Flying in Fog

Foggy Facts

“Caution, Sea Smoke approaching Ormond Beach.” 

Lifting my view limiting device while on a practice instrument approach at Dayton Beach (KDAB), a solid wall of white clouds rolled over the ocean toward my home airport at KOMN. I immediately canceled my practice instrument approach with KDAB approach requesting direct to KOMN. 

Fog is the most frequent cause of reduced surface visibility and is a persistent weather hazard. The speed with which fog can form makes it especially hazardous. In a matter of minutes, visibility can drop from Visual Flight Rules (VFR) to less than a mile. Fog can also reduce visibilities below Instrument Flight Rules (IFR) minimum approach requirements. Fog is composed of either water vapor or ice crystals. A small temperature-dew point spread must exist for fog to form. Dew point is the temperature at which air reaches saturation while relative humidity is an accurate measurement of moisture content in the air.

How is fog formed?

Fog forms when water vapor condenses in the air and the moisture becomes visible. Fog differs from a cloud in one significant way. A cloud forms when warm air rises and cools to the dew point. Fog forms in this same manner, only the warm air remains at the surface.

In the development of both clouds and fog, the water vapor condenses on condensation nuclei such as dust, ice, and salt. There are two ways fog can form:

  1. The air temperature cools to the dew point by contact with a cold surface or adiabatically.
  2. Moisture is added to the air until it contains the maximum amount of water vapor it can hold at a given temperature. 

What are the different types of fog?

There are many different types of fog and can be organized by how the air was cooled or moisture was added.  

Radiation Fog

Radiation fog is generally a shallow fog. Clear skies, little or no wind, and small temperature-dew point spread present favorable conditions for radiation fog. The ground radiates heat cooling the air nearest the ground to the dew point.

Ground Fog

Low, often dense fog is known as ground fog. The sky and clouds remain visible, and it does not extend to the base of any overhead clouds. Ground fog is a form of radiation fog. It describes the condition of visibility.

Advection Fog

Advection fog develops with a light wind moving moist air over a colder ground or water. It is most common along coastal areas but can develop in inland, as well.  This type of fog can form rapidly regardless of the time of day.

Upslope Fog

Upslope fog forms as a result of moist, stable air being cooled adiabatically as it moves up a sloping terrain. Once the upslope wind ceases, the fog dissipates. Unlike radiation fog, it can form under cloudy skies. Upslope fog can be dense and extend to high altitudes.

Frontal Fog

Relatively warm rain or drizzle falling through cool air causes precipitation-induced fog, or frontal fog. Evaporation from the precipitation saturates the cool air and forms fog. Precipitation-induced fog can become dense and continue for an extended period of time in no wind or light wind conditions. Precipitation-induced fog is often associated with warm fronts but can occur with slow moving cold fronts and with stationary fronts.

Evaporation Fog

Sea smoke, steam fog, or evaporation fog goes by various local names. Relatively cold, dry air moving over warmer water or moist land creates evaporation fog. The presence of sea spray and microscopic airborne salt crystals are contributing factors in the formation. This type of fog is very dense, appearing fluffy with a movement of rolling.

Valley Fog

Valley fog forms in mountain valleys during the winter. This is a radiation fog and confined by local topography. It can last for several days in calm conditions.

Ice Fog

Ice fog occurs in cold weather in very low temperatures. Water vapor sublimates turning directly from a vapor to frozen as ice crystals. Conditions favorable for formation are the same as for radiation fog except requiring cold temperatures. During blue sky conditions refraction of sunlight by the airborne crystals can create halos, often called diamond dust.

Freezing Fog

When the temperature at the surface is at or below freezing and the atmosphere is above freezing, freezing fog, also called hoar frost, forms. Water vapor in the above freezing atmosphere comes in contact with a solid surface that is below freezing, deposition occurs and ice forms on the solid surface. Water vapor deposited this way can create icy roadways, taxiways, and runways.

Hail Fog

Hail fog sometimes occurs in the vicinity of significant hail accumulations. As hail accumulates on the ground, it cools the air just above it. This type of fog is usually quite patchy and shallow. 

Landing in Fog

Modern auto-landing computers can put an aircraft down without the aid of a pilot. However, for safe operations on taxi or take-off, air traffic controllers must see the location of the aircraft on the ground. Thick fog conditions at many airports will prevent any taxi, take-off, or landing until conditions improve. 

My encounter with sea smoke fog ended uneventfully. Safety considerations are always first on my mind. If the fog arrived quicker than anticipated, I had two airport alternates west of our destination to which I could have diverted. On final approach, the fog rolled quickly toward the shoreline and approximately 10 miles east of the airport. I landed with 10 statute miles (SM) visibility. Within minutes, the thick blanket of fog rolled over the Grumman covering the entire airport. The tower reported zero visibility and declared a complete ground stop. There was no surface movement until the visibility improved. I shut the engine down on the taxiway and awaited ATC clearance. Twenty-five minutes later, visibilities improved enough for taxi, and I tucked the Grumman away in the hangar.

Foggy Facts

  • Fog forms when the dew point and air temperature is less than 2.5 °C.
  • When fog forms at a higher altitude it develops into a stratus cloud.
  • The National Weather Service issues an alert called a Dense Fog Advisory when widespread fog is developing or has developed.
  • Meteorologists consider Newfoundland, Canada one of the foggiest places on Earth, averaging 200+ foggy days every year.
  • During World War II, military aircraft developed what was called Fog Investigation and Dispersal Operation (FIDO). Returning fighter and bomber pilots burned enormous amounts of fuel flying alongside runways to evaporate fog providing visual cues to safely land their aircraft.

By Captain Judy Rice, Epic Ground School Instructor

Captain Judy Rice
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