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ecCharts offer forecast charts of aerosol optical depth and dust aerosol optical depth.  These, in effect, supply information regarding the reduction of transmittance of light by particulate matter in the lower layers of the atmosphere.  The charts can be used as an indication of areas where visibility is likely to be reduced because of dust or sand in the lower atmosphere, particularly where the winds are forecast to be strong and/or gusty.  There is no direct link between aerosol optical depths and visibility reduction due to rising sand and dust but they are strongly indicative of areas at risk, particularly where winds are forecast to be strong.  This is particularly helpful in data sparse areas (e.g. the Sahara).

Fig9.4.2-91: ecChart of NW Africa, T+57 VT 09UTC 10 March 2019, DT 00UTC 8 March 2019 showing forecast 10m winds, visibility, aerosol optical depth.  "Dust aerosol optical depth", though not shown on the map, is shown on the time series and probe diagrams.  Also shown are edited METAR reports for the same time (International station code (e.g. DAUA = Adrar), wind (e.g. 010/29 = N'ly 29kt), visibility (e.g. 0300 = 300m), present weather (e.g. BLSA = blowing sand) - visibility observations below 800m are shaded in red on the diagram.

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The pin (near 34N 05E) locates an upslope elevated area downwind of marshy lakes (the Chott Ech Chergui).  Some high-based showers were forecast in the area suggesting a local reduction in visibility from precipitation or local increase in humidity near the surface.   Forecasts of fairly high aerosol and dust aerosol optical in the area are not responsible for the IFS forecasts of locally reduced visibility.

Fig9.4.2-102: Meteosat IR Channel 10 (approx 11μm to 13μm).  09UTC 10 March 2019.  Some indication of the extent of the dust/sand is given by the light grey area on the satellite imagery.

Fig9.4.2-113: ecChart as Fig9.4.2-9 1 with the Meteosat the Meteosat IR Channel 10 shown in Fig9.4.2-10 2 superimposed.  This compares well with the area where higher aerosol optical depth are forecast.    

Fig9.4.2-124: ecCharts time series and probe data for Ouargla (DAUU) and Ardra (DAUA) taken from the ecChart presentation of forecast data based on DT 00UTC 8 March 2019 (as for Fig9.4.2-91). The METARs at 09UTC 10 March 2019 show the very reduced visibility at these stations.  The time series shows the IFS forecast visibility varies with time but nowhere approaches the observed visibility.  The aerosol optical depth and dust aerosol optical depth forecast data gives greater indication of reduction in visibility due to dust and sand.  However there is no direct link between the aerosol optical depths and visibility reduction due to sand and dust but they are strongly indicative of areas at risk particularly where winds are forecast to be strong.