Chapter 16 - Visibility

Question 1

Visibility

Answer 1

The greatest distance at which a black object situated near the ground, can be seen and recognised when observed against a bright background

OR

lights of 1000 candelas can be seen and identified against an unlit background

Question 2

Visibility is reduced by

Answer 2

Suspended water droplets or solid particles

Question 3

Fog

Answer 3

(FG)
visibility less than 1000m due to suspended water droplets

Question 4

Mist

Answer 4

BR
1000m to 5000m due to suspended water droplets

Question 5

Haze

Answer 5

(HZ)
up to 5000m due to suspended solid particles

Question 6

Radiation fog (FG)

Answer 6

Only forms over land and never sea
Requires high relative humidity + stability
Clear skies and light winds allow max radiative surface cooling overnight

WILL NOT OCCUR OVER BODIES OF WATER - due to water having a high specific heat capacity

Question 7

How is radiation fog formed

Answer 7

Low level air cooled to dew point by conduction under clear skies

Light winds 2-8kts —> cause gentle mixing of droplets and hold them in suspension

Fog up up to 1000ft but typically 500ft

If zero wind = droplets settle on surface and form dew instead

Question 8

Radiation fog dispersal

Answer 8

Surface heating causes thermal turbulence —> allow fog to lift
Increased turbulence = fog droplets lifted to for, low stratiform clouds
Increase in wind speed will also create turbulence/mixing to lift fog

Question 9

Valley fog

Answer 9

Air attached to higher ground is cooled via conduction at night

Now cold dense heavy air descends (katabatic) into valley

Question 10

Orographic / hill fog

Answer 10

Air forced to rise due to hill
As air rises it cools to dew point + becomes saturated

Dissipates by heating or change of air flow e.g downslope wind

Question 11

Frontal fog

Answer 11

Cold air meets warm air
Warm air with lower density forced above cold air
Warm rain falls through frontal surface into cold air
Relative humidity increases
air becomes fully saturated, generating stratus that can extend all the way down to the surface itself = creating fog.

Frontal fog can only be cleared via moving of frontal surface into cold

Question 12

Advection fog formation

Answer 12

Warm, moist stable air moves over colder surface which must have a temp below the DP of the mobile airmass

Surface air cools and condenses

Requires stability, high RH and wind below 15kt

Question 13

Arctic sea smoke (steam fog)

Answer 13

Cold stable air mass moves over warmer water

Evaporation occurs and air quickly becomes saturated as well as being warmed from below causing it to be less dense and to rise and cool adiabatically

Water vapour then condenses = fog droplets lifted

Question 14

Ice fog

Answer 14

Air temp -35degrees C
Sublimation of water vapour into ice crystals

Question 15

Freezing fog

Answer 15

Fog initially forms when Dew point temp above 0 degrees C but temp continues to fall

Fog droplets become supercooled due to lack of freezing nuclei

Causes rime ice on contact with sub zero surface

Question 16

Highest visibility in

Answer 16

Cirrus clouds
Reduces in alto clouds
And even less in cumulonimbus clouds

Question 17

RA

Answer 17

Moderate rain 3000m to 10km

Question 18

+RA

Answer 18

heavy rain - below 1000m

Question 19

SN

Answer 19

moderate snow - 1000m

Question 20

Drizzle

Answer 20

DZ
less than 3000m but could be less than 500m eg worse than rain

Question 21

+SN

Answer 21

Heavy snow can reduce visibility below 50m

Question 22

Blowing snow (BLSN)

Answer 22

Low drifting surface snow (DRSN)
can reduce visibility to a few metres (blizzards)

Drifting: snowflakes raised to height below 6ft/1.8m)

Blowing: snowflakes flakes raised to a height of 6ft/1.8m or more

Question 23

Haze (HZ)

Answer 23

Visibility reduced by solid particles

Biggest reduction occurs if inversion present —> solid particles remain trapped at lower levels

Question 24

Smog

Answer 24

Mixture of industrial smoke and fog

Question 25

Volcanic ash

Answer 25

Visibility affected 100s of miles downwind

Question 26

Dust (DU) and sandstorms (SS)

Answer 26

Min wind speed of 20 kts to lift dust/sand Heavy = reduced visibility to less than 200m Moderate = reduced visibility to less than 200m - 600m Sand grain size = 0.08mm-0.1mm

Question 27

Air to ground visibility or oblique or slant range visibility

Answer 27

Visibility from cockpit to ground surface

Question 28

Prevailing visibility

Answer 28

Visibility achieved or exceeded in at least 50% of horizons circle

Question 29

Runway visual range RVR

Answer 29

RVR given when met visibility drops below 1500m RVR midpoint measurement is normally taken between 1000m-1500m along the runway RVR is better than metvis Measured in meters Usually given by METARs, ATC and ATIS broadcasts RVR threshold measurement usually take 300m along runway from threshold point RVR stop end measurement usually taken 300m from end of runway

Question 30

Vertical visibility (VV)

Answer 30

Occurs when we have obscuration Measured in feet E.g VV002 = 200ft

Question 31

DR indicates

Answer 31

dust, sand or snow has been raised by the wind to a height of less than 2m

Question 32

BL indicates

Answer 32

dust, sand or snow has been raised by the wind to a height of less than 2m and that horizontal visibility has been reduced