FlyingWorx

Fog

Surface-based visibility hazards — types, forecasting, and operational planning.

Quick Reference

Key points

Short-answer refresher for returning pilots before diving into the full page.
  • Fog type tells you what clears it: radiation fog often lifts with sun or mixing, while advection fog can persist until the whole air mass changes.
  • Temperature-dew point spread, wind, terrain, and time of day predict fog better than any single station report read by itself.
  • For IFR planning, fog is a timing problem as much as a current-conditions problem, so compare forecast groups against recent METAR and RVR movement.

Standards & References

FAA doctrinal and ACS cross-reference

Use this box to line the topic up with the FAA’s primary instrument handbooks, the most relevant ACS task areas, and the knowledge, skill, and risk elements that usually drive checkride evaluation.

Instrument Rating Airplane ACS unless noted
IFH
  • IFH Ch. 8, Flight Planning: weather briefing, ceiling/visibility interpretation, and alternate thinking before launch.
  • IFH Ch. 9, IFR Flight: adapting arrivals and approaches when visibility and runway-environment acquisition become the limiting factor.
IPH
  • IPH Ch. 3, Arrivals: descent and arrival planning when ceiling and visibility narrow the likely runway and transition choices.
  • IPH Ch. 4, Approaches: minima use, runway-environment requirements, and approach choice when visibility is the controlling limit.
ACS Task References
  • I.F Weather Information.
On This Page

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.

For IFR dispatch, fog belongs in the personal-minimums conversation as much as the weather briefing. Pair this page with IFR Risk Management and Personal Minimums when deciding whether the destination and alternate picture still leave room for a missed approach, holding, or delay.

  • 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 the weather and approach requirements in 14 CFR § 91.169(c) and carry fuel for diversion, holding, and contingencies under 14 CFR § 91.167.
  • 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.

  • 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 prolonged 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.

  • 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; may be accompanied by low clouds and precipitation.
  • Pilot actions: Expect extended IMC on approaches through passes; 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 located above common valley fog layers.

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.

  • 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 and carry sufficient fuel for alternates.

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); visibility given in statute miles and vertical visibility as VV (in hundreds of feet). RVR groups appear as R06/0400FT.
  • 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 minima and to the alternate weather standard in 14 CFR § 91.169(c), then add your operator's or personal planning margins. Watch for temporary groups (TEMPO) that may affect go/no-go decisions.
Example METAR
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 ½ statute mile, fog present, vertical visibility 200 ft, temperature 2°C dewpoint 1°C, altimeter 29.92" Hg.

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

Decode: 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.

Learn More