11.fog-and-low-visibility. Fog and Low Visibility
Fog is a surface-based cloud composed of either tiny water droplets or ice crystals. It is one of the most insidious weather hazards to aviation because it can form rapidly, reduce visibility below operating minimums in minutes, and persist long after other weather has cleared. By definition, fog exists when visibility is reduced to less than 5/8 statute mile and the cloud base is at or near the surface; if visibility is greater, the same phenomenon is classified as mist or haze.
Fog forms when the temperature and dew point of the air become identical, or nearly so (typically within 5°F / 2.5°C). This may occur through cooling the air to its dew point or by adding moisture to the air. The five principal types of fog recognized in aviation meteorology are radiation, advection, upslope, steam, and precipitation-induced (frontal) fog.
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Radiation Fog (Ground Fog): Forms over land on clear, calm nights when terrestrial radiation cools the ground, which in turn cools the adjacent air to its dew point. It requires light winds (less than about 5 knots), high relative humidity, and clear skies. Radiation fog is shallow and most common in low-lying terrain and river valleys. It typically dissipates within a few hours after sunrise as solar heating raises the surface temperature.
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Advection Fog: Forms when warm, moist air moves horizontally over a cooler surface. Unlike radiation fog, it requires wind (typically 5–15 knots) to sustain itself and can occur day or night. Advection fog is common along coastal areas — for example, when moist Pacific air drifts over the cold California current. It is more persistent and often deeper than radiation fog and can move inland many miles.
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Upslope Fog: Forms as moist, stable air is forced up sloping terrain, cooling adiabatically to its dew point. It requires wind to push the air uphill. Upslope fog is common on the eastern slopes of the Rockies and Appalachians and may persist for days as long as the wind direction holds.
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Steam Fog (Sea Smoke): Forms when cold, dry air moves over warmer water. Water evaporates into the cold air and quickly recondenses as fog. Low-level turbulence and icing can be associated with steam fog.
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Precipitation-Induced (Frontal) Fog: Forms when warm rain or drizzle falls through cooler air, saturating it. It is most common with warm fronts but can occur with any front where the temperature spread between the airmasses is significant. This fog can be dense and widespread.
Ice Fog occurs in cold weather, typically when temperatures are well below freezing (−25°F / −32°C or colder), when water vapor sublimates directly into ice crystals. It is most common in the arctic regions but can occur anywhere given sufficiently cold, stable conditions.
Operational Considerations
Low visibility is a leading factor in weather-related accidents, particularly continued VFR flight into IMC and approach-and-landing accidents. Pilots should be familiar with the following criteria:
- Basic VFR weather minimums (14 CFR 91.155): In Class E airspace below 10,000 feet MSL, 3 statute miles visibility and cloud clearances of 500 below, 1,000 above, and 2,000 horizontal.
- Special VFR: May be requested in Class B, C, D, or E surface areas with at least 1 statute mile visibility and clear of clouds (day only for non-instrument-rated pilots).
- IFR alternate requirements (91.169): An alternate is required when forecast weather at the destination, from 1 hour before to 1 hour after ETA, is less than 2,000-foot ceilings or 3 SM visibility (the "1-2-3 rule").
Forecasting Fog
A useful rule of thumb is the temperature/dew point spread: when the spread is 5°F (about 3°C) or less and decreasing, fog formation is likely, especially overnight under clear skies with light winds. METARs and TAFs report visibility in statute miles and use abbreviations such as FG (fog), BR (mist, visibility 5/8 to 6 SM), HZ (haze), and FZFG (freezing fog). The remarks section often contains visibility trends and tower visibility.
Pilot Strategies
- Always check the temperature/dew point spread during preflight weather briefing.
- Be alert during the first hour after sunrise — radiation fog can be at its densest just before it burns off.
- Plan an out: maintain a usable alternate within fuel range and brief approach minimums before entering an area of marginal visibility.
- Remember that visibility from the cockpit on approach is often less than reported due to slant-range visibility and forward scatter from landing lights in fog.
- If unintentional flight into IMC occurs, immediately transition to instruments, maintain aircraft control (180° standard-rate turn to exit), and contact ATC for assistance.