Visibility Flashcards
Visibility is a measure of?
How transparent the atmosphere is to the human eye.
What units is horizontal visibility measured in?
- Up to 5000m in “Metres”
- > 5000m in “Kilometres”
- > 10km as “Greater than 10 km”
What units is vertical visibility measured in?
Feet
Vertical visibility is only given from ___________ when the sky is obscured
a staffed observation.
What is the prevailing visibility?
The greatest visibility over more than half the horizon.
When should the minimum visibility be provided alongside the prevailing visibility?
Where the min visibility is not prevailing AND the min vis <5000m
What two terms are used to denote visibility on a runway?
- Runway Visual Range (RVR)
- Runway Visibility (RV)
In Australia, which term is used exclusively to denote runway visibility measured by an instrumented system?
Runway Visual Range (RVR)
When MAY RV/RVR be reported?
When visibility is between 800m and 1500m
When MUST RV/RVR be reported?
When visibility is below 800m
Causes of reduced visibility fall into which groups?
Hydrometeors - associated with water
Lithometeors - associated with non-aqueous particles
Is the sun’s capacity to affect the ability to discern objects considered when measuring visibility?
No
List examples of hydrometeors
- Fog
- Mist
- Spray
- Haze (water)
List examples of lithometeors
- Sand
- Dust
- Pollen and Bacteria
- Smoke Haze
Flight beneath an inversion layer can be _________ with ___________
- turbulent
- poor visibilty
Above inversion the air is?
Clear and smooth
Define Fog
A concentration of water droplets or ice crystals reducing visibility to <1000m
The humidity is close to 100%
Define Mist
Mist is a concentration of water droplets or ice crystals reducing visibility to >1000m.
Humidity is >90%
What is fog caused by?
Cooling of air below its dew point temperature.
What factors affect how soon fog will form?
- Temp/Dew-point difference
- Rate of cooling
- Wind strength
What can cause cooling?
- an underlying cold ground or water surface
- the interaction of two air masses
- the adiabatic cooling of a moist air mass moving up a slope
- very cold air overlying a warm water surface
What does radiation fog require to form?
- Clear skies – to allow radiation cooling
- Light winds – to allow mixing of cold air
- High humidity – moisture content
- Condensation nuclei
Generally, explain what weather phenomena you would expect to see in no wind, light wind & stronger winds.
- No wind – Dew or frost
- Light wind – Fog/mist
- Stronger wind <7kts – Low stratus cloud
- Winds >10kts - Nil
What causes the dispersal of radiation fog?
- Earth’s surface heated by the sun
- Air warms and can absorb moisture
- Thick fog or cloud above can impede this process
- Wind increasing mixes and disperses.
What is required for Advection Fog to occur?
- Warm air flows over a cool surface
- Air is cooled below its dew-point
- Can occur suddenly
- May be more persistent than radiation fog.
What are the elements of sea fog?
- Advection fog that forms over the sea
- Tropical air moving poleward
- Moist air from land moving offshore over cooler sea.
Explain upslope fog
- Air moving up a slope cools adiabatically
- If cooled below dew-point fog will form
- If wind abates, fog will dissipate.
Explain frontal fog
- Cloud that extends to surface during passage of a front
- Air that becomes saturated by precipitation
- Generally ahead of a warm front but can form with slow-moving cold fronts.
Explain steam fog
- Caused by cool air over warm wet surface
- Over polar oceans it’s sometimes called “Sea smoke”
- Low-level turbulence and severe icing may be present.
Multiple RVR observations are always representative of what?
The touchdown zone, midpoint zone, and the roll-out/stop end zone respectively
What kinds of aerodromes are likely to have a transmissometer?
Aerodromes with precision approach systems
