Fog Types

Fog is simply a cloud whose base is at or very near the ground, thick enough to reduce surface visibility. In aviation reporting it is the phenomenon most likely to close a field to VFR operations without any dramatic weather, which is why understanding how each type forms and clears is a practical dispatch skill. The FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25) and the FAA Aviation Weather Handbook (FAA-H-8083-28) describe fog as forming by one of two mechanisms: cooling the air until it reaches its dew point, or adding moisture until the dew point rises to meet the temperature. The types are distinguished by which mechanism is at work and what supplies the cooling or the moisture.

Radiation fog, also called ground fog, forms on clear, calm nights when the ground radiates its heat away and cools, chilling the moist air in contact with it to the dew point. It requires little or no wind, a small temperature/dew-point spread, and clear skies that permit strong radiational cooling, so it is a stable-air, overnight phenomenon that settles into low ground and valleys. It typically burns off within a few hours after sunrise as the sun heats the surface and raises the temperature above the dew point — but a light wind can deepen it before it clears.

Advection fog forms when warm, moist air moves horizontally over a colder surface, such as sea air drifting over cold coastal land or over a cold ocean current. Unlike radiation fog it needs wind — up to roughly 15 knots deepens it rather than clearing it — and because it depends on air movement rather than overnight cooling, it can form at any time of day and persist for long periods, making it more operationally disruptive. It is common along coastlines and is a frequent cause of extended low visibility at seaside airfields.

Upslope fog forms when moist, stable air is forced up sloping terrain and cools adiabatically to its dew point as it rises. It requires wind to move the air upslope and can persist as long as that flow continues, which is why it can blanket the eastern slopes of mountain ranges for extended periods. Steam fog, sometimes called sea smoke or evaporation fog, forms when cold, dry air moves over much warmer water; moisture evaporates into the cold air and immediately recondenses, rising in wisps off the water surface. It is often associated with low-level instability and can carry a risk of icing and light turbulence. Precipitation-induced fog forms when relatively warm rain or drizzle falls through a cooler layer of air near the surface; evaporation from the falling precipitation saturates the cool air and forms fog, which is common ahead of a warm front and can produce widespread, persistent low visibility.

Across all types, the operational significance is the same: fog collapses visibility and often the ceiling, driving a field below VFR minima and complicating IFR approaches. EASA and ICAO reporting define fog as visibility below 1,000 meters, and the US METAR system uses the code FG for fog and BR for mist (visibility of five-eighths of a statute mile or more but still reduced); the distinction on a report tells a pilot how severe the restriction is.

For flight schools, fog is the classic reason a clear-looking morning still cannot support flying. A dispatcher who knows the local pattern — radiation fog settling into the valley overnight and burning off by mid-morning, or coastal advection fog that can sit for days — can set realistic start times and avoid stacking early lessons that will only be cancelled. Teaching students to read the temperature/dew-point spread on the evening METAR, and to recognize the calm, clear, humid setup that breeds radiation fog, turns fog from a surprise into a planned-around hazard.

The go/no-go impact is direct and unforgiving because fog attacks visibility, the parameter that most tightly bounds VFR training. Solo students, in particular, must never be dispatched into deteriorating visibility, and a field that is legally VFR at engine start can drop below minima before a student returns from the practice area. Schools that document the observed conditions against each dispatch decision protect themselves in an audit and give instructors the data to teach the difference between mist and fog, and between fog that will lift and fog that will linger.

Aviatize's Smart Planning & Booking module lets a dispatcher see the day's flying against each airport's current conditions, so a morning of radiation fog or a persistent advection event can be managed by holding and rebooking affected lessons in one place rather than fielding a string of individual cancellations. When visibility sits below a school-defined training minimum, affected bookings can be flagged for dispatcher review before anyone drives to the field.

Aviatize's Compliance & Auditing and Digital Data & Records modules keep the observed conditions attached to each flight record, so a school can demonstrate in a Part 141 or Part-ATO audit that solo and cross-country flights were dispatched only when visibility met the applicable minima.