Dense fog is covering roads in the north and visibility is very poor.

浓雾笼罩了北部的公路，能见度很低。

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Widespread ice on untreated roads and "very poor visibility" during heavy snow showers would make driving conditions dangerous, the Met Office warned.

BBC: NEWS | UK | Temperatures 'will keep falling'

Underwater visibility in the Foyle is very poor at the best of times making identification difficult.

BBC: Experts to examine River Foyle, Derry, wreck

"Because of the poor visibility and high winds we ran a very real risk of going over the edge, " he said.

BBC: Pen y Fan rescue in blizzard conditions on Beacons