Eurocontrol's Brian Flynn explains how the ash cloud is being monitored as passengers wait at airports in northern Europe.
Thousands of passengers have had their flights cancelled because of drifting ash from an Icelandic volcano.
Airports affected include Londonderry, Glasgow, Edinburgh, Prestwick, Durham Tees Valley, Newcastle and Carlisle, air traffic services company Nats said.
The UK's emergency response committee, Cobra, met to discuss the knock-on effects of the ash cloud.
Air traffic management body Eurocontrol said about 500 flights were cancelled across Europe on Tuesday.
UK air traffic control service Nats said the ash cloud would continue to affect flights from some airports in Scotland and northern England from 1900 BST on Tuesday until 0100 BST on Wednesday.
http://www.bbc.co.uk/news/uk-13513981
Note: 24 May 2011 Last updated at 15:51 GMT