Transcript Visibility and Fog

Chapter 09
Fog Formation
Lesson 30/31
Types of Fog
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Radiation Fog
Smoke Fog (Smog)
Advection Fog
Thaw Fog
Arctic Sea Smoke (Steam Fog)
Frontal Fog
Hill 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%.
– Plentiful supply of condensation nuclei.
Mist
• 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%.
– Plentiful supply of condensation nuclei.
Haze
• Haze:
– caused by solid particles in suspension.
– visiblity in haze may reduce to fog levels but
generally it is better and has an upper limit of
5000m for reporting purposes.
– airborne solid particles from industrial
processes are the most frequent cause of haze as
well as fires.
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
Radiation Fog
• 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 Radiation 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.
Radiation 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
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.
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.
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.
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.
Sea smoke over Thurso
(Acknowledgments to A. Moar)
Arctic Sea Smoke
• 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.
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.
Orographic Fog/Hill Fog
• Also referred to as “cloud covering
hills.”
• Formed by cloud in contact with high
ground
–forming orographic stratus due to
forced ascent over high ground or,
–simply high ground extending into an
existing low cloud layer.
Hill Fog
• If forecast the visibility is automatically
assumed to be less than 200m.
• Usually encountered in warm sectors with
Tropical Maritime air.
Orographic/Upslope Fog
• Radiation Fog advecting up a slope
• If forecast the visibility is automatically
assumed to be less than 200m.
• Usually encountered in warm sectors with
Tropical Maritime air.
Orographic Fog
• Hill fog will clear with lifting of the cloud base or passage
of the warm sector.
• Upslope fog will clear either by an increase in wind speed
or by insolation