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1.Aviation industry is full of accidents involving the loss of thousands of lives and economic loss worth billions of dollars. Critical analysis of the aviation accident revealed that nearly 90 percent of the accidents are due to the human factor. Explanation of the human factor is a long discussion and if analyzed is due to the error of judgment. A good judgment is only possible with adequate knowledge and it is only the correct understanding of these phenomenons which makes pilots to sail through the hazards. One of the factors in flying is the environmental factor and within this the most common factor is visibility. Aim

2.To teach the visibility and its limiting factors.
3.The lecture will be covered in the following sequence:-
b.Factors Affecting Visibility.
c.Effect of Moon or Sun.
d.Oblique Visibility.
e.Descriptive Terms.
f.Measurement by Day and Night.
g.Causes of Poor Visibility.
h.Local Trends of Visibility.
i.Comprehension Check.

4.Visibility.Greatest horizontal distance at which objects can be seen and recognized by an observer with normal sight and under conditions of ordinary day light visibility. 5.The greatest distance at which lights of 1,000 candelas can be seen and identified against an unlit background. (Wikipedia) 6. The ability, as determined by atmospheric conditions and expressed in units of distance, to see and identify prominent unlighted object by day and prominent lighted object by night. (Federal Aviation Regulations) a.Flight Visibility. The visibility forward from the cockpit of an aircraft in flight. (Figure 1) b.Ground Visibility. The visibility at an aerodrome as reported by an accredited observer. c.Runway Visual Range. The range over which the Pilot of an aircraft on the centerline of a runway can see the runway surface marking of the lights delineating the runway or identifying its centerline. (Figure 2) 7.Fog.Fog is a cloud that is in contact with the ground. A cloud may be considered partly fog; for example, the part of a cloud that is suspended in the air above the ground is not considered fog, whereas the part of the cloud that comes in contact with higher ground is considered fog. (Figure 3) There are different types of fog, but all of them are formed by the cooling of moist air below its dew point. These are classified according to the way in which this cooling is brought about. (Wikipedia) 8.Mist.Mist is a phenomenon of small water droplets suspended in air. The only difference between mist and fog is visibility and amount of water droplets. Seen from a distance, mist is bluish or bluish grey in colour (Figure 4). Mist formation is often a phenomenon of humid air. (Wikipedia) 9.Haze.Haze is an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. Haze is usually brownish or bluish in colour (Figure 5) and is often a phenomenon of dry air. (Wikipedia) 10.Dust In Suspension.Dust suspended in air over a large area for a longer period reducing visibility below 4 kilometer. Dust in suspension is usually brownish in colour(Figure 5). (PMD) 11.Difference Between Fog, Mist and Haze. The international definition of fog is a visibility of less than 1 kilometre (3,300 ft); mist is a visibility of between 1 kilometre (0.62 mi) and 2 kilometres (1.2 mi) and haze from 2 kilometres (1.2 mi) to 5 kilometres (3.1 mi). Fog and mist are generally assumed to be composed principally of water droplets, haze and smoke can be of smaller particle size. . (Wikipedia) 12.Pakistan Meteorology Department has following parameters to differentiate between Fog, Mist and Haze:-

VisibilityRelative Humidity
FogLess than 1 km95% and above
MistMore than 1 Km butless than 95% but
Less than 4 Kmmore than 85%
HazeMore than 4 KmLess than 85%

Factors Affecting...
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