There have been long delays on UK roads because of thick fog across the UK.
Lots of flights have been cancelled or delayed at many airports too in the UK and across Europe.
You've been sending your pictures into the Newsround website. Here's some of the best.
Lewis, who's 10 sent this foggy view from his house in West Sussex.
Amy, who's 9, sent in this picture of her street in Coventry
Fred from Hertfordshire says this is how his back garden looked for most of today.
One school in north London sent this picture of the misty, spooky fog that took over their playing field.
But yesterday was the hottest November day ever recorded in Britain.
The hottest place was in mid Wales.