Ch 16 - Visibility

Question 1

Visibility is Reduced by:

Answer 1

Water Vapour
Pollen 
Pollution
Haze
Fog
Etc

Question 2

Fog and Mist Different because:

Answer 2

Fog vis. 0<1000m 100%RH

Mist vis. 1000-5000m 95%RH and above

Question 3

Radiation Fog

Answer 3

Occurs early morning
Overnight the ground cools, conduction cools the air in contact with the surface, dew point is eventually reached until it is 100% saturated, then cooled further until it condensates

Need; calm winds(2-8kts), clear night, mostly land, high relative humidity

If wind speed increases though out the day, stratus clouds may form low level, if there is a lot of convective activity, may form stratocumulus

Question 4

Valley Fog

Answer 4

Surface temp cools, cools air in contact with it, cool air sinks into the valley = katabatic wind, air moving down, sits in the bottom of the valley, keeps cooling and if saturated, becomes fog

Question 5

Advection Fog

Answer 5

Advection is the horizontal movement of air

Needs a stronger wind speed of 15kts

Warm moist air mass over a cool surface, lower layer cools, condenses = Fog

Can be day or night, over land or sea

Usually in winter and can be lifted to form ST/SC

Question 6

Evaporation Fog (artic smoke, steam fog, sea smoke)

Answer 6

Occurs over the seas/oceans - relatively warm water with a cool air mass over the top, surface water evaporates, instantly cools past dew point, condenses = fog

Capped by an inversion - which must be present

Most common; in winter with light winds

Question 7

Frontal Fog

Answer 7

Dz/RA falls from tropical maritime air into cold polar air , tries to evaporates, latent heat absorbed (cools atmosphere), decreases past the dew point, condenses = Fog

Can extend 200nm in front of approaching warm front

Question 8

Orographic Fog (Hill Fog)

Answer 8

Air rises, cools, reaches dew point, condenses and the air on the surface turns to fog

Question 9

Freezing Fog

Answer 9

Has to be below 0C and uses SCWD

Question 10

Ice Fog

Answer 10

Fog made up of ice crystals

Temp has to be roughly ~-30C and mainly occurs in polar regions

Question 11

Precipitation Limits

Answer 11

Mist (BR) 1000m. <5000m. >95%
Haze (HZ). 1000m. <5000m. <95%
Fog (FG). 0m. <1000m. 100%
Heavy Snow (+SN). <50m. 200m
Moderate Snow (SN). 200m. 1000m
Light Snow (-SN). >1km
Heavy Rain (+RA). <1000m. 3000m
Moderate Rain (RA). 3000m. 5000m
Drizzle (DZ) 500m. 3000m
Blowing Snow. <1m

Question 12

Pollution / Haze on Visibility

Answer 12

Haze - created in high pressure systems, inversions, trapped pollutants/solid particles (dust,smoke,sand,salt) reduces vis to 1000-5000m

Smog - smoke and fog mixed together in an inversion

Question 13

Volcanic Ash Clouds

Answer 13

Can be 100’s nm down wind of the volcano and can cause engines to flame out

Can also lead to 0 visibility

Question 14

Sand and Dust Storms

Answer 14

DS and SS

Up to 3000ft AGL

Develops in 2 ways; updrafts caused by surface heating or by moderate wind (15-20kts)

Inside can be lighting due to the static charge

Question 15

Blowing Snow - White Out

Answer 15

Can reduce visibility to 0m

Caused by; wind lifting upon the snow to a point where you cant see a thing

Question 16

Slant Visual Range

Answer 16

Is the visual range from the flight deck

On an approach with fog or mist; your visibility can be distorted and reduced significantly from the flight deck

Question 17

Meteorological Visibility

Answer 17

The greatest distance you can see a black object against a bright background or an bright object (1000 candelas) against an unlit background

Question 18

Prevailing Visibility

Answer 18

The prevailing visibility is the visibility that is taking up half or more of the view/horizon

The greatest visibility value reached within half of a horizon circle

If <800m it is updated ever 50m change
If 800m < 5km it is updated for every 100m change
If 5km<10km it is updated every 1km

Question 19

Ground Visibility

Answer 19

RVR - The Runway Visual Range

There is an instrument (transmissiometer - Fwd Scatter Meter) to measure how far down the runway you can see. (Humans can do it based in the distance or amount of lights you can see delinearating from the flight deck.

RVR tends to be the same, if not more than the prevailing visibility

Question 20

Transmissiometer

Answer 20

Measures the RVR between 50m and 1500m

Only see the RVR on METARs or TAFs when the vis is between 50 and 1500m

At 2.5m above the runway
No more than 120m away from the runway

3 Different positions; Touch down RVR(300m from threshold), Mid-Point RVR(1000/1500m) and Stop End RVR (300m from other threshold)

Question 21

RVR Codes

Answer 21

After RVR:
U = Increasing
D = Decreasing
N = No change

Before RVR:
P = More than
M = Less than

V = significant variations

VC = In the vicinity (8km)

Question 22

Vertical Visibility

Answer 22

The distance one can see upwards into a surface based obstruction or mac height from which a pilot inflight can recognise the ground through the surface based obstruction.

If they cannot tell you:

VV///

Question 23

Category Approaches

Answer 23

Depend on RVR

Cat 1 approach minima table ILS

Full facilities and lights, the RVR must be higher than the specified amount