JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
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 ⅝ of a
statute mile. Fog reduces horizontal and vertical visibility and may extend over a large area.
Fog that extends 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 that 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. It 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.
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
minute water droplets that are light enough to remain suspended in the air. If the condensed
water particles form in sufficient amount near the surface, the resulting condition is fog. For fog
to form, three conditions must be satisfied: (1) condensation nuclei must be present in the air, (2)
the air must have a high water content (a low dew point spread), (3) and light surface winds must
be present.
Recall from Chapter 2 that when the air temperature is equal or nearly equal to the dew point
temperature, there is a low dew point spread, and the air is close to saturation. Once saturation is
achieved–either through the cooling of the air or through the evaporation of water into the
atmosphere–water will condense from the vapor state into water droplets or ice crystal.
Version 3.2/Dec 08 4-21