1. CGS Ground School Meteorology Visibility
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2. Visibility
Visibility is defined as "the greatest horizontal distance at which objects can be seen and recognised, by an observer on the ground, with normal sight, under conditions of normal daylight illumination".
The observer will measure the visibility around 360° of their location, the value measured or exceeded over half of the horizon will be recorded as the meteorological visibility. This is known as the prevailing visibility.
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3. Prevailing visibility
12km
5km
12km
2500m
5km
5km
10km

4. Prevailing visibility
Visibility is defined as "the greatest horizontal distance at which objects can be seen and recognised, by an observer on the ground, with normal sight, under conditions of normal daylight illumination".
The observer will measure the visibility around 360° of their location, the value measured or exceeded over half of the horizon will be recorded as the meteorological visibility. This is known as the prevailing visibility.
In this example the maximum visibility is 12km but that doesn’t make up 50% of the horizon. A large area has 10km visibility and a few smaller areas are less. In this case the prevailing visibility would be 10km with an area of 2500m to the East.
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5. Runway visual range
Runway Visual Range (RVR) is the maximum distance that lights or runway markers can be seen from the touch down point of the runway. When this range is greater than 1500m, a RVR figure is not required.
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1500m
1000m
500m
100m

6. Visibility classifications
Visibility is given in metres up to 5000m, and kilometres thereafter.
A visibility of <1000m is classified as FOG.
A visibility affected by water droplets, but >1000m is classified as MIST.
A visibility affected by solid particles, but >1000m is classified as HAZE.
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7. Effect of fog/haze layers
Fog or haze layers can cause confusion for pilots when the meteorological visibility appears different from that which they are experiencing. This is known as slant visibility.
Consider the following example:
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8. Effect of fog/haze layers
Visibility in the fog layer is 800 m.
Visibility above the fog layer is > 25 km.
If the pilot of the aircraft looks down, his visibility through the clear air is not impeded, but he can still only see 800m
through the fog.
>25km
The effect is that to the pilot the visibility seems much greater than 800 m.
>25km
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800m
>800m
800m

9. Effect of fog/haze layers
As the aircraft descends the visibility appears to reduce.
By the time the aircraft is just above the fog layer the visibility appears less than 800m.
>25km
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800m
>800m
<800m

10. Effect of fog/haze layers
Should the aircraft enter the fog whilst trying to approach the airfield the field of view is reduced still further.
Although the visibility is still 800 m, the apparent air - ground
visibility is much less.
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800m
600m

11. Fog
Fog is effectively cloud in contact with the ground and is formed in the same way (ie the airmass is cooled below its dew point).
Fog can be considered under the following headings:
a. Radiation fog.
b. Advection fog.
c. Hill fog.
d. Frontal fog.
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12. Radiation fog
On a clear night the ground will radiate its heat rapidly and its temperature will fall. The air in contact with the ground will cool by conduction.
If it cools below its dew point then condensation will occur.
If there is no surface wind then the condensation will be confined to a very shallow layer of air (air being a poor conductor of heat).
The condensation therefore settles on the ground as dew (or rime frost if temperature is below freezing).
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13. Radiation fog
On a clear night the ground will radiate its heat rapidly and its temperature will fall. The air in contact with the ground will cool by conduction.
If it cools below its dew point then condensation will occur.
If there is a light wind (<10 kts) then turbulence will spread the cooling effect through a deeper layer of air.
Condensation therefore occurs throughout this layer and radiation fog is formed extending up to several hundred feet.
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14. Radiation fog
On a clear night the ground will radiate its heat rapidly and its temperature will fall. The air in contact with the ground will cool by conduction.
If it cools below its dew point then condensation will occur.
If the wind is stronger (>10 kts) then turbulence will spread the cooling over a greater depth and the airmass may not cool to its dew point.
Fog may not form, but the airmass will cool adiabatically and low stratus may form.
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15. Radiation fog
Ideal conditions for the formation of radiation fog are therefore:
Clear skies - allowing rapid cooling of the ground.
Moist airmass - requiring little cooling to reach dewpoint.
Light winds - providing turbulence over a shallow height band.
These conditions are often associated with anticyclones or ridges of high pressure.
In the UK radiation fog can occur throughout the year, but is most common between October and March when the short days and long nights cause the ground to remain cool.
Because the cool air is more dense, radiation fog will tend to flow down slopes and into valleys and other low lying areas. High ground and tall buildings may protrude through the fog layer.
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16. Radiation fog
When a dew or frost has been formed in nil wind conditions, fog may form after dawn, as thermal heating causes a turbulent layer to form.
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17. Radiation fog
Radiation fog is usually dispersed by solar radiation, which warms the ground and hence the air in contact with the ground.
This increases the turbulence and mixes the lower, saturated air mass with the warmer, drier air above.
Initially this may cause the fog to get thicker, before further mixing causes the air to rise further.
As the air rises further it cools adiabatically and low stratus forms.
This may cause airfields at higher altitudes, that have been clear of the low lying fog, to become fog bound.
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18. Radiation fog
Radiation fog may also be cleared by freshening winds increasing the turbulence and mixing the air as previously described.
It may also be cleared by the passage of a drier airmass.
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19. Advection fog
Advection fog is formed when a moist airmass flows over a cold surface, which reduces its temperature to its dew point.
In the UK advection fog over the land is uncommon, but may occur during winter or spring when a warm airmass (typically behind a warm front) flows over an area of severe frost or snow. This is known as thaw fog.
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RH 75%
RH 100%

20. Advection fog
Advection fog over the sea can occur all year round. It is formed when a warm airmass flows over a colder area of sea and cools to its dew point. It may also form during the spring or early summer when a warm airmass from over the land moves out over the comparatively cold sea.
Advection fog is usually dispersed by the passage of a drier airmass, although advection fog over land may also clear in strong winds when turbulence mixes the air to form low stratus.
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RH 75%
RH 100%

21. Hill fog
Hill fog is often formed orographically, when a moist airmass is cooled adiabatically as it is forced up the hill.
It may also be formed elsewhere and simply drift onto the hill.
Hill fog is particularly prevalent on high ground near to coasts, where the airmass is more likely to be moist.
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22. Frontal fog
If continuous rainfall from a front causes the lower airmass to become nearly saturated, then any turbulence will cause the formation of very low stratus cloud or fog.
On a warm front this fog may be less than 100ft thick.
The fog is usually short lived and disperses within 1-2 hours as the front passes. As
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23. THE END
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