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 2019/05/06 Templ ref: PPT-ISO-colour.001 Doc Ref no:

Low Cloud and Poor Visibility 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.

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 precipitation and climatology. Limiting factors for fog formation are strong winds and warm surfaces.

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 Radiation and Advection fog are well studied across the globe as they are common Radiation fog happens overnight when the air cools down in very calm winds until it reaches saturation. Advection fog takes place when moisture or rather existing low cloud is pushed from the east i.e Mpumalanga to Gauteng because of a ridging high pressure. Up-slope fog is a combination of radiation and advection helped by topography. Low cloud that already exist in the SE is lightly pushed over an already cooled surface ,fog takes place as the cloud is elevated up in FAOR(Higher elevation) Ridging high pressure system – from the East, South or over the extreme southern parts of the country, is the best situation when fog may develop on the Highveld.

See the wind direction change, for the next three figures. Explain the High pressure system situated to the east of the country, with the anti-cyclonic flow. Mist already along the escarpment. See the wind direction change, for the next three figures. Figure 1: Conceptual model of a ridging high pressure system at 2100Z.

From the previous figure the High is still ridging brining in the moisture from the east, but pushed in a bit further inland. Wind direction change Figure 2: Conceptual model of a ridging high pressure system at 0100Z (the following morning).

Again reiterate that the High has been pushing in the low cloud/fog over the escarpment and since now reached FAOR. Wind direction change. Figure 3: Conceptual model of a ridging high pressure system at 0300/0400Z (time FAOR usually experiences fog).

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A shallow surface inversion is most likely to produce fog patches. Low cloud/ Fog clears when the surface temperature reaches, or exceeds the inversion temperature. Showing them the inversion and how it works since we mentioned that fog dies off once the air temperature reaches that of the inversion. Can also mention, that ground previously moistened by precipitation prompts fog formation. A shallow surface inversion is most likely to produce fog patches. Figure 4: Tephi-gram showing the inversion layer over Irene on 19th of February 2019.

Bullet number 7: Timing would appear to be important Bullet number 7: Timing would appear to be important. It is preferable for ridging to take place overnight and be off the KZN coast between 00:00 and 06:00UTC. Showing them the inversion and how it works since we mentioned that fog dies off once the air temperature reaches that of the inversion. Figure 5: Low level wind profile over FAOR from AMDAR data.

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. Explain the process of forecasting low cloud and how a Tphi helps along with METARs and used the Tphi for the 19th as an example and explain the inversion As time increases, the difficulty to forecast a small scale hazardous weather event increases. In order to produce more accurate TAFs, improved knowledge of the events are required. Better TAFs will lead to earlier fog warnings, fewer false alarms, accurate timing, better public image, improved flight planning, increased preparedness and ultimately profitability.

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

On the 4th of June at 0600Z Figure 6: Day Natural Colour Satellite Image, showing fog over FAOR on 4th of June 2018 at 06:00UCT.

Roof top view How the roof top view on a good visibility day vs the 4th of June when there was fog. Figure 7: Good visibility viewed from FAOR roof (left) and poor visibility in fog (right).

Aviation Website Observations Satellite FOG A Explaining to them how the FOG A RGB in the website can be used to identify the low cloud ONLY at night ,using TAFs to guide further as to when it will clear. ICT was informed in fixing the FOG A on the website Figure 8: FOG Satellite Image on Aviation Website.

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