When & where: Common along coastlines and over large bodies of water where warm maritime air moves over colder surfaces; can persist for many hours or days.
• Characteristics: Often deeper and more extensive than radiation fog; can reduce ceilings and visibility significantly and persist into daytime.
• Pilot actions: Treat as a significant operational hazard — check TAFs and coastal METAR trends, plan alternates, and avoid peninsulas or routes that prolong time in the fog bank when possible.

Upslope Fog

Upslope fog forms when moist air is lifted mechanically up a terrain slope and cools to its dew point. It is common on windward slopes of mountain ranges and can lead to continuous low ceilings along the slope and surrounding areas.

• When & where: Occurs with stable, moist airflow perpendicular to rising terrain; typical in foothills and mountain passes.
• Characteristics: Can be spatially extensive along windward slopes and persist as long as the upslope flow continues; may be accompanied by low clouds and precipitation.
• Pilot actions: Expect extended IMC on approaches through passes; avoid low-level penetrations across ridgelines when icing or turbulence is present, and brief alternates with higher, unaffected fields.

Valley Fog

Valley fog is a localized form of radiation fog that settles into depressions and river valleys, often persisting until daytime heating or a change in wind mixes the layer out.

• When & where: Develops overnight in topographically low areas with poor drainage and cool surfaces; common in river valleys and basins.
• Characteristics: Highly localized; surrounding higher-elevation airports may remain VFR while valley floors are IMC.
• Pilot actions: Check airport elevation versus valley floor reports; prefer alternates at airports located above common valley fog layers and monitor sunrise trends.

Sea / Steam Fog

Sea (or steam) fog forms when cold air moves over warmer water, causing moisture to evaporate into the cold air and saturate it. It can form quickly and reduce visibility dramatically over coastal waters and downwind shores.

• When & where: Cold-air outbreaks over warm water bodies — common in fall and early winter around coastlines and large lakes.
• Characteristics: Can form rapidly, be dense and patchy, and extend offshore; often accompanied by low-level turbulence and wind shear near the surface.
• Pilot actions: Exercise caution for low-level operations and ferry flights over water — plan routes that minimize exposure, carry sufficient fuel for alternates, and brief passengers and crew on potential diversions.

How to read METAR/TAF for fog

METARs and TAFs contain the key weather information pilots use to detect and plan for fog. Look for visibility, fog/mist codes, runway visual range (RVR), and ceiling/vertical visibility reports. Pay special attention to changes (BECMG, FM) and probability groups (PROB30/40) in TAFs.

• METAR cues: 'FG' = fog, 'BR' = mist (lower impact), 'TS' = thunderstorm; visibility given in statute miles (or meters) and vertical visibility as 'VV' (in hundreds of feet). RVR groups appear as R06/0400FT (runway 06 RVR 400 ft).
• TAF cues: Forecasted visibility reductions or 'FM' (from) and 'BECMG' (becoming) groups indicate when fog is expected to develop or dissipate; 'PROB30' indicates a 30% probability of the condition.
• Decision factors: Compare forecasted minima to approach/alternate minima and your operator's required planning margins. Watch for short-term trends and temporary groups ('TEMPO') that may affect go/no-go decisions.

Example METAR (decode)

METAR KJFK 181651Z 18005KT 1/2SM FG VV002 02/01 A2992 RMK SLP134
                    

Decode: At KJFK on the 18th at 1651Z, wind 180° at 5 kt, visibility 1/2 statute mile, fog present, vertical visibility 200 ft, temperature 2°C dewpoint 1°C, altimeter 29.92" Hg.

Example TAF (decode)

TAF KJFK 181130Z 1812/1918 17005KT P6SM -RA BR
 FM181800 18006KT 2SM FG VV002
                    

Decode: TAF issued on the 18th at 1130Z valid 12Z-18Z on the 18th-19th; initial conditions VFR with light rain and mist, but from 1800Z expect wind 18006KT, visibility 2 SM with fog and vertical visibility 200 ft — plan alternates and minima accordingly.