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CEILING AND VISIBILITY
The CEILING is the height above ground or water of the lowest layer of clouds and is reported as BKN (broken) or OVC (overcast). A minus sign preceding the designation BKN and OVC indicates that the sky cover is thin. BKN and OVC do not constitute a ceiling. If clouds are present but cannot be distinguished because of obscuring phenomena, the ceiling is reported as obscured (X). A sky condition reported as partially obscured (-X) indicates that some span of the sky or cloud layer is visible through the obscuration
VISIBILITY is one of the most important elements of weather from the standpoint of aircraft operations. It, in conjunction with ceiling, determines whether an airplane is open to traffic. VFR operations generally require a minimum of three miles visibility and a ceiling of 1000 feet.
Restrictions to visibility would include cloud, precipitation fog, haze, smoke, blowing dust or snow. Blowing snow, dust and sand can produce very poor visibility conditions. Blowing snow can be responsible for optical illusions. Precipitation, in the form of rain, snow and drizzle, appreciably reduces visibility. Drizzle which occurs in stable air is often accompanied by fog or smog.
Reduced visibility is a function of the stability of the air. If the air is stable, impurities that contribute to haze are trapped in the lower levels. Stable air is also favorable to drizzle and fog. If the air is unstable, vertical currents scatter the haze panicles but cause blowing snow and dust which also contribute to reduced visibility.
Visibility means the distance at which prominent objects may be seen and identified by day, and prominent lighted objects by night.
FLIGHT VISIBILITY is the average range of visibility forward from the cockpit of an airplane in flight.
SLANT RANGE VISIBILITY is the distance a pilot can see over the nose of the airplane towards the ground. It is sometimes called approach visibility.
GROUND VISIBILITY is the visibility at an airport as reported by an accredited observer.
PREVAILING VISIBILITY is the distance at which objects of known distance are visible over at least half the horizon. It is reported in miles and fractions of miles.
RUNWAY VISUAL RANGE (RVR) represents the distance a pilot will be able to see the lights or other delineating markers along the runway from a specified point above the centerline that corresponds to eye level at the moment of touchdown. RVR is reported in hundreds of feet. A device, called a transmissometer that is installed adjacent to the runway, samples a specified pinion of the atmosphere and converts the sample into an estimate of the runway visual range. The reading derived from a transmissometer located adjacent to the runway threshold is reported as RVR -A.; that from a transmissometer located adjacent to the runway midpoint is reported as RVR.B. RVR information is available from ATC, the control tower and the flight service station. The actual RVR reading is provided to pilots if the RVR is less than 6000 feet.
VMC AND IMC
Visual meteorological conditions (VMC) is a term used by meteorologists to indicate that visibility, distance from cloud and ceiling are equal to or better than the minimum under which flight according to the visual flight rules (VFR) may be conducted.
Instrument meteorological conditions (IMC) is a term that indicates that visibility, distance from cloud and ceiling are below minima and flight can be conducted only under instrument flight rules (IFR).
Click here to go to the Flight Environment-Weather table of contents
The basis for this section is the Flight Training Manual by Transport Canada. However, the text was modified for US users and readers by Dr. Claudius Carnegie of the ALLSTAR website. Any questions should be directed to Dr. Carnegie at email@example.com directly.
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Updated: 12 March, 2004