Storm Ophelia left a distinctive red imprint on skylines across western Europe even as it headed north along the west coast of Ireland.
The hurricane, whose impact stretched over a vast area, whipped up sands from the Sahara desert and particles from wildfires in Spain and Portugal into the upper atmosphere. This darkened skies beyond its immediate path and gave them a distinctive pale red colour.
The sun shone red and the sky darkened to a foreboding orange and brown in many areas. Social media users shared pictures of ominous-looking clouds blocking out the sun, prompting London’s Science Museum to joke on Twitter: “It’s not the apocalypse!”
The strange reddish sky reported over parts of the United Kingdom may have appeared to some as a sign of impending apocalypse or a celestial Instagram filter, but it was a dust phenomenon and became a gigantic filter that reduced sunlight.
The effort was most dramatic in the London region but was seen from along the eastern coast of Ireland, Met Éireann forecaster Harm Luijkx confirmed.
The Irish weather forecasting service was most occupied, however, with the path of Ophelia as it turned into a subtropical storm and the associated winds speeds likely to cause destruction.
The red sky effect is "very spectacular and of topical interest, but nothing to worry about," Mr Luijkx added.
"Ophelia originated in the Azores, where it was a hurricane, and as it tracked its way northwards it dragged in tropical air from the Sahara," BBC weather presenter Simon King explained in trying to reassure viewers.
“The dust gets picked up into the air and goes high up into the atmosphere, and that dust has been dragged high up in the atmosphere above the UK.”
The red sky effect "captures people's imagination because it's so striking," said Dr Liz Coleman of NUIG School of Physics and Centre for Climate and Air Pollution Studies. Its dramatic effect arises because it scatters light so high in the upper atmosphere. Such events have occurred in Ireland in the past, including the deposit of dust from the Sahara, she said.
Saharan dust leads to deterioration in air quality, and so has human health implications, she said. It can also affect visibility, requiring for example streetlights to be turned on in daytime, which is considered a form of pollution.
“This hurricane was unusual because of its track north from the Azores. They normally go westwards. Our main interest is erratic climate behaviour, and the likelihood of more extreme events. That is the angle we are most concerned about,” she added.
Ophelia also pulled in unusually warm air from Spain and North Africa, which is why temperatures reached the early 20s over the weekend – at one point temperatures across much of Britain were more than 25 degrees, Mr Luijkx confirmed.
The same southerly winds that brought us the recent warmth also drew dust from the Sahara to our latitudes, and the dust scattered the blue light from the sun letting more red light through, much as at sunrise or sunset.