Visibility Flashcards
Definition of visibility
A black object observed against a bright background
Or
A light of 1000 candelas can be seen against an unlit background
Fog Visibility
Less than 1000 metres (water droplets/solid particles)
Mist
1000m to 5000m due to water droplets/solid particles
Haze
Up to 5000 metres due to solid particles
Radiation fog
Forms overnight over land (HP areas) (non tropical regions)
Terrestrial radiation cools surface
High relative humidity meaning cools below dew point
Requires light winds 2/8 knows for mixing
Common in clear skies/long nights
Autumn/temperature regions/col over land
Valley Fog
Surface air cooled by radiation
Cold/dense air descends into valley (katabatic effect)
Air saturated
Warm Front - Frontal Fog System
Warm air mass rises over colder air mass
Extensive rain into cold front
Moisture is added into colder air (relative humidity increases)
Fog forms just as base of warm front (saturation point)
Rain lowers cloud base
Sea Fog/Sea Mist
Winter - Warmer air from sea moves over costal areas that is colder to form coastal fog
Summer - Warmer air on land advected over cooler sea cools below dew point and becomes saturated
Artic sea smoke (Steam Fog)
Found at very high polar latitudes
Very cold air from land advected over non frozen ocean that is warmer
Cold stable air mass over warmer ocean causes evaporation and vapour condenses to produce wispy fog
Temp difference between air and water has to be more than 10 degrees
Ice Fog
Air temp below -35 ice crystals suspended in air
Freezing fog
Small SCWD cause rime ice on contact with sub zero surface
Visibility in high/mid/low level clouds
High - 1000m+ (ice crystal small cloud droplets)
Mid - 20 - 1000m
Low level - 10-30m (denser/bigger cloud droplets)
Visibility in precipitation
RA - Moderate rain - 3-10km
+RA - Heavy - below 1000m
SN - Mod Snow - 1000m
DZ - Drizzle - 500m
+SN - Heavy Snow - 50m
Requirements for Dust (DU/DS) and Sandstorms (SS) + visibility
Minimum wind speed 15-20kt to lift dust and sand
Heavy dust storm can reduce visibility to less than 200m
Mod dust storm - 200-600m
Air to Ground Visibility Illusion
Horizontal ground visibility appears greater at higher altitude
Surface visibility less
Reporting Visibility ranges METAR
50m steps up to 800m
100m steps from 800m to 5000m
1km steps from 5000m to 10km
Reporting Visibility in TAF
50m up to 800m
100m from 800m to 5000m
1000m from 1000m to 10km
RVR minimum and requirements
Reported when visibility below 1500m
Measured at threshold/midpoint/stop point
Mid point/stop point given if threshold below 400m
Steps:
25m up to 400m
50m from 400m to 800m
100m from 800m
Better than MET vis
RVR reporting times for ATC and METARS
METARS report averages over 10 minutes
ATC will report current RVR to within 1 minute
How is RVR Measured
By a Transmissometer ranging from 50m to 1500m
RVR codes
U = Increasing
D = Decreasing
N = No Change
Vertical Visibility (VV) Reports
Reported in METAR when cloud base not visible due to fog etc
Report in 100ft AGL up to 2000ft
30m steps up to 600m
RVR Mid Point Measurement taken between….
1000m and 1500m
RVR Is usually ….. the reported ground or met visibility
Greater than
RVR stop end measurement is normally taken about ….. from end of runway
300m
Prevailing Visibility
The greatest visibility which is reached within at least half the horizon circle
RVR values are usually given by
ATC
ATIS
METAR
RVR threshold measurement is normally taken about
300m
Oblique Visibility
Point directly below ac to the further viewable object on the surface
RVR is measured in
Metres or feet
Primary visibility is reported in
Metres or status miles depending on region
What height does steam fog typically extend to
500ft
Visible imagery - what is it/advantages/disadvantages
Produced by sun rays reflecting off clouds
+ easy to interpret
- not availed continuously due to lack of sunlight at night
Infrared cloud detection - what is it/advantages and disadvantages
Temperature of cloud will determine the wavelength of radiation emitted
+ availed for 24 hours a day
- if clouds near surface are approx same temp as land surface can be difficult to distinguish
-
Vertical Visibility reported in up to what distance
Reported in 30m steps up to 600m
Blowing Snow can reduce visibility to a minimum value of
1 meter
Best way to dissipate Orographic fog?
A downslope wind
How long does radiation fog last and what height does it extend to
Around 30 minutes after sunrise
Maximum thickness of 500ft
Sand Grain dimensions and wind speed
0.8mm and W/V of at least 20kt
Fog will dissipate with
Low pressure
Unstable conditions
High winds
Oreographic fog with:
- downslope
- increase temperature
- increase altitude
