There are a total of nine Volcanic Ash Advisory Centers (VAACs) around the world. The London center is run by Britain's Met Office, the country's national weather service.

The other centers are in Toulouse in France, Anchorage, Alaska, and Washington DC in the US, Montreal in Canada, Darwin in Australia, Wellington in New Zealand, Tokyo in Japan and Buenos Aires in Argentina.

The Volcanic Ash Advisory Centers are part of an international system set up by the International Civil Aviation Organization (ICAO).

Responsibilities

The centers analyze and publish observations of volcanic ash from volcano observatories and other organisations, satellites and aircraft in flight.

They also issue warnings and, since the mid 1990s, publish volcanic ash advisory messages, identifying areas of volcanic ash and their predicted movements.

The London center is responsible for monitoring and forecasting the movement of volcanic ash over the United Kingdom, Iceland and the north-eastern part of the North Atlantic Ocean.

Although this is a relatively small area, it includes some of the busiest airways in the world.

As millions of stranded passengers have now experienced at first hand, a volcanic eruption in Iceland can quickly affect a large area of airspace, as strong winds spread the ash from the volcano.

In such an event, the London VAAC liaises closely with the Icelandic weather service, which is in close contact with the Nordic Volcano Institute.

The data the center collects comes from seismic surveys carried out by the Nordic Institute, eyewitness accounts of current volcanic activity and from analyses of satellite pictures.

ng (based on information from the British Met Office)

Editor: Susan Houlton