Fog VS. Stratus

CHAPTER FIVE

AVIATION WEATHER

Surface based obscuring phenomena classified as few, scattered, or broken also present a slant

range visibility problem for pilots on approach for a landing but normally to a lesser degree than

when the celestial dome is completely hidden. Thus, partial obscurations are not considered

ceilings.

511.

FOG VS STRATUS

Fog-related low ceilings and reduced visibility are among the most common and persistent

weather hazards encountered in aviation. Since fog occurs at the surface, it is primarily a hazard

during takeoff and landing.

Fog is a visible aggregate of minute water droplets that is based at or within 50 feet of the

surface, is greater than 20 feet in depth, and reduces the prevailing visibility to less than 5/8 of a

statute mile. Fog reduces horizontal and vertical visibility and may extend over a large area.

Fog extending no more than 200 feet in height is considered shallow fog and is normally

reported as a partial obscuration. Since the fog may be patchy, it is possible visibility will vary

considerably during the approach and rollout. RVR may not be representative of actual

conditions in this situation if the measuring equipment is located in an area of good visibility.

One of the most serious problems with shallow fog stems from the abundance of cues available

at the start of the approach. You may see the approach lighting system and possibly even some

of the runway during the early stages of the approach. However, as the fog level is entered, loss

of visual cues may cause confusion or disorientation. In these conditions, you should not rely

entirely on visual cues for guidance. Bring visual cues into your instrument cross-check to

confirm position, but maintain instrument flight until visual cues can provide sufficient

references for landing.

Dense fog normally causes a total obscuration. You will not normally see visual cues during the

early portion of an approach. Strobe lights and landing lights may cause a blinding effect at

night. Transitioning to land in a total obscuration involves the integration of visual cues with the

instrument cross-check during the latter portion of the approach.

A layer of low clouds forming a ceiling is usually formed from stratus clouds. Stratus, like fog,

is composed of extremely small water droplets or ice crystals suspended in the air. The main

distinction between fog and stratus is that a stratus layer is not surface based. Stratus is above

the ground (greater than 50 feet AGL) and does not reduce the horizontal visibility at the surface.

An observer on a mountain enveloped in the layer would call it fog, while one farther down the

slope would call it stratus. In fact, the requirements for formation of fog contain many of the

same items listed in the requirements for cloud formation.

512.

FOG FORMATION

The formation of fog or cloudiness of any type is dependent on the air becoming temporarily

supersaturated (contains more moisture than the air can hold at that temperature). Once the air

reaches a supersaturated state, the excess moisture in the air condenses out of solution into

5-24

Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds