CEILING AND VISIBILITY (METEOROLOGY NOTES) | Meteorology Notes


CEILING AND VISIBILITY (METEOROLOGY NOTES)

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 measure of the degree of transparency of the atmosphere. In effect, visibility is the measure of atmospheric clarity, or obscurity. This can be caused by water droplets- cloud, fog, rain or solid particles- sand, dust or smoke, or by a mixture of both- smog, ice in form of crystals, hail or snow will also reduce visibility. Poor visibility is usually associated with stable conditions, an inversion and light winds. 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. An aviator’s interest in visibility arises because he wants to know how far off he will be able to see various things- landmarks, targets, obstructions, beacon lights, other aircraft, runways etc. while he is in flight or when he is about to start an approach for landing. Although all these questions cannot be fully answered by Met Personnel sitting on ground, however, Met reports of visibility from ground stations provide an aviator with vital information, which may prepare him mentally for a difficult landing or a diversion. 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

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 (METEOROLOGY NOTES)

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).

Measurement and Reporting of Visibility

In very generic terms, Meteorological Optical Range (MOR), or more simply, ‘ Met Vis’ is the greatest horizontal distance at which a dark object can be recognized by an observer with normal eyesight, or at which lights of specified candle power can be seen by night, as reported by an accredited observer.

During day, measurements are made by reference to suitable objects at known distances from an observing position. By night, if a suitable arrangement of lights of known candlepower is not available, the visibility value obtained has to be the daytime equivalent in terms of atmospheric obscurity. This is achieved by viewing lights of known candlepower from a known distance through a variable filter in an instrument called a Gold’s Visibility Meter.

Reporting of Visibility

Visiblity is recorded by noting visible landmarks in different directions at different distances. The lowest of these values is reported, except when

The difference between max and min values is at least 50% of the min observed visibility

The min Vis is less than or equal to 1500 m and Max Vis is greater than 5000 m.

Exception 1

The difference between max and min values is at least 50% of the min observed visibility

Example

Observed Values:  Min- 1500 m NE & Max – 2000 M SW

The difference between Min and Max observed Vis is less than 50% of the min (i.e. 750m)

Therefore the Vis reported will be 1500 m.

Observed Values:  Min 1500 m NE/ Max – 2500 m SW

The difference between Min and Max = 1000m> 750 m which is 50% of the min Vis

Therefore the reported Vis will be as 1500m NE/ 2500 m SW

Exception2

The min Vis is less than or equal to 1500 m and Max Vis is greater than 5000 m.

Example

1200 m NE & 6000 m SW

This will be reported as

1200m NE/ 6000 m SW

Steps For Reporting Visibility

Upto 800 M Rounded Down To The Nearest 50 M
800 M- 5000 M Rounded Down To The Nearest 100 M
5000 M – 9999M Rounded Down To The Nearest 1000 M
10 Km or More Reported as 9999

Reporting Of RVR (METEOROLOGY NOTES)

The Runway Visual Range is the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its centerline. This value is normally determined by the visibility sensors, located alongside and higher than the centerline of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity. It is common practice to use a transmissometer or forward scatter meter as the RVR visibility sensor. A transmissometer measures the transmittance of the atmosphere over a baseline distance while a forward scatter meter measures the extinction coefficient of the atmosphere. RVR is then derived fromequations that also account for ambient light and runway light intensity based on expected detection sensitivity of the pilot’s eye.

Meteorology Notes

Trnsmissometer

 

 

 

 

 

 

 

 

 

Transmissometer

 

RVR is reported when the normal visibility is 1500 m or less, or when shallow fog is reported in forecast. Three readings can be given, one each for touch-down zone, mid- point and stop-end.

e.g. R10L/ 500 400 600

Reporting Of RVR

If only two readings are given, the first relates to touch down point and the second is prefixed as:

500 stop end 550

600 mid point 400

Mid point and/or stop end values are suppressed when:

They have equal or higher values than Touch Down and a vlue above 400 meters

Example.  300/450/600 would be reported as R 300

If the values are 800 Meters or greater

Example.  1000/850/900 would be reported as R 1000

Rvr values of less than 50 m are reported as M0050

RVR values greater than 1500 , are reported as P1500

A trend over the preceding 10 min may be added giving an indication of increasing (U) or decreasing (D) readings: R10L/1000D

Varying RVR coding

Example: 10 min RVR for RW 28L varying between 300m and 400 m would be coded R28L/0300V0400

Steps in Reporting Of RVR

          25 m Step if RVR , 400 m

          50 M Step if 400 < RVR < 800 m

          100 m Step if RVR > 800 m

 

Oblique Visibility

Oblique visibility is the distance along the ground from a point directly beneath the aircraft and the most distant point a pilot can see.

In a deep haze layer visibility will change with a change of height as follows.

Change of height within the Layer

An increase of height inside the haze layer will reduce the oblique visibility.

Change Of Height Above The Layer

An increase in height when the aircraft is already above the haze layer will increase the visibility.

Visibility Obscuring Phenomenon (METEOROLOGY NOTES)

Haze

Atmospheric obscurity due to moisture, dust or smoke, wherein visibility is reduced to 2-5 Km. It is called Moist Haze if RH > 75%. In case RH < 75% it is called Dust Haze or Smoke Haze.

Dust Haze

Widespread dust in suspension reducing visibility to to less than 5 Km. RH is less than 75%.

This type of dust haze is common over Northwest India in summer (May-Jun). The haze extends even upto Northeast India, but visibility progressively improves as one goes further east from UP. Very often this type of haze prevails in Northern India in the late winter months. In this case the haze originates from the arid stretch from Iraq to Rajasthan. Due to the widespread nature of the dust haze diversions are difficult. Airfields in the south of the dust fetching are are the most suitable for diversions.

Dust Raising Winds

Strong surface winds of speed exceeding 20 kt and raising dust, reducing visibility to less than 5 Km. RH is less than 75%, Vertical extent, 3-5 Km. During night, wind speed reduces, as a result dust particles settle down, leading to slight improvement in visibility.

Dust Storm

Meteorology Notes

Dust storm

 

 

 

 

 

 

 

 

 

 

 

Sudden increase in wind speed associated with drop in visibility less than 1 Km for short durations of about half an hour, generally followed with light or little precipitation. It generally occurs in the season when dust particles are suspended in the air. The visibility ia a dust storm may range from less than 10 m to 1000 m. Very poor visibility lasts for short duration of about half an hour, however, the dust may remain suspended for many hours giving only partial improvement in visibility. If the storm is followed by light shower, the improvement in visibility is rapid.

Dust Devil

Meteorology Notes

Dust Devil

 

 

 

 

 

 

 

 

 

 

 

A swirling motion or whirl of dust of small dimensions seen on hot summer afternoons moving at random and lasting for short periods.

Moist Haze

Due to the condensation of water vapour, consequent to cooling of air layers near the ground, moist haze, mist or fog may occur. Moist haze usually occurs in the early morning hours and soon dissipates after the sun rises and solar radiation increasers. Mainly occurs in winters.

Smoke Haze

Smoke emitted from industries, vehicles and residential places spreads as a haze layer, especially when there is existence of inversion layer and light winds. Light winds causes diffusion and the inversion layer keeps the smoke within that layer (Upto 3000 ft). Visibility may reduce to 1500 m, but is usually between 2-4 Km. Its again mainly a winter hazard (Strong Inversion Layers, Light Winds, Clear Skies at night).

Precipitation

Meteorology Notes

Precipitation

 

 

 

 

 

 

 

 

 

 

 

The visibility in rain is dependent on the size of the raindrops and number of raindrops in a given volume.

Shower

Precipitation in the form of large water droplets from clouds of vertical development. It lasts for shorter periods than rain and falls over a limited area at a time.

Rain

Precipitation from clouds of medium size water drops.

Drizzle

Precipitation in the nature of spray from clouds consisting of minute droplets.

Light rain has little effect, moderate rain is associated with visibility 3-5 Km, while in heavy rain the visibility may temporarily reduce to less than 1 Km. In drizzle the visibility may be as low as 3 Km, but is usually 5-6 Km. Drizzle is a hazard not just because of poor visibility but also due to association of very low stratus clouds. Heavuy shower may cause a significant reduction in visibility.

Fog

Meteorology Notes

Fog

 

 

 

 

 

 

 

 

 

 

 

A fog may be described as cloud resting on ground. It may be considered as moist haze reducing visibility to less than 1 Km or as clouds forming within 50 ft from the surface. Fog is classified according to the manner in which it is formed

Radiation Fog

The formation of Radiation Fog depends on the cooling of the ground and the layers of air near the ground in night. It is essential that the air layers are cooled below dew point. During cold nights with little or no wind, radiation fog may develop. If the radiation fog is thicker than 20 ft it is called ground fog

Meteorology Notes

Radiation Fog

 

 

 

 

 

 

 

 

 

 

 

Conditions for Radiation Fog

High Relative Humidity.   This is necessary, so that little cooling is required to reach the dew point.

Clear Sky.    This is also necessary for maximum radiational cooling of the ground.

Light Wind.   This enables the cooling to extent to sufficiently deeper layers and bring the temperature at all levels in this layer down to dew point. A wind speed of 2-7 Kts is usually favourable for fog formation.

Advection Fog

Meteorology Notes

Advection Fog

 

 

 

 

 

 

 

 

 

 

Warm moist air moves over cold surface. Winds upto 15 Kts required. Winds above 15 Kts leads to the formation of low stratus. Mainly in coastal areas.

 

 

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