Visibility And Fog

Question 1

Define RVR and characteristics

Answer 1

The distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface marking delineating the runway identifying the centreline.

Max distance a pilot 15ft above runway in touchdown area can see marker boards by day or runway lights by night
Reported when RVR less than 1500m
Measured with transmissometers

Question 2

Explain the effect of altitude on visibility

Answer 2

Reduction in aerosols with increase in altitude as aerosols are closer to the surface.

Visibility increases with an increase in altitude

Slant range also increases with altitude

Question 3

State difference between fog, mist and haze

Answer 3

Fog = low level cloud with visibility less than 1000m
•Mist = low level cloud with visibility 1000m to 5000m
•Haze = solid aerosols with visibility less than 5000m

Question 4

Describe the principle of formation of radiation fog with respect to the required met conditions

Answer 4

Forms over cold land with high RH
Forms at night and early morning (dusty to dawn)
Clear skies and moist air allow radiation to escape
Wind 2-8kts

Question 5

What is the process of radiation fog formation (include factors from previous) (4)

Answer 5

Land loses heat at night through outgoing terrestrial radiation and cools down.

If cloudy above, it’ll absorb outgoing radiations and re-radiate a significant portion of this energy back to earth, slowing the cooling process and preventing fog. Thus clear skies needed.

Moist atmosphere required, greater the moisture the less cooling required for air to reach saturation. If air is too dry there may not be enough cooling to reach saturation point halting cooling process.

Little wind required as no wind frost will form. 2-8 kts allow distribution of cooling effect mixing the low-level moisture evenly. Too strong, mixing is over a deeper laying and there isn’t enough cool to reach saturation.

Question 6

Describe required met. Conditions for advection fog.

Answer 6

Forms over cold sea or land
Warm moist air moving over cold surface
Wind 5-25 kts
High RH so little cooling required to condense

Question 7

What is formation of advection fog

Answer 7

Warm moist air moves to an area where surface is substantially cooler, during the travel it cools and by conduction the latter of air in contact with the surface cools.

Turbulence within the flow distributes the cooling effect and eventually the moist airmass is cooled to saturation point and advection fog forms.

Question 8

Factors affecting extent of radiation fog

Answer 8

Forms in valleys because of katabatic effect
Occurs in anticyclones, ridges and cols due to subsidence
Seasonal change in surface temp
Direction of wind

Question 9

Factors affecting extent of advection fog

Answer 9

Seasonal temperature change
Direction of wind
Encouraged by advection of tropical maritime air

Question 10

Factors affecting timing of radiation fog

Answer 10

Forms in autumn or winter
Surface is cooler than air
Night and early morning due to lowest temperatures
Latest time it can form is 30 minutes after sunrise

Question 11

Factors affecting timing of advection fog

Answer 11

Forms over land in winter and early spring
Over sea in late spring and early summer
Can occur anytime of year when tropical maritime air moves over sea with temperature below dew point

Question 12

Factors affecting dispersal of radiation fog

Answer 12

Insolation (solar radiation) causing convection lifting the fog and evaporating the lower layers

Strong wings lifting the fog to form stratus cloud

Question 13

Factors affecting dispersal of advection fog

Answer 13

Change of air mass form wind direction change

Winds greater the 15kts lift the fog to form stratus cloud