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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


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