Download Visibility and Fog

yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts
no text concepts found
Types of Fog
Radiation Fog
Advection Fog
Smoke Fog (Smog)
Hill Fog
Frontal Fog
Thaw Fog
Arctic Sea Smoke (Steam Fog)
• Fog:
– is composed of small water droplets in suspension (or ice
crystals in ice fog).
– Visibility is reduced to less than 1000 m.
– Relative humidity is generally 100%.
• Mist:
– is also caused by small droplets in suspension.
– Visibility is 1000m or more.
– METAR and TAF codes impose an upper limit of 5000
– RH is at least 95% but less than 100%.
• Haze:
– caused by solid particles in suspension.
Radiation Fog
• Caused by long wave radiation cooling from ground
at night.
• Favourable conditions are
– High relative humidity
– little or no cloud
– light winds, 2-8 knots.(or 1-5 kt.)
• Fog dispersal caused
– by incoming insolation heating environment to above
saturation temperature, or by
– turbulent mixing in the boundary layer with increasing
wind and lifting fog into low stratus or by
– mixing with dry air from above.
Synoptic Conditions for Rad. Fog
• These conditions are often met in anticyclones, ridges and cols,
in late autumn, winter and early spring.
• Radiation fog is usually between 300 and 1000 ft in depth.
• Cloud cover arriving early in the night will arrest the heat loss, so
that the radiation fog will not develop further, and may clear.
Rad. Fog Cont.
• Suitable diversion airfields in radiation fog would be:
– A high level airfield if early in the night.
– A coastal airfield with onshore wind.
– An airfield in the lee of high ground,
there is at least 7 - 8 kt of
Advection Fog
• Formed by warm moist air moving over a colder surface below dew
point of the air.
• Can form over sea as well as land.
• Wind strength is not material to the formation of advection fog.
• Sea Fog which has formed over the sea may drift inland, as does the
HAAR on east coasts of the UK in summer.
• Depth of advection fog is typically 1,000-1,500 ft.
• A change of airmass is usually necessary for the clearance of
advection fog.
Smoke Fog (Smog)
• A form of radiation fog.
• Formed when
– A marked low level inversion exists.
– A source of pollution.
• The inversion traps industrial and domestic pollution in the lower
• Fog visibilities may be found at slightly less than 100% relative
Hill Fog
• Also referred to as “cloud covering hills.”
• Formed by cloud in contact with high ground
– forming orographic stratus due to forced acsent over high ground or,
– simply high ground extending into an existing low cloud layer.
• If forecast the visibility is automatically assumed
to be less than 200m.
• Usually encountered in warm sectors with
Tropical Maritime air.
Frontal Fog
• Formed by the lowering of frontal cloud to the surface (eg.
at warm front passage).
• Forms ahead of a warm front by the saturation of air due
to the continuous rain.
Thaw Fog
• Thaw fog is a particular case of advection fog.
• Warm air arriving over a snow-covered surface will often
produce widespread fog while the snow melts
• The melting snow surface (at 0°C) both cools the air and
increases the moisture content as it melts and evaporates.
• It is a particular feature of central and eastern European plains in
the spring.
Arctic Sea Smoke
• Also known as steam fog or frost smoke.
• Occurs when very cold air moves over a relatively warm water
• The very cold air mixes with the warm air above the sea surface to a
depth of about 500 feet, cooling to below dew point temperature.
• There must also be a marked inversion.
• Occurs frequently in winter
– over the north-west Atlantic near Greenland or northern Canada when cold
air moves over the relatively warm ocean.
– very cold Siberian air moves over the sea by the Kamchatka Peninsula off
eastern Russia, and
– when cold katabatic air moves down the mountains into the fjords of
Meteorological Visibility
Meteorological Visibility
• Is a measure of the clarity of the atmosphere.
• It is more properly known as the Meteorological
Optical Range (MOR).
• Visibility (Met Vis)
– is the greatest horizontal distance at which suitable
objects can be recognised for what they are in daylight or
– at which lights of specified intensity can be seen at night
by a person with normal sight.
• Where the visiblity is variable the lowest value is
Visiblity Reporting
• Lowest value is normally reported for Met Vis.
• In METARS the lowest value is always given and
the highest values may be reported under certain
• Runway Visual Range (RVR) reporting begins when
the Met Vis or the RVR fall to below 1500 m.
• RVR is the maximum distance in direction of takeoff or landing at which designated runway markers
or lights can be seen from a centreline height
corresponding to the eye-level of the pilot on touchdown.
Visibility Reporting (cont.)
• RVR reporting begins when the horizontal visibility
or the RVR is less than 1500 m.
• At a/d’s with Instrumented RVR (IRVR) systems,
RVR may also be reported when
– the observed value is at or below the maximum
reportable value (usually 1500m) or
– when shallow fog is forecast or reported.
• RVR is passed to a/c before take-off and during
approach to land.
• Changes in RVR are passed to a/c during approach
or if pilot reports or ATC observation indicate the
RVR is worse than that indicted by equipment
Visibility Reporting (cont)
• RVR is reported by human observer (OBS) or by
IRVR systems.
• Multi site IRVR systems report touchdown (TDz),
midpoint (MID) and stop-end (END) values.
• Mid-point and/or stop-end values are suppressed
– they are 800 m or more or
– they are equal to or higher than the touch-down zone
unless they are less than 400 m.
Visibility Reporting Problems
• Lack of reference objects e.g. at sea or level
snow areas.
• Depends on size, shape and colour of the
• Object illumination and background.
• Transparency of wind shield.
• Visibility varies in layers and in direction.
• Slant visual range in flight.
• Met vis and RVR may vary widely.
In-Flight Visibility in Poor
• Inside an obscuring layer
– Increasing height decreases circle of visibility.
– Decreasing height increases circle of visibility.
In-flight visibility above a layer
of poor visibility
• Above an obscuring layer
– decreasing height decreases circle of visibility
– increasing height increases circle of visibility
In-flight visibility on the
• On the approach
– Airfield clearly visible from directly above.
– On glide path forward visibility lost due to slant
range through obscuring layer.
– Contact with airfield will be lost.
– Instrument approach will be necessary.
Effect of Precipitation on Visibility
• Drizzle commonly restricts visibility to between 500 metres and
3,000 metres.
• Light rain has virtually no effect.
• Moderate rain can reduce visibility to 3-10 km.
• Heavy rain showers of temperate latitudes rarely cause a reduction
below 1,000 metres.
• Snow and blown snow are both very effective in restricting visibility
to less than 1,000 metres.
• Heavy snow may lead to visibilities of the order of 50-250 metres.
Flight in Rain
• Rain on the windscreen can have two opposite effects.
• It can scatter light and so reduce its intensity.
– the runway lighting may appear further away than it is or,
• It may cause the runway lights to “bloom” thus
– they appear larger and closer than they really are.
• Care must be taken to avoid an inadvertent descent well below a safe
approach path, when subject to such illusion.
• In heavy rain a film of water on the windscreen can cause refraction.