Visibility and Fog Flashcards
Describe runway visual range
The distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings delineating or identifying its centre line.
How is the runway visual range obtained?
Sensors positioned near the threshold (for the touchdown zone) and at the midway point and stop-end of the runway.
How does an increase in altitude affect one’s visibility?
- The brightness of the sky decreases with height. This factor becomes more apparent at high altitudes such as the Tropopause.
- At high altitudes, the lack of dust, haze and cloud will make it harder for the eye to focus.
- The eye will focus between close and long distances, for that reason, it is difficult to see and locate objects such as other aircraft flying at altitude.
List the requirements in order for radiation fog to form?
- High relative humidity (little cooling needed to saturate the surface air).
- Clear sky (so T.R. can readily escape).
- Light wind (mixing within the surface air layer is assured).
- Generally stable atmosphere (so that mixing and cooling are confined to a shallow layer).
Discuss why light wind is a factor that affects the extent of fog?
- Necessary to maintain radiation fog.
- Prevents them from lifting into stratocumulus clouds.
Discuss why subsidence is a factor that affects the extent of fog?
Subsidence that accompanies an anti-cyclone is likely to produce stability of air, thus preventing the fog bank from lifting off the surface.
Discuss why clear skies are a factor that affects the extent of fog?
Associated with anticyclones prevent T.R. from escaping.
When does radiation fog usually form?
Night
Is it possible for radiation fog to form after sunrise, if so how?
- Yes
- When the RH is high (but a lack of mixing at right prevented its formation).
- The mixing from solar input after sunrise and demand for latent heat as some surface moisture evaporates may cause radiation fog to form after sunrise.
Regarding radiation fog, how can mixing intensify?
After sunrise, the particles in the friction layer warm-up and move around more.
What will happen to the fog bank when mixing intensifies?
Top of the fog bank will experience this turbulence, causing the height of the fog bank to increase.
What will initially happen when there is solar input into the fog? What does the process require?
- Encourages droplets at and near the top to evaporate.
- Requires latent heat, which is drawn from wherever it is available, including from the air above the bank.
What will cause the steepening of the ELR regarding radiation fog? (dispersal of radiation fog)
- Solar radiation has a limited ability to penetrate moisture and also has some warming effect of the water content within the fog bank.
- These factors combined causes a steepening of the ELR.
What causes a cloud to form with regards to ELR and radiation fog? (dispersal of radiation fog)
- Fog will lift the surface in places, causing a few holes to appear and direct solar input onto the surface which further steepens the ELR.
- This causes a layer of cloud to form, taking the form of low stratus but can turn into stratocumulus.
How does surface air cooling occur regarding advection fog?
- Whereas radiation fog involves the relatively rapid cooling of the surface.
- Advection fog requires a surface that is already cold so that when moist air is transported over it, the air can cool to temperatures lower than dewpoint.
