Fog Formation Patterns
Understanding how and why fog forms to predict its impact on your flights
Why Pilots Need to Understand Fog
Fog is simply a cloud at ground level—water droplets suspended in air reducing visibility. But for pilots, fog represents one of the most common and disruptive weather phenomena. Fog can form rapidly, persist unexpectedly, and strand VFR pilots at airports that were clear an hour earlier.
Understanding the different types of fog, their formation mechanisms, and local patterns allows you to anticipate fog events rather than being surprised by them. This knowledge directly translates to better go/no-go decisions and smarter alternate planning.
Key insight: Fog is a local phenomenon. General area forecasts may not predict fog at your specific airport. Learn your airport's fog patterns—some airports fog up while nearby fields remain clear due to terrain, water bodies, or urban heat.
The Temperature-Dewpoint Connection
All fog formation comes down to one principle: when air cools to its dew point temperature, water vapor condenses into visible droplets. The two ways this happens are:
Cool the Air
Radiation fog, upslope fog, and precipitation-induced fog all form by cooling the air to reach the dew point. The temperature drops to meet the dew point.
Add Moisture
Steam fog forms by adding moisture to cold air, raising the dew point to meet the temperature. Evaporation from warm water into cold air.
Reading the Temperature-Dewpoint Spread
In a METAR, when the temperature and dew point are within 3°C (5°F) of each other, fog or low clouds are likely. When they're equal, saturation has occurred.
↳ Only 1°C spread - fog very likely overnight
Types of Fog
Radiation Fog (Ground Fog)
The most common fog type in aviation, radiation fog forms on clear, calm nights when the ground loses heat through radiation. The ground cools the air above it until it reaches the dew point. This fog typically forms in valleys and low areas where cold air collects.
Formation Requirements
- • Clear skies - allows maximum radiational cooling
- • Light winds - below 5 knots (gentle mixing deepens fog)
- • High humidity - small temperature-dewpoint spread
- • Long nights - autumn/winter favored over summer
- • Moist ground - after rain, near water sources
When It Forms
Typically develops in the hours before dawn when temperatures are lowest. Deepest just before sunrise.
When It Lifts
Usually 2-4 hours after sunrise. Burns off from edges first, lifts from ground upward. Thick fog takes longer.
Advection Fog
Advection fog forms when warm, moist air moves horizontally over a cooler surface. Unlike radiation fog, advection fog can form at any time of day, in any wind condition, and can persist for days. It's common along coastlines and over cold ocean currents.
Classic Scenarios
- • San Francisco Bay - Pacific air over cold California Current
- • Great Lakes - warm summer air over cold spring lake water
- • UK/Northern Europe - Atlantic air over cold land in winter
- • Gulf Coast - warm Gulf air moving over cold winter land
Key Difference from Radiation Fog
Advection fog doesn't "burn off" with morning sun—it requires a wind shift to move the moist air mass away. It can persist for days when the synoptic pattern maintains the airflow pattern.
Upslope Fog
When moist air is forced up a slope (by wind), it cools adiabatically. If it cools to the dew point, fog forms on the hillside or mountain. This is particularly common east of the Rockies when easterly winds push Gulf moisture up the High Plains.
Where It Occurs
- • Eastern Colorado/New Mexico - easterly upslope flow
- • Appalachian Mountains - Atlantic moisture meeting ridges
- • UK Pennines - moist westerly flow
- • Any elevated airport - with moist, upslope wind component
Steam Fog (Sea Smoke)
Steam fog forms when very cold air moves over much warmer water. Evaporation from the warm water adds moisture to the cold air faster than it can mix away, causing instant condensation. It appears as "steam" rising from water surfaces.
Common Situations
- • Cold arctic air over relatively warm ocean
- • Cold autumn/winter mornings over warm lakes
- • Seaplane bases and waterfront airports
Usually shallow (under 100 feet) but can reduce visibility significantly at water-level airports and seaplane operations.
Precipitation-Induced Fog
When rain falls through cool air near the surface, evaporation from the falling rain adds moisture to the air, raising the dew point to meet the temperature. This is common ahead of warm fronts where warm rain falls into cool surface air.
Frontal Fog
Watch for this ahead of warm fronts. The TAF might show improving conditions after frontal passage, but the fog can form rapidly as rain begins and persist until the front passes and the airmass changes.
Fog in METARs and TAFs
METAR Fog Codes
| Code | Meaning | Visibility |
|---|---|---|
| FG | Fog | Below 5/8 SM (1000m) |
| BR | Mist | 5/8 SM to 6 SM |
| BCFG | Fog patches (bank) | Varies by location |
| PRFG | Partial fog | Part of airport |
| FZFG | Freezing fog | Deposits ice |
| MIFG | Shallow fog | Under 6 feet deep |
TAF Fog Prediction
TAF KSFO 161130Z 1612/1718 28008KT P6SM FEW025
TEMPO 1702/1708 1SM BR
FM170800 VRB03KT 1/4SM FG VV002
BECMG 1711/1713 28008KT P6SM FEW020
This TAF shows fog developing overnight: first mist (BR) from 02Z-08Z, then dense fog (FG) with 1/4 mile visibility and vertical visibility 200 feet starting at 08Z, improving back to clear by 13Z after morning heating.
TAF Limitations
TAFs are issued only for airports with weather stations. Many smaller airports don't have TAFs—you must use nearby TAFs plus local knowledge. Also, fog is notoriously difficult to forecast precisely; be prepared for the TAF to be wrong.
Predicting Fog Yourself
While TAFs provide fog forecasts, understanding the conditions yourself lets you second-guess unrealistic forecasts and plan better.
Radiation Fog Checklist
Check these conditions in the evening to predict overnight fog:
- 1 Temperature-dewpoint spread: Less than 5°C? Fog likely. Equal? Fog certain.
- 2 Sky conditions: Clear or scattered high clouds? Maximum cooling possible.
- 3 Wind forecast: Calm to 5 knots overnight? Light mixing deepens fog.
- 4 Recent rain: Moist ground evaporates into cooling air, raising humidity.
- 5 Local terrain: Valley or low area? Cold air drains and pools there.
The 4°F Per Hour Rule
On a clear, calm night, temperature typically drops about 4°F (2°C) per hour after sunset. If sunset temperature is 60°F and dew point is 52°F, expect to reach the dew point approximately 2 hours after sunset. Fog will form shortly after. This is a rough estimate— actual cooling rates vary with humidity, wind, and cloud cover.
Notable Fog-Prone Airports
Some airports are famous for fog. Knowing these patterns helps with flight planning:
KSFO - San Francisco International
Classic advection fog from May-October. Pacific air moves over the cold California Current (10-15°C colder than surrounding ocean). Fog banks form offshore and push through the Golden Gate gap, especially in late afternoon. Can persist for days.
KDEN - Denver International
Radiation fog in autumn when Front Range cools under clear skies. Also upslope fog with easterly winds pushing Gulf moisture up the High Plains. Can close the airport when all parallel runways go below CAT III minimums.
EGLL - London Heathrow
Both radiation fog (Thames Valley) and advection fog (Atlantic air). November is notorious. Heathrow maintains CAT IIIb capability but delays are common when fog reduces capacity.
Central Valley, California
"Tule fog" - dense radiation fog from November-February. Can reduce visibility to near zero over huge areas. KFAT, KMOD, KSMF all affected. Often doesn't lift until afternoon or may persist for days under high pressure.
Operational Decisions
VFR Pilot Strategies
- • Early departures: Leave before dawn fog develops
- • Late arrivals: Wait for fog to burn off (check TAF BECMG times)
- • Alternate planning: Pick alternates away from fog-prone areas
- • Phone ahead: Call destination FBO for real conditions
IFR Considerations
- • Approach minimums: Know your lowest approach available
- • Alternate requirements: 600-2 / 800-2 rule may require distant alternates
- • Fuel planning: Budget for holds and missed approaches
- • Takeoff minimums: Part 91 none, but consider engine-out options
The No-Takeoff Trap
Just because Part 91 allows takeoff in zero visibility doesn't mean you should. If you have an engine failure after takeoff in fog, you cannot return to the runway. You cannot see to land anywhere. Many experienced IFR pilots set personal minimums for takeoff—commonly requiring enough visibility to make the runway if the engine fails on initial climb.
Key Takeaways
- Radiation fog forms on clear, calm nights—burns off with morning sun
- Advection fog can form anytime, in any wind, and persist for days
- Temperature-dewpoint spread under 3°C = fog is likely or imminent
- Learn your local airports' fog patterns—nearby airports can vary dramatically
- Just because you can legally take off in fog doesn't mean you should