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Low cloud and poor visibility FAOR South African Weather Service

Low cloud and poor visibility FAOR South African Weather Service. Airports Management Centre. Developed by ACSA prior to 2010 Improve airport performance Collaborate service providers :SAWS SAWS : Physically present October 2018 Two formal briefings a day Numerous informal briefings.

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Low cloud and poor visibility FAOR South African Weather Service

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  1. Low cloud and poor visibility FAOR South African Weather Service

  2. Airports Management Centre Developed by ACSA prior to 2010 Improve airport performance Collaborate service providers :SAWS SAWS : Physically present October 2018 Two formal briefings a day Numerous informal briefings Templ ref: PPT-ISO-colour.001 Doc Ref no:

  3. Most frequent between March and August. FAOR with high elevation is more prone to fog than surrounding Aerodromes/Airfields: *FAOR is a Category II airport. Not common in summer but still possible, depending on the general circulation pattern. Frequent between 04:00-07:00am in summer. Frequent between 05:00-08:00am in winter. Average occurrence is two or three days per month. Low Cloud and Poor Visibility

  4. Types of Fog and Formation • Fog (FG) is defined as: vapor condensed to small water droplets in the lower atmosphere, reduced vis to below 1Km. • Mist (BR) defined as: Reduced visibility between 1 and 5 Km. • Reduced visibility, due to fog, is influenced by numerous factors. - Fog prone areas, can be divided into two categories namely; orography and meteorological. • Orographic factors: terrain, altitude, topography, slope and aspect. • Meteorological factors: pressure, wind speed, low level inversion, previous precipitationand climatology. • Limiting factors for fog formation are strong winds and warm surfaces.

  5. Types of Fog and Formation • Common: Radiation and Advection fog. • Radiation fog: Overnight when air near the ground is cooled to saturation. Most common after it has rained and the weather clears, resulting in a cloud free sky. • * Advection fog: Moisture advected from the E/NE to FAOR. • * Up-Slope fog: Low cloud pushed from the SE and trapped by a low level inversion due to wind change from SW to E . • * Low cloud advected from the NW in the 2nd/3rd day of a ridging high pressure system (not easy to forecast). * Due to a ridging high pressure system

  6. Figure 1: Conceptual model of a ridging high pressure system at 2100Z.

  7. Figure 2: Conceptual model of a ridging high pressure system at 0100Z (the following morning).

  8. Figure 3: Conceptual model of a ridging high pressure system at 0300/0400Z (time FAOR usually experiences fog).

  9. Templ ref: PPT-ISO-colour.001 Doc Ref no:

  10. Figure 4: Tephi-gram showing the inversion layer over Irene on 19th of February 2019.

  11. Figure 5: Low level wind profile over FAOR from AMDAR data.

  12. Forecasting • Low cloud and poor visibility are small scale weather events. • Numerical Prediction Models along with observations used to forecast such as midnight Tephigram and METARs • Advection fog easier to forecast than the other types • Aviation forecasts need to be issued in a timely fashion . • (Not well predicted within a forecast period of 12 to 18 hours.) • TAF updated every 6 hours and validity of 36 hours. • Forecast difficulty is proportional to time.

  13. Case: 4TH June 2018 Up-slope fog. Very light winds. Dew point depression was 0 at times Visibility 100m for 1hour.

  14. Figure 6: Day Natural Colour Satellite Image, showing fog over FAOR on 4th of June 2018 at 06:00UCT.

  15. Roof top view Figure 7: Good visibility viewed from FAOR roof (left) and poor visibility in fog (right).

  16. Aviation Website Figure 8: FOG Satellite Image on Aviation Website.

  17. Templ ref: PPT-ISO-colour.001 Doc Ref no:

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