Visibility And Fog

Question 1

Define runway visual range (RVR)

Answer 1

• Max distance a pilot 15ft above the runway in the touchdown area can see marker boards by day or runway lights by night.
• Reported when RVR is below 1500m
• Measured with Transmissometers

Question 2

Explain the effect of altitude on visibility.

Answer 2

• Reduction in aerosols with an increase in altitude
• Visibility increases with an increase in altitude
• Slant range also increases with altitude.

Question 3

State the 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 and advection fog with respect to required meteorological conditions.

Answer 4

• Radiation fog forms over cold land with high RH.
• Forms at night and early morning
• Clear skies and moist air allow radiation to escape
• Wind 2 to 8kts
• Advection fog forms over cold sea or land
• Warm moist air moves over cold surface
• Wind 5 to 25kts
• High RH so little cooling required to condense

Question 5

Describe the principle of formation of radiation and advection fog with respect to factors affecting the extent of the fog.

Answer 5

• Radiation fog forms in valleys because of katabatic effect
• Occurs in anticyclones, ridges and cols due to subsidence
• Seasonal changes in surface temperature
• Direction of wind

Question 6

Describe the principle of formation of radiation and advection fog with respect to factors affecting the timing of the fog.

Answer 6

• Radiation fog forms in autumn and winter
• Surface is cooler than air
• Night and early morning due to lowest temperatures
• Latest time is can form is 30 minutes after sunrise
• Advection fog forms over land in winter and early spring
• Over sea in late spring and early summer
• Can occur anytime of year when air moves over sea with temperature below dew point

Question 7

Describe the principle of formation of radiation and advection fog with respect to factors affecting the dispersal of the fog.

Answer 7

• Radiation fog
• By insolation causing convection, lifting the fog
• Will also evaporate the lower layers
• By stronger winds lifting the fog to form stratus cloud
• Advection fog is affected by a change of air mass from wind direction change.
• Winds greater than 15kt lift the fog to form St cloud