1. Visibility
ATC Chapter 5

2. Aim
To learn about Visibility and how it affects us as pilots.

3. Objectives Define Slant visibility and horizontal visibility
Define Runway Visual Range
Explain factors affecting visibility
Describe fog and mist formation

4. Background
Visibility is extremely important for us as visual pilots to know whether a flight can be carried out safely or legally.
If the visibility decreases below a specified amount en-route, we would not be able to see where we are going, therefore would not be able to fly.
If the visibility at our arrival aerodrome is below minimums, an alternate would be required.

5. 1. Slant and horizontal vis.
Slant Visibility and Horizontal Visibility
If an aircraft is approaching an airfield in low visibility above a layer of stratus or fog, the pilot may be able to see the airfield from directly above the airfield
As soon as the aircraft is on approach to land, the airfield may be hard to see because the observer is looking through a thicker layer of weather, this is known as slant visibility
Horizontal visibility is the visibility given at the same level to the station.
Visibility reported at an aerodrome, or given in a forecast is horizontal visibility
Pilot’s should exercise extreme caution when operating on the border of VMC as they may not be able to land as they see slant visibility

6. 2. Runway Visual Range
Runway Visual Range is defined as the maximum distance along a runway that can be seen from a position corresponding to the average pilots eye level at touchdown
RVR is broadcast at certain aerodromes with precision approaches ie the ILS.
RVR is given in metres along a runway from touchdown zone.

7. 3. Factors affecting Visibility
There are numerous factors both natural and man made which affect how far we can see.
These factors include:
Fog and mist
Precipitation
Spray from the sea
Smoke haze
Dust and sand storms

8. 3. Factors affecting Visibility
Precipitation
Precipitation can occur in many different ways and in many different forms
It can occur as rain, drizzle, hail or snow
The degree of the reduction in visibility will depend on the amount and size of the water droplets
If flying in rain, the visibility can reduce to as little as 50m in very heavy rain and up to over 10km in drizzle.
Hail is virtually invisible so the risks of flying in hail is not the reduction in visibility
Visibility in snow can vary from below 1000m in heavy snow.
It is very important to study the weather available to us and make decisions whether flight can be undertaken through showers.

9. 3. Factors affecting Visibility
Spray from the sea
Spray from the sea is not possible from normal flight over water as you would have to be very low for it to affect you.
Spray from the sea reduces visibility for those seaside airports and is not actually water that reduces visibility, it is the salt that is deposited on windscreens after it is evaporated from the water.

10. 3. Factors affecting Visibility
Smoke Haze
The content of smoke is usually carbon and sulphur
These together are dark in colour and reduce visible light
If there was an inversion, soot can build up to very high concentrations and reduce visibility sometimes below VMC!

11. 3. Factors affecting Visibility
Dust and Sand Storms
A dust storm is defined as a condition in which the visibility is reduced to below 1000m by raised dust,
The requirements for formation of a duststorm are:
A long period of dry weather
Surface wind exceeding 15kts
Unstable air (steep lapse rate)
The steeper the lapse rate, the greater the depth in which the dust is raised.

12. 3. Factors affecting Visibility
Inversions
A temperature inversion can act as a blanket in which vertical convection is stopped.
That is air that is warmed starts to rise but meets air equal in temperature and stops.
Particles that are trapped in the inversion will stay there as they will not be able to rise any further.
These particles could be anything from smoke, haze or pollution.
These particles can act as condensation nuclei and if the relative humidity is high enough, fog can form.
If the condensation nuclei is from smoke, and the relative humidity is high enough, smog will form (combination of smoke and fog)
Smog can be found mostly in industrial areas.

13. 4. Fog
Fog is the most frequent cause of low visibility at airports, as such it is one of the most important hazards to aviation.
In general, fog is low level cloud and can severely reduce the visibility to less than 1000m
The relative humidity in fog is 100%
Mist is much the same but the visibility is not less than 1000m and the relative humidity is just below 100%.
The different types of fog which we will discuss are:
Radiation fog
Advection fog
Frontal fog
Upslope fog
Steam fog

14. 4. Fog
Radiation Fog
Radiation fog is caused when the air is cooled to below the dewpoint due to heat loss from radiation.
Conditions favourable for the formation of radiation fog are:
A cloudless night (allowing the land to lose heat by radiation to the atmosphere)
Cold land surface
A high relative humidity (only a little bit of cooling is required for air to reach saturation)
Light winds (promotes mixing of air)
The presence of condensation nuclei

15. 4. Fog
Radiation Fog
Radiation fog is dissipated when the sun heats the earth up causing the air above it to also warm
If the fog is very thick it can act as a blanket insulating the ground. The ground will not be able to warm, therefore the air above it will not warm. In this scenario the fog can last throughout the day
Fog can dissipate if the wind strength is increased therefore creating enough turbulence to drag warmer air down into the fog layer.

16. 4. Fog
Advection Fog
Advection is to do with the horizontal movement of air.
When a moist air mass moves horizontally and comes into contact with a colder surface the air is cooled from below
If the cooling is significant enough to cool the air to below the dewpoint, fog will form.
Fog formed in this way is termed advection fog
Moist air may flow from a relatively warm sea to a cold air mass to form advection fog

17. 4. Fog
Frontal Fog
Frontal fog occurs at the boundary of two air masses rather than within a single air mass as previously discussed with radiation and advection fog.
It occurs as cloud reaching or descending to the surface at the frontal edge, or forms in the precipitation when the air becomes saturated

18. 4. Fog
Steam Fog
Steam fog forms when a cold moist air mass flows over a much warmer water body
The evaporation of water from the warm water surface causes the air above the surface to become saturated.
Because the surface is warmer, convection currents give the steamy appearance.

19. 4. Fog
Upslope Fog
When moist is forced up a slope or mountain it is forced to cool adiabatically.
If the moist air reaches the dewpoint, fog will form.
If the wind stops the fog will dissipate.

20. Questions?