Usually, the prevailing south-westerly wind would take the ash cloud away from Europe and over the polar region, but a high-pressure area is bringing it over Europe, where it is sitting and barely moving as the ash spreads.

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According to Dr Elsner, this change is related to sudden shifts in the position of the long-term area of high pressure that sits near the Azores.

ECONOMIST: Eye on the storm | The

With high pressure now established up from the southwest in the eastern North Atlantic - and an area of low pressure being held further west of Ireland - this year's festival continues on a settled and increasingly warm note.

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If the high pressure area expands to the south or the south-west, storms can be pulled to the west, hitting Mexico and the northern Gulf coast.

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