Chapter 16 - Visibility

Question 1

What is Meteorological visibility and what is it measured in?

Answer 1

Greatest known distance which an object can be seen by an observer under conditions of normal day observation.
m up to 5000m then km up to 10km.

Question 2

What is Runway Visual Range and when is it reported?

Answer 2

Maximum distance that can be seen in the direction of the runway. 
Reported only when less than 1500m.
Intervals of 25m up to 400m
Intervals of 50m up to 800m
intervals of 100m up to 1500m

Question 3

Where is RVR measured?

Answer 3

At the touch down zone, the mid point and the stop end.

Question 4

What letters are used in RVR reporting and what do they mean?

Answer 4

M - RVR less than 50m
P - RVR more than 1500m (equipment cant measure)
U - Increasing
D - Decreasing
N - no change

Question 5

What defines mist and what is it’s code?

Answer 5

liquid water droplets or ice crystals
Visibility more than 1000m
RH more than 95%
BR

Question 6

What defines Fog and what is its code?

Answer 6

RH close to or at 100%
Visibility less than 1000m
FG
Below 0degC is freezing fog (FZFG)

Question 7

What defines Haze and what is its code?

Answer 7

Solid particulates
Qualified by type of particulates
HZ

Question 8

What are snowstorms and sandstorms and what kind of visibility can be expected?

Answer 8

Gradient surface winds that pick up sand or snow respectively.
Visibility:
Snow - 50m
Sand - 10m

Question 9

What is a transmissometer?

Answer 9

RVR measuring equipment with at least 2 units. A higher signal means higher visibility.

Question 10

What is a forward scatter meter?

Answer 10

RVR measuring equipment with 1 unit. A higher signal means lower visibility.

Question 11

What are the characteristics of orographic/hill fog?

Answer 11

Essentially low cloud on high ground
visibility less than 200m
Needs stable air

Question 12

What are the characteristics of frontal fog?

Answer 12

Ahead of a warm front
Precipitation saturates cold air beneath front
Fog passes with front

Question 13

What are the characteristics of steam fog/arctic sea smoke?

Answer 13

Cold stable air moving over warmer water/sea
slow to form
clears with increase in wind speed and dryer air.

Question 14

What are the characteristics of radiation fog?

Answer 14

Only over land
Common in winter
Dissipates with surface heating in the day
Common in cols in all seasons
Up to 1000ft think
Wind will initially lift radiation fog to form low stratus cloud before it dissipates

Question 15

How does wind affect radiation fog?

Answer 15

Calm winds won’t produce fog, instead dew will form on surface.
3-8kts will form radiation fog.
More than 10kt wind will lift fog into low stratus cloud.

Question 16

What are the characteristics of advection fog?

Answer 16

Most common fog
Moist warm air moving over cold surface
Common in Newfoundland, North sea and Kamchalka peninsula
Required wind of up to 15kts, high RH and surface temperatures below dew point
Disperses when wind increases and surface heating occurs.

Question 17

What is vertical visibility and how is it determined and what is it measured in?

Answer 17

Distance observer can see vertically to infinite ceiling.
Determined by sensor algorithm at automated stations.
Measured in 100s of ft.

Question 18

When is RVR smaller than meteorological visibility?

Answer 18

Never RVR is either the same as or further than meteorological visibility.