15. Visibility Flashcards
Visibility
Poor visibility is a hazard, but in particular most dangerous at which 2 stages of flight
- LANDING
- TAKE-OFF
2
Visibility
Apart from the obvious difficulty seeing the ground, what is another associated risk of water drops in rain, mist or fog on the windshield
DIFFRACTION
- Makes the angle of the approach appear differently
- Makes you think you are closer to touchdown or at a higher approach angle
2
Visibility
What are the 2 ICAO terms given to visibility, which are often different readings for the same given point in space
- FLIGHT VISIBILITY
- AERODROME VISIBILITY
- A mist layer may be quite shallow and easy to see through from the air but quite dense and difficult to see through on the ground
3
Visibility
Aerodrome visibility can be reported as which 2 of the following;
[ ] Runway Visibility
[ ] Runway Visual Range
[ ] Ground Visibility
[ ] Prevailing Visibility
[ ] Measured Visibility
[ ] Runway Visibility
[X] Runway Visual Range
[ ] Ground Visibility
[X] Prevailing Visibility
[ ] Measured Visibility
3
Visibility
“The visibility value that is reached or exceeded within at least half the horizon circle or within at least half of the surface of the aerodrome. These areas could comprise contiguous or non-contiguous sectors”
This is a definition of what visibility
PREVAILING VISIBILITY
- Used to give aerodrome visibility
- Represents roughly the average visibility
5
Visibility
There are caveats in place if visibility in one or more directions at an aerodrome are significantly lower than the prevailing visibility. When visibility in any direction matches which 2 criteria will they be reported;
[ ] Where the visibility in any direction is either less than the prevailing visibility and less than 2000m
[ ] Where the visibility in any direction is either less than the prevailing visibility and less than 1500m
[ ] Where the visibility in any direction is either more than the prevailing visibility but less than 1500m
[ ] Where the visibility is less than 50% of the prevailing visibility and less than 5000m
[ ] Where the visibility is less than 50% of the prevailing visibility and less than 8000m
[ ] Where the visibility is less than 75% of the prevailing visibility and less than 5000m
[ ] Where the visibility in any direction is either less than the prevailing visibility and less than 2000m
[X] Where the visibility in any direction is either less than the prevailing visibility and less than 1500m
[ ] Where the visibility in any direction is either more than the prevailing visibility but less than 1500m
[X] Where the visibility is less than 50% of the prevailing visibility and less than 5000m
[ ] Where the visibility is less than 50% of the prevailing visibility and less than 8000m
[ ] Where the visibility is less than 75% of the prevailing visibility and less than 5000m
5
Visibility
Where visibility is fluxtuating rapidly and the prevailing visibility cannot be determined, what visibility will be reported
LOWEST VISIBILITY with NO DIRECTION
- When carrying out a non-precision approach or circle to land approach, pilots should bear in mind that visibility in certain sectors could be up to 50% less than reported
- However, the reassurance is that the mimum value woudl be no less than 1500m
5
Visibility
How should the following extract from a METAR be interpreted;
METAR EGGD 1250 1800 0800NE
Weather at time 1250, prevailing visibility is 1800m, with worst visibility being 800m to the North East
5
Visibility
The methods of measuring RVR and prevailing visibility are quite different. What is the difference in terms of what is measured between the two
- RVR measures one specific direction if LESS than 1500m
- Prevailing visibility will be an average and the minimum values
- whilst there can be no direct comparison between the 2, it can be said that RVR is liklely to be the same as or greater than the lowest reported value of prevailing visibility
6
Visibility
The methods of measuring RVR and prevailing visibility are quite different. RVR measures one specific direction if LESS than 1500m and prevailing visibility will be an average and the minimum values. Whilst no direct comparison can be made, this means that RVR is likely to be the same or MORE or LESS than the lowest reported value of prevailing visibility
MORE
6
Visibility
What are the requirements for RVR reporting;
- In steps of ____m up to 200m
- In steps of ____m up to 800m
- In steps of ____m up to 1500m
- Not reported when RVR is ____m or more
- 25m
- 50m
- 100m
- 1500m
Visibility
On most automatic RVR systems, the distance recordings are given at 3 points, TOUCHDOWN, MID-POINT, STOP END (or ROLL OUT).
Typically, mid-point and stop end values are only reported when the following conditions are met;
- The Mid-point and stop end values are less than the touchdown value and less than ____m OR;
- If their values are in themselves less than ____m
- 800m
- 400m
- If the touchdown value is 800m but the midpoint is 900 and stop end is 1100, despite the midpoint and stopend being less than 1500m, they themselves are not less than the touch down point so not reported
- If the touchdown point is 1200m and the midpoint is 900m and stop end 1000m, despite the midpoint and stopend being less than the touchdown, the touchdown point is not less than 800m so they are not reported.
7
Visibility
Irrespective of the type of obscruation i.e.mist, haze, smoke, dust or sand, is not normally reported if the visibility is more than;
[ ] 2500m
[ ] 3000m
[ ] 4000m
[ ] 5000m
[ ] 2500m
[ ] 3000m
[ ] 4000m
[X] 5000m
8
Visibility
“caused by water droplets or ice crystals, visibility less than 1000m and relative humidity is taken to be 100%”
This is the definition of what visibility condition
FOG
8
Visibility
“caused by water droplets or ice crystals, defined as visibility at least 1000m but not more than 5000m. Relative humidity is assumed to be around 95%”
This is the definition of what visibility condition
MIST
8
Visibility
“consists of supercooled water droplets as temperature as falledn below 0℃”
This is the definition of what visibility condition
FREEZING FOG
8
Visibility
“consists only of ice crystals. Very rare and usually foundonly in polar regions, occuring at temperatures below -20℃”
This is the definition of what visibility condition
ICE FOG
8
Visibility
“consists of solid particles if visibility is less than 1000m, qualified as a type of pollution”
This is the definition of what visibility condition
HAZE
- Haze consists of solid particiles if visibility in haze is less than 1000m, and is qualified by type of polution i.e. Smoke Haze
8
Visibility
- The Harmattan is what kind of sandstorm
- The Haboob is what kind of sandstorm
- GRADIENT WIND
- CONVECTIVE
8
Visibility
“exists in 2 forms. In way of example Harmattan is considered a gradient version, and Haboob is a convective version”
This is the definition of what visibility condition
SANDSTORM
- Sandstorms exist in two forms. The gradient wind sandstorm like the Harmattan and the Haboob, which is a convective sandstorm
8
Visibility
“Snow at temperatures well below zero, picked up by strong surface winds”
This is the definition of what visibility condition
BLOWING SNOW
8
Visibility
“formed when the surface cools at night and the cold surface cools the air in contact by conduction, bringing it below its dew point”
This is the definition of what sort of fog
RADIATION FOG
- radiation fog only occurs over land
- Heat loss at night is through long wave radiation, hence radiation fog
- Industrial contamination icnreases the probability of fog formation by providing nuclei for condensation
9
Visibility
What time of day is most likely for radiation fog to form
DAWN
10
Visibility
The favouring conditions for radiation fog are;
[ ] Over land
[ ] Over sea
[ ] At night
[ ] Early evening
[ ] Clear or nearly clear skies
[ ] Heavy stratiform clouds
[ ] Light winds (2kts to 8kts)
[ ] Strong winds
[ ] Low relative humidity
[ ] High relative humidity
[X] Over land
[ ] Over sea
[X] At night
[ ] Early evening
[X] Clear or nearly clear skies
[ ] Heavy stratiform clouds
[X] Light winds (2kts to 8kts)
[ ] Strong winds
[ ] Low relative humidity
[X] High relative humidity
- If the air is very still cooling by conduction will be limited to a very thin layer of air in which the water condensing out will fall as heavy dew, so there will be no fog.
- Strong winds create turbulent mixing and will lift the fog into low cloud or disperese it.
- If there is strong cloud cover, the heat will not radiate away
- If there is low humidity, there is no moisture in the air to condense into fog
10
Visibility
Radiation fog layers are typically up to 500ft or 1000ft thick
500ft
10
Visibility
Where favorable conditions are met for a typical scenario where radiation fog would form i.e. over land, at night, light winds, cloudless night, hight relative humidity, but the dew point temperature is below 0℃,
- What is likely to occur rather than radiation fog forming at surface level
- When the night gives way to daytime, if it is still air conditions, with the heating from the sun, what may occur at surface level
- FROST AT SURFACE
- HEAVY DENSE LOW LEVEL FOG
- The frost will occur over night but when the daytime gives way to sunrise and heating, where still air conditions exist this will cause low level but very dense fog
- This is a process of the frost/ice sublemating back into water vapour as the sun heats the surface.
- With no wind conditions to lift or mix the vapour, it will sit at ground level in a dense thick layer as fog
11
Visibility
- Radiation fog will clear more quickly in the SUMMER or WINTER months
- What are 2 locations synonymous with radiation fog at what times of year respectively
- SUMMER
- (a) ARABIAN GULF (BAHRAIN) WINTER (b) NW EUROPE WINTER (Polar maritime airmasses)
- The ground will heat faster, which expands the air more, which means it can hold more moisture
ARABIAN GULF
* Arabian Gulf is directly beneath the sub-tropical high pressure system with light winds and clear skies at night
* Temperatures fall over the desert areas are significant
* During the day sea breezes bring cool moist air over coastal regions
* Radiation fog forms in the coastal areas and gently drifts out to sea
POLARMARITIME
* Polar maritime air originates from cold polar areas and passes over warmer ocean surfaces
* This collects moisture and patches of air warm, which rise and since are warmer than surrounding are, are unstable
* Associated with convective clouds and good visibility
* If the polar maritime air lies over land in winter, convective activity may die away at night, which leaves cool stable moist air masses with clear skies
11
Visibility
Advection fog occurs under what conditions;
- COLD & MOIST or WARM & MOIST ar masses move over a WARM or COLD surface
- Adevection fog requires NO WIND or 15kt WIND
- The majority of advection fog is formed from warm air flowing from TROPICAL or SUB-TROPICAL regions towards the POLAR or ARCTIC front zone
- Advection fog forms over LAND or SEA or LAND & SEA
- Advection for forms at DAY or NIGHT or DAY & NIGHT
- WARM & MOIST / COLD SURFACE
- 15 KTS WIND
- SUB-TROPICAL to POLAR FRONT
- LAND & SEA
- DAY & NIGHT
- The term “advection” is the clue in that it is “advancing” i.e. is moving.
- It is therefore a given that no wind is inaccurate as it must move.
- The logic of fog is that air masses are reaching the dew point. It could be from air heating to reach the dew point, but advection fog is associated with WARM & MOIST air
- Therefore, it must be assumed it is warm air cooling to the dew point, meaning it must come from hotter climates to colder.
- Radiation fog will only form at night over land where as advection fog can form over land or sea due it being associated to cooling rather than heating (i.e does not require the sun), and therefore can happen day or night
14
Visibility
What type of fog is most commonly associated with North Western Europe as a result of air masses moving from the Azores high, in particular around the northern Bay of Biscay
ADVECTION FOG
- warm air from the Azores high arrives over the cold sea in the northern Bay of Discay
14
Visibility
Advection fog formed over sea can persist when blown over land when the land is COLD or WARM, so typically associated with which season
- COLD LAND
- WINTER
- If the air is moist and still cooling, whilst it has cooled over the sea, as the breeze blows the mass over land, if the land mass is still colder than the air mass, it will continue to cool and as such, the advection fog will persist
- Warm winds blowing over cold ground can also form advection fog
15
Visibility
- If the gap between the surface temperature and the dew point temperature is significant, high winds will do what to advection fog
- If the surface temperature and dew point temperature are the same, advection fog will persist up to wind speeds of ____kts
- LIFT IT (Becoming low Stratus Cloud)
- 30 to 40 KTS
15
Visibility
“a fog type that occurs off West Africa, where there is a welling up of very cold deep water at the southern end of the Cnaries current. In the NH winter season, warm dry wind, called the Harmattan in this region, flows out of the Sahara high pressure region and passes over the forest of the Niger basin, picking up water and becoming warm and moist. On passing offshore over the Canaries Current, the fog forms and sea breezes during the day bring it back onshoar affecting coastal airfields”
This is a well known case of what type of fog
ADVECTION FOG
Visibility
“Steam fog formed when cold air passes over a warm surface i.e. cold poar air flows off of the ice cap or from a glacier over relatively warm open water”
This is a definition of what sort of fog
ARCTIC SMOKE
aka SEA SMOKE
aka STEAMING FOG
- If warm moist air were being warmed from below, normally it would be expected that this would cause instability and the warmed moist air to rise
- In order to prevent this, there must be a marked low level temperature inversion that keeps it close to the surface
- i.e. the warm moist air flows off of the ice cap onto a cold sea. The air above it warmer than it so acts like a lid keeping it down
- It cools against the colder sea, creating the steaming fog and cannot rise away due to the inversion
- Generally occurs from surface to 500 ft
16
Visibility
Greenland and Norwegian Fjords are synonomous with this type of fog
ARCTIC SMOKE
aka SEA SMOKE
aka STEAMING FOG
Visibility
Arctic Fog (aka Sea Smoke aka Steaming Fog) is generally what thickness layer i.e. from what height to what height
FROM SURFACE to 500 FT
16
Visibility
“fog formed as a result of cold saturated air ahead of warm air mixing together associated with the passage of a given point”
This is the definition of what sort of fog
FRONTAL FOG
Visibility
“fog caused by a wind moving warm moist air up a hill, cooling it adiabatically”
This is the definition of what sort of fog
OROGRAPHIC FOG
(aka upslope fog)
Visibility
- The Harmattan haze (sandstorm) is associated with what sort of winds
- The Haboob sandstorm is associated with what sort of winds
- Sandstorms associated with gradient winds are typically NARROW or WIDESPREAD and no thicker than the boundary layer
- Sandstorms associated with gust fronts are typically NARROW or WIDESPREAD and can extend up to 6000ft
- GRADIENT WINDS
- GUST FRONTS (Thunderstorms)
- WIDESPREAD
- NARROW
21
Visibility
For snow to lift in strong winds (Blowing Snow);
- The snow must have fallen and remained at temperatures well below zero, typically between ____℃ to ____℃
- Moderate winds lift the snow to low levels, typically between ____ to ____
- The lifting of snow to low levels brings what hazard
- Strong winds will lift snow to high levels (boundary layer) bring what hazard
- -5℃ to -20℃
- FEW CM to FEW M
- LOSS OF GROUND DETAIL / INABILITY TO JUDGE HEIGHT
- WHITE OUT
22
Visibility
- Visibility would be considered better in RAIN or DRIZZLE
- DRIZZLE or RAIN indicates very little lifting action in STABLE or UNSTABLE air
- DRIZZLE or RAIN indicates greater lifting action in STABLE or UNSTABLE air
- STABLE or UNSTABLE air clear pollutants, making vertical visibility better than the opposed
- RAIN
- DRIZZLE / STABLE
- RAIN / UNSTABLE
- UNSTABLE
- Rain is associated with unstable lifting air where more condensing is taking place and droplets becoming bigger, falling
- Drizzle is associated with more stable air. The dew point is reached but the water droplets are not really lifting away, making visibility harder
- Unstbale air lifts pollutants away with it, making visibility clearer
22
Visibility
“The maximum distance in the direction of takeoff or landing at which the runway, or specified lights delineating the runway, can be seen from a position on the centreline corresponding to the average eye level of a pilot at touchdown”
This is the defintiion of what visibility
RUNWAY VISUAL RANGE
(RVR)
6
Visibility
The typical measurement of snow is what per unit time
MM PER HOUR
23
Visibility
“typically very heavy and wet snow that has formed as a result of passing through a super cooled cloud”
This is the definition of what sort of snow
RIME SNOW
23
Visibility
- If the temperature aloft goes just above freezing WET (RIMED) or DRY snow will fall
- If the temperature aloft is below freezing WET (RIMED) or DRY snow will fall
- WET
- DRY
- When temeperature aloft is just above freezing, the snow will partially melt, increasing its water content therefore creating wet snow
- If the temperature aloft is below freezing, the water content will be low and dry snow will fall
23
Visibility
“The maximum distance in the direction of takeoff or landing at which the runway, or specified lights delineating the runway, can be seen from a position on the centreline corresponding to the average eye level of a pilot at touchdown”
This is the defintiion of what visibility
RUNWAY VISUAL RANGE
(RVR)
6
Visibility
Visibility is anticipated to be better in WET (RIMED) or DRY snow for the same given equivilant snow fall
WET (RIMED)
- Visibility increases as a factor of terminal velocity i.e. the wet snow will fall faster
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