Iceland volcano: Ash cloud shuts down 500 flights, expected to affect Scandinavian airports Wednesday

High concentrations of volcanic ash from the Grimsvotn volcano in Iceland are wafting over northern parts of Britain and have forced the cancellation of 500 flights across Europe, the European air traffic center said Tuesday.

Eurocontrol also predicted more cancellations Wednesday as the cloud drifts toward Denmark, southern Norway and southwest Sweden. But the agency also expects the number of future flights affected by the cloud will be relatively low.

Airlines such as British Airways, KLM, Aer Lingus, Loganair, Ryanair and others that canceled flights Monday and Tuesday have been scrambling to keep passengers informed of operations using Twitter, Facebook and websites, telling passengers not to come to the airport if their flight has been canceled. Continental and United have issued fee waivers for travelers heading to, from or through airports in Ireland and Scotland. And SAS warned passengers that flights to and from the U.S. will take about an hour longer because of a change in routes over the Atlantic Ocean to dodge the cloud.

Photos: Huge volcano eruptions from the past

How exactly do airlines gauge the danger of the ash cloud? Britain’s Meteorological Office charts the direction of the ash plumes and issues forecasts on where they will spread. The highest concentrations (more than 4,000 micrograms per cubic meter) show up as red zones on forecast maps -- a signal that airspace may be closed because of hazardous conditions.

RyanAir, however, disputed the danger of red zones and the dire predictions. The Dublin-based budget carrier said in an online statement that a one-hour test flight from Glasgow to Inverness in Scotland found “no visible volcanic ash cloud or any other presence of volcanic ash.”

The airline asked authorities to reopen airspace over Scotland based on what it called a “verification flight.” But air officials in Britain say the test flight didn’t fly through the red zone. See the report below featuring Ryanair CEO Michael O’Leary.