Visibility
Kinds of visibility
Four kinds: forward from the cockpit, at the aerodrome, around the horizon, and straight up. Then why visibility matters to the traffic.
The visibility forward from the cockpit of an aircraft in flight.
The visibility at an aerodrome, as reported by an accredited observer or by automatic systems.
The greatest horizontal distance seen over at least half of the horizon circle.
It is taken as the representative visibility at a particular location.
How far up an observer or a sensor can see into an obscuring layer, such as fog or heavy snow.
When the sky is obscured, it replaces the cloud groups in a meteorological report. Given in feet, as in MET 001.
- VFR flightsNeed it to see and avoid other aircraft and obstacles.
- IFR flightsNeed some visibility too: to taxi around the aerodrome, and to see the runway in the final stage of the flight.
- Procedures and separationLow visibility can change the procedures, the separation methods and the minima.
- Delays and diversionsPoor visibility can cause delays and diversions. They delay other aircraft and passengers, and add to the controllers’ workload.
Why visibility is reduced
ExamTwo kinds of particle in the air: water, and solid material that is not ice.
Liquid or solid water particles. They can greatly reduce visibility.
For example, mist and fog, and precipitation: rain, drizzle and snow.
Solid material that is not ice.
For example, volcanic ash. The five kinds are under Lithometeors, below.
Fog and mist
ExamBoth are water droplets in the air. The visibility tells them apart.
The smaller the difference between temperature and dew point, the nearer the air is to mist or fog: see MET 003.
Kinds of fog
ExamSeven kinds, and how each forms.
Conditions: night, little or no cloud, little wind.
The land cools down by radiating heat. The moist air near the ground loses heat to the cold ground by conduction and condenses.
The night must be long enough.
Conditions: warm, moist air over a cold surface.
The warm air loses heat and condenses. Sea fog is one example.
Conditions: moist air is forced up by rising ground (orographic lifting).
The air is cooled by expansion and becomes saturated (MET 004).
Conditions: rain ahead of a warm front or an occlusion (MET 005).
Some of the falling rain evaporates, and the colder air below the main cloud bank becomes saturated. The fog forms in that colder air.
Cold, dense air settles into the low part of a valley, condenses and forms fog.
Extremely cold air passes over a warm, moist surface.
The two mix, and the air becomes saturated.
Fog of supercooled water (MET 003).
It forms a layer of ice or rime on impact.
Lithometeors
Five kinds of solid particle, each with its report code.
- Small particles suspended in the air, produced by combustion.
FUSmoke - Small particles of earth or other matter, raised into the air by the wind.
DUDust - Particles of sand, raised into the air by the wind.
SASand - Extremely small, dry particles, invisible to the naked eye (air pollution).
HZHaze - Fine particles of rock powder from a volcano. They can stay in the air for a long time.
VAVolcanic ash
Haze is dry. That is what separates it from mist, which is water droplets.
Observing and reporting visibility
The units visibility is given in, and the two sensors that measure it.
Runway visual range
RVR (runway visual range): when it is reported, where it is measured, and how it reads in a report.
R26L/0900- R
- RVR
- 26L
- Runway 26L
- 0900
- 900 m
RVR runway 26L, 900 metres.
R01R/P2000- R
- RVR
- 01R
- Runway 01R
- P
- More than
- 2000
- 2 000 m
RVR runway 01R, more than 2 000 metres.
METAR codes
ExamThe codes for the weather in a METAR, column by column. FBL, MOD and HVY are the words of plain language and local reports.
FBLLightMinus sign in a METARMODModerateNo sign in a METARHVYHeavyPlus sign in a METARVCVicinityWithin 16 km of the aerodrome, but not on it
MIShallowLess than 2 m above the groundBCPatchesPRPartialDRLow driftingDust, sand or snow raised by the wind to less than 2 mBLBlowingRaised by the wind to 2 m or moreSHShowersTSThunderstormFZFreezing
DZDrizzleRARainSNSnowSGSnow grainsPLIce pelletsGRHailGSSmall hail or snow pellets
BRMistFGFogFUSmokeVAVolcanic ashDUDustSASandHZHaze
PODust or sand whirlsSQSquallFCFunnel cloudDSDust stormSSSandstorm
The precipitation codes are each explained in MET 006. CAVOK and its conditions are in MET 001.