Visibility

Question 1

When does the evaporation fog form?

Answer 1

When relatively warm water evaporates into cool air.

Question 2

When does the advection fog form?

Answer 2

When a relatively warm, moist, and stable air passes over a cool surface.

Question 3

When is the additional item of minimum visibility included in the meteorological reports?

Answer 3

When the visibility in one direction is less than 1.5 km or less than 50% of the prevailing visibility.

Question 4

What does the letter N mean in runway visual range reports?

Answer 4

It means that the runway visual range is fluctuating and is showing neither and upward or downward tendency.

Question 5

What types of fog exist?

Answer 5

» radiation fog

» advection fog

» evaporation fog

» frontal fog

» orographic fog

Question 6

When does the frontal fog form?

Answer 6

Frontal fog forms ahead of a frontal surface when relatively warm rain evaporates into relatively cold air.

Question 7

Define: hygroscopic particle.

Answer 7

A water-seeking category of condensation nuclei.

Question 8

Define: mist.

Answer 8

Visibility of more than 1 km and but less than 5 km.

Question 9

Define: haze.

Answer 9

Reduction in visibility by solid particles.

Question 10

How widespread can frontal fog be?

Answer 10

Hundreds of kilometres.

Question 11

Define the format in which runway visual range is reported.

Answer 11

RX/Y

X = Runway

Y = Visibility

Question 12

Where does the thickest fog tend to occur?

Answer 12

In industrial areas (many hygroscopic particles).

Question 13

Define: runway visual range.

Answer 13

Visibility at the surface near a runway in the direction of landing.

Question 14

What are the conditions that must be present for the evaporation fog to form?

Answer 14

» air must be much cooler than the water

» an inversion must be present

Question 15

When does the radiation fog form?

Answer 15

Overnight, when the ground loses heat by radiation and cools.

Question 16

Define the steps used in reporting visibility.

Answer 16

visibility < 800 m: 50 m

800 m < visibility < 5 km: 100 m

5 km < visibility < 10 km: 1 km

Question 17

What does the letter D mean in runway visual range reports?

Answer 17

It means that the runway visual range is fluctuating and is showing an downward tendency.

Question 18

What visibility is indicated in the meteorological reports if the prevailing visibility cannot be determined due to rapid fluctuation?

Answer 18

The lowest visibility observed.

Question 19

When does the orographic fog form?

Answer 19

When humid air is forced by high ground to rise.

Orographic fog may be freezing fog, due to low temperatures associated with high ground.

Question 20

Define: prevailing visibility.

Answer 20

The greatest visibility value which is reached within at least half the horizon circle (or aerodrome surface).

Question 21

What are the ideal conditions for the formation of radiation fog?

Answer 21

» High relative humidity

» Light winds

» Clear skies

» Long nights

Question 22

Define the steps used in reporting runway visual range.

Answer 22

RVR < 400 m: 25 m

400 m < RVR < 800 m: 50 m

800 m < RVR: 100 m

Question 23

There are several types of fog. How is fog classified?

Answer 23

According to the processes which produce it.

Question 24

What does the letter U mean in runway visual range reports?

Answer 24

It means that the runway visual range is fluctuating and is showing an upward tendency.

Question 25

When does the advection fog disperse?

Answer 25

When there is a change of air mass or strong winds lift it into low cloud.

Question 26

Visibility is normally poorer in drizzle than in rain. Why?

Answer 26

Because in drizzle there is a greater density of droplets.

Question 27

How deep can radiation fog be?

Answer 27

From several to about 1,500 feet.

The average tends to be about 500 feet.

Question 28

When is fog most likely to form?

Answer 28

When the temperature is at its lowest, which occurs just after sunrise.

Question 29

What does the letter M mean in runway visual range reports?

Answer 29

It means that the runway visual range is below the minimum value that can be determined by the system.

Question 30

How else is evaporation fog called?

Answer 30

» arctic smoke

» sea smoke

» arctic sea smoke

» water smoke

» warm water fog

» frost smoke

Question 31

Define: fog.

Answer 31

Visibility of less than 1 km.

Question 32

Define: condensation nuclei.

Answer 32

Condensation nuclei provide surfaces upon which water vapour can condense to create cloud/fog droplets.

Question 33

What is the averaging period of runway visual range measurements?

Answer 33

» 1 minute for towers

» 10 minutes for meteorological reports

Question 34

Can we always assume good approach visibility if surface visibility was reported excellent?

Answer 34

No, because surface visibility does not necessarily equal in-flight visibility.

Question 35

Can mist/fog form if relative humidity is not 100%?

Answer 35

Yes.

Question 36

At how many points along the runway is runway visual range measured?

Answer 36

Three:

» threshold

» mid-point

» end

Question 37

How else is orographic fog called?

Answer 37

» hill fog

» upslope fog

Question 38

Below which visibility is runway visual range reported?

Answer 38

2 km.

Question 39

When does the radiation fog disperse?

Answer 39

Radiation fog will disperse if conditions for its formations are reversed or cancelled:

» increased temperature

» thermal turbulence

» high wind speed

Question 40

What does the letter P mean in runway visual range reports?

Answer 40

It means that the runway visual range is above the maximum value that can be determined by the system.