Llansadwrn (Anglesey) Weather Wx-Watch from the observer's notebook

On 25th February with pressure on 1014 mb there was a slow-moving low SW of Ireland. Pressure had been high over Europe with a ridge extending to S Scandinavia. It was a mostly cloudy morning with a slight ESE'ly wind with a patch of rain over the Irish Sea but it continued dry here although the cloud thickened. The afternoon remained mainly cloudy and dry until 2000 GMT when there was a shower of rain that amounted to only 0.3 mm. This contained a deposit of dark-grey coloured dust tinged with orange. Earlier in the day dust deposition was reported from, Guernsey (Channel Islands). Following trajectory analysis done on the NOAA Air Resources Laboratory's Website Ready Model (Stephen Burt, 2003) it seems that the dust originated from the Great Western Desert in Algeria. The analysis suggests that dust was raised between 20 - 22 February, travelled to Anglesey over the next 3 - 4 days and was washed out from a parcel of air at around 1500 m.

On the next day, 26 February, some spots of rain at times early on had left the ground damp; deposits of the dark-grey coloured dust could still be seen and a sample was collected. Surfaces used to observe dust deposition were routinely cleaned or replaced with new. Pressure 1006 mb was steady but the high pressure over Europe (1034 mb) was declining. Warm frontal cloud over North Wales and the Midlands was slow-moving and brought the first rain with a succession of cold fronts to follow. The wind was a light SE'ly at times and the cloud occasionally showed signs of breaking up when it became brighter. The afternoon was brighter as the cloud remained high and thinned for a while before thickening again by evening. There was light rain from 2130 to 2315 GMT, amounting to 1.6 mm, that contained moderate to heavy amounts of dark-orange or reddish-brown dust. Preliminary indications, looking at the SKIRON model dust forecast, published by the University of Athens, showed that dust was in a narrow band of wet deposition from Algeria through the Pyrenees, W of France up to the Irish Sea. A later trajectory analysis (Stephen Burt, 2003) confirmed that some of the dust originated this time from the Great Eastern Desert in Algeria. It was likely to be of mixed origin being part from the Great Western Desert and part from the Great Eastern Desert where it borders Libya and Tunisia. The amount of fall was one of the largest seen here and equivalent that seen on 14 October 2001.

On the 20th a cold front with convective storms, associated with a low near the Balearic Islands, was lying along North African. The dust could have been raised along the North African continent between Morocco and Tunisia between 20 - 22nd February. Subsequently in a strong S'ly flow of air, generated between complex low pressure (980 mb) SW of Ireland and (993 mb) W of the Bay of Biscay and persistent high (1040 mb) over E Europe on the 22nd, was transported over Spain and W France to Anglesey.

During the same event dust was deposited in rain in Essex during the morning of the 26th and in the Wirral on the afternoon of the 27th. The fall of dusts was reported in the Times Newspaper on 5 April; " ... What made this episode even more interesting, and possibly unique, is that the dust came in two different colours, from different regions of the Sahara."

References

Diary entry on 25th and 26th February 2003.

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