London’s Heathrow airport was forced to cancel some departures on Monday amid thick fog and freezing temperatures.

There may be “minor changes” to the schedule as a result of poor visibility at the airport as well as across the southeast of England, a Heathrow spokesperson said in an email. Airlines are working to rebook passengers on alternative services later in the day, with the fog expected to begin clearing at 10 am. 

About 30 flights, or 5% of Heathrow’s departures, were canceled, with British Airways being the most affected, according to data from tracking website FlightAware.