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Fog Types in Aviation: How Fog Forms, How to Predict It, and How to Fly in Foggy Conditions

Updated: 5 days ago

Fog has caused more aviation accidents than almost any other weather phenomenon you can think of. It looks innocuous from the ground — a soft gray haze that "should burn off." But fog can reduce visibility from 10 miles to 1/4 mile in 30 minutes, persist for hours or days, and trap aircraft below minimums when conditions are otherwise calm and benign. The infamous Tenerife disaster — the deadliest accident in aviation history — happened in fog. Multiple GA fatalities every year are attributed to scud running below low ceilings or attempting takeoff with inadequate visibility.


This post covers fog formation, the six main types pilots need to recognize, how to predict fog formation before flight, regional patterns, and how to fly safely when fog is or might be present.



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What Fog Actually Is

Fog is a cloud at ground level. The same physics that creates clouds in the sky creates fog at the surface — water vapor in the air condenses into tiny suspended droplets. The difference is location, not formation mechanism.


For fog to form, two things must be true:

  • The air must contain moisture — water vapor from evaporation of bodies of water, plant transpiration, or precipitation.

  • The air must reach saturation — temperature and dewpoint must converge so that the air can no longer hold the moisture in vapor form.


Saturation occurs when the temperature equals the dewpoint. The various types of fog differ primarily in HOW they reach saturation — through cooling, through moisture addition, or through mixing of different air masses.


The temperature-dewpoint spread is the single most important predictor of fog. When the temperature-dewpoint spread is small (under 5°F or so), saturation can occur with relatively small temperature changes. When the spread is wide, more dramatic changes are needed.


The Six Types of Fog

1. Radiation Fog (Ground Fog)

The most common type and the one pilots encounter most regularly.


Formation:

  • Clear skies allow ground to radiate heat into space at night

  • Surface and the air directly above it cool rapidly

  • If cooling brings air temperature down to dewpoint, fog forms

  • Calm or light winds (3-5 knots typically) maintain the fog — too much wind disperses it, too little doesn't provide enough mixing


Required conditions:

  • Clear skies (no insulating cloud cover)

  • Light winds (3-7 knots typical)

  • Sufficient surface moisture

  • Long nights for cooling (more common in fall through spring)


Where it forms:

  • Valleys and basins (cold air drains downhill and pools)

  • Flat terrain with adequate moisture

  • Areas surrounded by water or vegetation

  • Airport surfaces with grass or moist ground


When it dissipates:

  • Solar heating after sunrise warms ground

  • Wind picks up and disperses the fog

  • Typically dissipates by mid-to-late morning

  • In valleys with significant fog or under inversions, can persist into early afternoon

  • The "Tule fog" of California's Central Valley can persist for days under stagnant conditions


Pilot implications:

  • Predictable based on overnight conditions

  • Often patchy initially, then becoming dense

  • Common at airports surrounded by farmland, lakes, or rivers

  • Plan early morning departures conservatively


2. Advection Fog

Forms when warm, moist air moves horizontally over a cooler surface. Doesn't depend on nighttime cooling, so it can form at any time of day or night.


Formation:

  • Warm, moist air moves over cold surface (water or land)

  • Air in contact with cold surface cools to dewpoint

  • Wind continues to advect more warm moist air, which continues to cool


Required conditions:

  • Warm, humid air mass

  • Cool surface (cold ocean current, snow-covered ground, cold land)

  • Wind to maintain the air movement

  • Persistent temperature differential


Where it forms:

  • Pacific coast — warm Pacific air over cold California Current

  • North Atlantic coast — warm air from south meeting cold North Atlantic

  • Great Lakes regions during seasonal transitions

  • Areas where warm moist air moves over snow-covered ground


When it dissipates:

  • Wind shifts away from the source of warm moist air

  • Solar heating warms the cold surface

  • Less affected by sunrise than radiation fog

  • Can persist for days under sustained conditions


Pilot implications:

  • Less predictable than radiation fog

  • Can form rapidly when wind direction shifts

  • Highly persistent — sometimes days

  • Major operational impact at coastal airports

  • Famous for affecting San Francisco, Seattle, Boston, San Diego


3. Upslope Fog

Forms when moist air is forced up sloping terrain and cools to saturation as it rises.


Formation:

  • Wind pushes moist air up a mountain slope or hillside

  • Adiabatic cooling occurs (3°C per 1,000 feet)

  • If cooling reaches dewpoint, fog forms

  • The fog "wraps" around the higher terrain


Required conditions:

  • Moist air mass

  • Wind blowing toward higher terrain

  • Sufficient elevation for adequate cooling

  • Often associated with frontal systems pushing air against terrain


Where it forms:

  • Eastern slopes of the Rocky Mountains

  • Appalachian Mountains

  • Coastal mountains and ranges

  • Anywhere terrain rises significantly above surrounding lowlands


When it dissipates:

  • Wind direction changes and stops pushing moist air upslope

  • Air mass changes

  • Significant warming

  • Can persist for days under sustained upslope flow


Pilot implications:

  • Particularly dangerous for VFR pilots in mountainous terrain

  • Higher elevation airports may be IFR while lower airports are clear

  • Mountain pass flying can become hazardous

  • Common in the Rocky Mountain west during eastward-flowing moisture


4. Steam Fog (Evaporation Fog)

Forms when cold air moves over warmer water. The opposite mechanism of advection fog.


Formation:

  • Cold, dry air over warmer water surface

  • Water evaporates into the cold air

  • The cold air rapidly becomes saturated

  • Steam-like vapor rises from the water


Required conditions:

  • Cold, dry air mass

  • Warmer water (lake, river, ocean)

  • Significant temperature difference between water and air


Where it forms:

  • Great Lakes in fall and early winter (cold continental air over warm lake water)

  • Rivers and streams during cold winter mornings

  • Ocean during cold air outbreaks over warmer water

  • Particularly dramatic over geothermal features


When it dissipates:

  • Air warms toward water temperature

  • Wind disperses the rising vapor

  • Usually localized and patchy


Pilot implications:

  • Often visible from the air as wisps or columns rising from water

  • Generally patchy and localized rather than widespread

  • Can suddenly reduce visibility for aircraft over water

  • Common visual feature in fall and winter aviation


5. Ice Fog

Forms in extreme cold when water vapor sublimates directly into ice crystals rather than condensing into liquid droplets.


Formation:

  • Very cold temperatures (typically below -20°F or -30°C)

  • Water vapor sublimates directly into ice

  • Tiny ice crystals suspended in the air

  • Common where additional moisture is added (vehicle exhaust, breathing, industrial emissions)


Required conditions:

  • Extreme cold

  • Some moisture source

  • Calm or light winds

  • Common in arctic and very cold continental areas


Where it forms:

  • Alaska, northern Canada

  • Northern continental U.S. during severe cold snaps

  • Around airports with significant exhaust emissions in extreme cold

  • High-elevation areas in winter


Pilot implications:

  • Severe visibility reduction

  • Frost can accumulate on aircraft surfaces from the ice fog

  • Airport operations significantly affected

  • Less common than other fog types but extreme when present


6. Precipitation (Frontal) Fog

Forms when warm rain falls through cold air below a warm front, evaporating partially and saturating the cold air at the surface.


Formation:

  • Warm front lifts warm air over cold surface air

  • Warm rain falls from above, passes through cold air below

  • Some rain evaporates into the cold air

  • The cold air becomes saturated and fog forms


Required conditions:

  • Active warm front

  • Warm precipitation falling from aloft

  • Cold air mass at surface

  • Often associated with steady rain or drizzle


Where it forms:

  • Anywhere warm fronts encounter cold surface air

  • Common in winter and early spring

  • Often widespread across regions experiencing frontal passage


Pilot implications:

  • Often combined with low ceilings and steady precipitation

  • Can produce IFR conditions over very large areas

  • Frequently associated with freezing rain and severe icing risk

  • One of the most operationally limiting weather conditions for entire regions


Predicting Fog Before Flight

Fog is one of the most predictable weather phenomena if you know what to look for.


For radiation fog:

  • Clear skies overnight

  • Light or calm winds

  • Sufficient ground moisture (recent rain, dew, river/lake nearby)

  • Long nights (more time for cooling)

  • Temperature-dewpoint spread less than 5°F


For advection fog:

  • Wind direction bringing warm moist air over cold surface

  • Significant temperature differential between air and surface

  • Persistent wind direction

  • Coastal locations with cold currents


For upslope fog:

  • Wind direction pushing moisture toward higher terrain

  • Synoptic situation favoring upslope flow

  • Active frontal systems with onshore flow


Key METAR/TAF clues:

  • VV (Vertical Visibility) indicates fog or low ceiling obscures the sky

  • FG in METAR = fog (visibility less than 5/8 statute mile)

  • BR in METAR = mist (visibility 5/8 to 6 statute miles)

  • HZ = haze

  • FU = smoke

  • Temperature/dewpoint spread of 0-3°F or 0-2°C in METAR = fog likely

  • TAFs often forecast fog with specific timing — read them carefully


The Temperature-Dewpoint Convergence

One of the most useful pilot techniques: track temperature and dewpoint trends through ATIS or weather updates.


During the evening:

  • Temperature drops as the sun sets

  • Dewpoint stays relatively constant

  • Spread narrows

  • When spread approaches 0, fog is imminent


Trend monitoring:

  • 75°F / 70°F = 5°F spread, low fog risk

  • 70°F / 67°F = 3°F spread, fog forming likely

  • 65°F / 64°F = 1°F spread, fog very likely

  • 62°F / 62°F = 0°F spread, fog present or imminent


By tracking ATIS broadcasts at airports along your route, you can monitor the convergence in real time and make better decisions.


Fog Dissipation: When Will It Clear?

Knowing when fog will clear is as important as recognizing when it forms.


Radiation fog dissipation:

  • Solar heating of ground warms the lowest layer

  • Mixing causes fog to "lift" into low stratus, then dissipate

  • Typical timing: 1-3 hours after sunrise on a clear day

  • Faster dissipation with stronger sunshine and breeze

  • Slower in deep valleys or under inversions

  • "Burn off by noon" is a common expectation but not guaranteed


Advection fog dissipation:

  • Requires wind direction change or air mass change

  • Doesn't reliably burn off with sun alone

  • Can persist for days

  • Coastal advection fog often clears inland during the day, returns at night


Upslope fog dissipation:

  • Requires wind direction change

  • Doesn't dissipate with sunshine alone

  • Can persist as long as upslope flow continues

  • Often associated with multi-day weather patterns


Steam fog dissipation:

  • Localized and usually clears quickly with daytime warming

  • Can return with renewed cold air masses


Ice fog dissipation:

  • Requires significant warming

  • May persist for days or weeks in sustained cold

  • Reduced when calm conditions disperse the source moisture


Precipitation fog dissipation:

  • Clears as the warm front passes through

  • May take hours to days depending on system speed


Regional Fog Patterns Pilots Should Know

Pacific Coast (Northern California, Oregon, Washington):

  • Frequent advection fog from cold California Current

  • Particularly intense July-September

  • Marine layer extends inland during summer mornings

  • "Sea of clouds" can extend up to several thousand feet


Northeast Coast (Maine to Long Island):

  • Advection fog when warm southerly air meets cold Atlantic

  • Common during summer heat waves

  • Coastal Maine experiences persistent fog days


Great Lakes Region:

  • Steam fog over lakes in fall (cold air over warm water)

  • Lake-effect snow with associated visibility reduction

  • Advection fog when warm moist air moves over cold lake


Central California Valley:

  • Tule fog — radiation fog that persists for days under inversions

  • Most severe November through February

  • Can affect entire San Joaquin and Sacramento Valleys

  • Visibility can drop to 1/8 mile or less


Appalachian Mountains:

  • Upslope fog with eastward-flowing moisture

  • Common in winter and spring

  • Affects mountain airports from Georgia to Maine


Rocky Mountain East Slope:

  • Upslope fog with east winds bringing Plains moisture into the foothills

  • Particularly affects Denver, Colorado Springs, Cheyenne

  • Often precedes frontal passages


Alaska and Northern Canada:

  • Ice fog in extreme cold

  • Persistent throughout deep winter

  • Major operational challenge for arctic flying


Operational Considerations

Pre-flight:

  • Check current and forecast visibility at departure, destination, and alternates

  • Note temperature-dewpoint spread for fog potential

  • Look at TAF for forecast visibility throughout your flight window

  • Consider seasonal patterns for your region


Approach planning:

  • Determine the minimums required for the approach

  • Check whether the approach is possible at expected visibility

  • Consider alternates if visibility is marginal

  • Arrive with extra fuel for potential holds or diversions


During flight:

  • Listen to ATIS at destination and en route airports

  • Watch for trending conditions — improving or deteriorating

  • Have alternate plans ready before they're needed

  • Plan extra time for unexpected delays


If conditions are below minimums:

  • Don't try to push it — fog deteriorates rapidly

  • Divert early to maintain options

  • Wait it out on the ground if you're already there

  • Patience is cheaper than incidents


On the Written Test

Fog appears consistently on weather knowledge tests. The most commonly tested topics:

  • Definition of fog (visibility less than 5/8 SM)

  • Radiation fog formation conditions

  • Advection fog vs. radiation fog

  • Upslope fog associated with terrain

  • Steam fog conditions (cold air, warm water)

  • Temperature-dewpoint spread as fog predictor

  • METAR and TAF abbreviations (FG, BR, HZ, etc.)


Predictors:

  • Temperature-dewpoint spread under 5°F

  • Clear skies, light winds (radiation)

  • Coastal locations with onshore flow (advection)

  • Wind toward higher terrain (upslope)

  • Cold air over warm water (steam)

  • Warm front with cold surface (precipitation)


METAR codes:

  • FG — fog (visibility < 5/8 SM)

  • BR — mist (visibility 5/8 to 6 SM)

  • HZ — haze

  • VV — vertical visibility (no defined ceiling)



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Author: Nathan Hodell

CFI, CFII, MEI, ATP, Creator and CEO

Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.



 
 
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