@reaganmart via Instagram

A layer of fog on a Monday morning is a regular sight for Londoners at this time of year, just before the real winter begins. But this week the fog has been so dense that flights were canceled not just in London, but also in other major western European cities, including Amsterdam and Frankfurt, the BBC reports. Travel warnings were issued for today as well, with authorities concerned that the fog could thicken overnight.

Londoners took some strange, beautiful pictures of their shrouded city:

On Sunday, just before the fog set in, the UK recorded its hottest-ever November day with temperatures reaching 72.3 degrees Fahrenheit in Wales. Earlier this year, the UK also recorded its hottest July day ever.