Download Visibility and Fog

FOG
Types of Fog
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Radiation Fog
Advection Fog
Smoke Fog (Smog)
Hill Fog
Frontal Fog
Thaw Fog
Arctic Sea Smoke (Steam Fog)
Fog/Mist/Haze
• 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
m.
– 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
wind.
providing
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
atmosphere.
• Fog visibilities may be found at slightly less than 100% relative
humidity.
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
surface.
• 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
Scandinavia.
Visibility
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
reported.
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
conditions.
• 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
when:
– 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.
• 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
Conditions
Summary
• 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
Summary
• Above an obscuring layer
– decreasing height decreases circle of visibility
– increasing height increases circle of visibility
In-flight visibility on the
approach
Summary
• 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.