At Heathrow's Terminal 5, the queue to re-book cancelled flights with British Airways stretches almost the entire length of the terminal building.
One family, about three-quarters of the way up, said they had already been waiting an hour and a half.
They were due to fly to Frankfurt this morning but their flight was cancelled. They don't know whether they may be able to get on a flight tonight or have to wait until tomorrow morning.
Airport staff are moving through the queue handing out bottles of water.
Despite the disruption, and the huge number of frustrated passengers waiting for news, there is a surprising calm in the terminal as people wait patiently to discover if and when they will be able to leave.
More top news
Manchester United want Renato Sanches and Everton slap £50m price tag on Ross Barkley with Spurs interested.
Shoppers who don't bring their own bag will need to buy a bag for life costing 8p or 10p.
Temperatures will reach around 28 Celsius (82F) at their highest.