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Chapter 09 Fog Formation Lesson 30/31

Chapter 09 Fog Formation Lesson 30/31. Types of Fog. 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).

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Chapter 09 Fog Formation Lesson 30/31

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  1. Chapter 09Fog FormationLesson 30/31

  2. Types of Fog • Radiation Fog • Smoke Fog (Smog) • Advection Fog • Thaw Fog • Arctic Sea Smoke (Steam Fog) • Frontal Fog • Hill Fog

  3. 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.

  4. 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.

  5. 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.

  6. 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

  7. 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.

  8. 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.

  9. 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, providing there is at least 7 - 8 kt of wind.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. Sea smoke over Thurso(Acknowledgments to A. Moar)

  16. 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.

  17. 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.

  18. 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.

  19. Hill Fog • If forecast the visibility is automatically assumed to be less than 200m. • Usually encountered in warm sectors with Tropical Maritime air.

  20. 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.

  21. 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

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