Visibility

Question 1

Definition of visibility

Answer 1

A black object observed against a bright background

Or

A light of 1000 candelas can be seen against an unlit background

Question 2

Fog Visibility

Answer 2

Less than 1000 metres (water droplets/solid particles)

Question 3

Mist

Answer 3

1000m to 5000m due to water droplets/solid particles

Question 4

Haze

Answer 4

Up to 5000 metres due to solid particles

Question 5

Radiation fog

Answer 5

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

Question 6

Valley Fog

Answer 6

Surface air cooled by radiation
Cold/dense air descends into valley (katabatic effect)
Air saturated

Question 7

Warm Front - Frontal Fog System

Answer 7

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

Question 8

Sea Fog/Sea Mist

Answer 8

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

Question 9

Artic sea smoke (Steam Fog)

Answer 9

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

Question 10

Ice Fog

Answer 10

Air temp below -35 ice crystals suspended in air

Question 11

Freezing fog

Answer 11

Small SCWD cause rime ice on contact with sub zero surface

Question 12

Visibility in high/mid/low level clouds

Answer 12

High - 1000m+ (ice crystal small cloud droplets)
Mid - 20 - 1000m
Low level - 10-30m (denser/bigger cloud droplets)

Question 13

Visibility in precipitation

Answer 13

RA - Moderate rain - 3-10km
+RA - Heavy - below 1000m
SN - Mod Snow - 1000m
DZ - Drizzle - 500m
+SN - Heavy Snow - 50m

Question 14

Requirements for Dust (DU/DS) and Sandstorms (SS) + visibility

Answer 14

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

Question 15

Air to Ground Visibility Illusion

Answer 15

Horizontal ground visibility appears greater at higher altitude
Surface visibility less

Question 16

Reporting Visibility ranges METAR

Answer 16

50m steps up to 800m
100m steps from 800m to 5000m
1km steps from 5000m to 10km

Question 17

Reporting Visibility in TAF

Answer 17

50m up to 800m
100m from 800m to 5000m
1000m from 1000m to 10km

Question 18

RVR minimum and requirements

Answer 18

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

Question 19

RVR reporting times for ATC and METARS

Answer 19

METARS report averages over 10 minutes
ATC will report current RVR to within 1 minute

Question 20

How is RVR Measured

Answer 20

By a Transmissometer ranging from 50m to 1500m

Question 21

RVR codes

Answer 21

U = Increasing
D = Decreasing
N = No Change

Question 22

Vertical Visibility (VV) Reports

Answer 22

Reported in METAR when cloud base not visible due to fog etc

Report in 100ft AGL up to 2000ft

30m steps up to 600m

Question 23

RVR Mid Point Measurement taken between….

Answer 23

1000m and 1500m

Question 24

RVR Is usually ….. the reported ground or met visibility

Answer 24

Greater than

Question 25

RVR stop end measurement is normally taken about ….. from end of runway

Answer 25

300m

Question 26

Prevailing Visibility

Answer 26

The greatest visibility which is reached within at least half the horizon circle

Question 27

RVR values are usually given by

Answer 27

ATC
ATIS
METAR

Question 28

RVR threshold measurement is normally taken about

Answer 28

300m

Question 29

Oblique Visibility

Answer 29

Point directly below ac to the further viewable object on the surface

Question 30

RVR is measured in

Answer 30

Metres or feet

Question 31

Primary visibility is reported in

Answer 31

Metres or status miles depending on region

Question 32

What height does steam fog typically extend to

Answer 32

500ft

Question 33

Visible imagery - what is it/advantages/disadvantages

Answer 33

Produced by sun rays reflecting off clouds
+ easy to interpret
- not availed continuously due to lack of sunlight at night

Question 34

Infrared cloud detection - what is it/advantages and disadvantages

Answer 34

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
-

Question 35

Vertical Visibility reported in up to what distance

Answer 35

Reported in 30m steps up to 600m

Question 36

Blowing Snow can reduce visibility to a minimum value of

Answer 36

1 meter

Question 37

Best way to dissipate Orographic fog?

Answer 37

A downslope wind

Question 38

How long does radiation fog last and what height does it extend to

Answer 38

Around 30 minutes after sunrise
Maximum thickness of 500ft

Question 39

Sand Grain dimensions and wind speed

Answer 39

0.8mm and W/V of at least 20kt

Question 40

Fog will dissipate with

Answer 40

Low pressure
Unstable conditions
High winds

Oreographic fog with:
- downslope
- increase temperature
- increase altitude