Visibility Flashcards
VFR minimum conditions?
Visibility greater than 5km
Vertical cloud distance 1000ft
Horizontal cloud distance 1.5km
4 non-atmospheric causes of low visibility
- Glare from the sun or a windscreen
- Contaminated windscreens
- Scratched windscreens
- UV damaged (acrylic) windscreens
5 atmospheric causes of low visibility
- Water droplets
- Ice crystals
- Smoke
- Chemical pollutants
- Volcanic ash
What visibility does fog imply?
Less than 1000m
What visibility does mist/haze/smoke/dust/sand imply?
1000m - 5000m
5 types of fog?
- Radiation fog
- Advection fog
- Evaporation/steam fog/artic smoke
- Frontal fog
- Hill/upslope fog
What are the ideal conditions for radiation fog?
High RH, light winds (2-8kt), clear skies
High pressure system over land
Nighttime temperature inversion
3 ways radiation fog disperses
- Temperature increases above dew point
- Wind mixing dry and saturated air
- May lift into a low St or Sc
What causes advection fog?
When a parcel of warm, moist air moves horizontally over a colder surface
What type of areas in advection fog associated with?
Cool sea areas adjacent to coasts in the summer
What causes Steam Fog/Arctic Fog/Sea Smoke?
Very cold air sitting over a warm, moist surface
As soon as water evaporates it condenses causing fog
What causes frontal fog?
Rain forces the cloud base down
Warm front over a cold front creates an inversion
What causes orographic/hill fog?
Air forced up a slope condenses and forms clouds, close to the surface these are fog
What is freezing fog?
Fog at a temperature below 0 degrees
What does freezing fog contain?
Supercooled water droplets which freeze on impact
What is fog below 2m depth?
Mist
What is the weather abbreviation for patchy fog?
BCFG
What is the weather abbreviation for partial fog?
PRFG
What is the visibility in cirriform clouds?
1000m+
What is the visibility in Ac and As?
20 - 1000m
What is the visibility in Ns and Cb?
10 - 20m
What is the visibility in Sc and St?
10 -30m
What is the visibility in Cu?
Less than 20m
What is smoke?
Suspension of small particles produced by combustion
What is haze?
Suspension of extremely small, dry particles invisible to naked eye and sufficiently numerous to give the air opalescent appearance
What is dust haze?
Suspension of dust or small sand particles, raise from the ground prior to the time of observation
What wind speed do sand and dust storms require?
15kt+
How high can sand and dust storms reach?
Dust storms - 15000ft
Sand storms - few hundred metres (heavier)
Visibility in a heavy dust storm?
Less than 200m
What happens to air-to-ground visibility when there is fog?
Reduces as you descend
VFR operating minima on the ground?
Cloud ceiling greater than 450m/1500ft
Ground visibility greater than 5km
How does a control tower report visibility observed?
Whatever it is for at least half of the horizon
For visibility reporting what are the different increments?
0 - 800m = 50m increments
800m - 5km = 100m increments
5km - 10km = 1km increments
10km+ = 10km increments
What does the USA use for reporting visibility?
Statute miles
What is the runwa visual range (RVR)?
The range over which the pilot of an aircraft on the centreline of a runway can see the runway surface/edge lights/centreline lights
Where are RVR detectors placed?
2.5m above the runway within 120m
Usually threshold, mid point and stop-end point
When is RVR reported?
When RVR is below 1500m
What do U, D and N indicate when reporting RVR?
U - upward visibility trend
D - downward visibility trend
N - neutral trend
What is the ceiling also known as?
Vertical visibility (VV)
