People across parts of the UK had their coldest November night since 2010 on Wednesday - and have three more days of snow and ice warnings ahead of them.
In lower-lying Prestwick, in Ayrshire, the overnight temperature fell to -5.5C (22F) and dropped to a chilly -6.1C (21F) in the Cumbrian town of Keswick.
Freezing fog is making travel treacherous in parts of the Midlands, north-west England and Northern Ireland, and could develop elsewhere.
The Met Office issued weather warnings for snow and ice from 17:00 GMT on Wednesday for parts of northern and eastern Scotland, north-east England and Yorkshire.
In the capital, Sadiq Khan activated a "severe weather emergency protocol" for London - measures designed to protect rough sleepers, with temperatures expected to drop below freezing overnight.
The amber alert indicates that the cold weather is likely to affect the whole health service, with the potential for the entire population to be at risk, the agency says.
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Yay, I love it when we have recent-record-breaking temperatures in both the summer and in the winter! Makes me feel so warm and fuzzy about all those burning fossil fuels. /s