Even though some say it's been a massive overreaction by badly prepared safety regulators - one airline even claims the official scientific findings are simply wrong - hundreds of flights were canceled Tuesday as winds blew the cloud of ash from the Grimsvotn volcano over Scotland. Experts say that particles in the ash could stall jet engines and sandblast planes' windows.
The only comfort for frustrated passengers and airlines is that officials in Iceland said the amount of ash being released by the volcano was decreasing, and officials don't expect the disruption to be as bad as last year, when millions were stranded after the eruption of Iceland's Eyjafjallajokull volcano.
Nonetheless, British authorities said concentrations of ash in the skies over Scotland were high.
"All the data we are receiving confirms our forecasts, that there is high-density ash over Scotland," said Barry Grommett, spokesman for Britain's weather agency.
But Irish budget airline Ryanair challenged the results, saying it had sent its own airplane into Scottish airspace and found no ash in the atmosphere.
"Exactly as we predicted, we encountered absolutely no problems, Ryanair's chief executive Michael O'Leary told The Associated Press."There's no cloud over Scotland. There's no dusting of ash on the airframe or the wings. The airspace over Scotland should never have been restricted in the first place."
Still, Ryanair was forced by Irish authorities to cancel all 68 flights in and out of Scotland for the rest of Tuesday. Seven other airlines - most of them regional carriers - also grounded their Scottish flights.
The problem also affected Sweden, where 10 domestic flights were canceled Tuesday evening. The country's aviation officials say they expect medium concentrations of ash over its western coast late Tuesday, including over its second-largest city Goteborg.
In Edinburgh, meanwhile, several hundred passengers faced either a patient wait or overnight stays in the city. Among the crowds at the airport were soccer fans heading to Dublin for the international match between Scotland and Ireland.
The main international body representing carriers, the International Air Transport Association, complained to the British government about the way it had handled the issue, saying it should have had Cessna planes ready to carry out tests, instead of relying on the weather service.
U.K. airspace was not closed, but some airlines would rather not take risks and were willing to follow official advice. EasyJet had 113 cancelations Tuesday in and out of Scotland, Newcastle and Northern Ireland. British Airways grounded 92 flights in total, and Dutch airline KLM canceled a total of 42 flights. A spokesman for Aer Lingus, said it had canceled 22 flights between Ireland and Scotland.
The Grimsvotn volcano began erupting on Saturday, sending clouds of ash high into the air. The amount of ash spewing from the volcano tapered off dramatically on Tuesday, however, said Elin Jonasdottir, a forecaster at Iceland's meteorological office. She added that because the plume has decreased in height - it's now at about 16,000 feet - the ash won't travel far and will most likely fall to the ground near its source.
Earlier there were fears that the ash cloud might also affect France, but French civil aviation authority DGAC on Tuesday sounded an optimistic note.
"Forecasts for the next few days are promising, French airspace should be affected only very marginally by the volcanic ash," it said in a statement. "No closure of French airspace is currently envisioned."
The ash cloud forced President Barack Obama to shorten a visit to Ireland on Monday, and has raised fears of a repeat of huge travel disruptions in Europe last year.
Last year, European aviation authorities closed vast swaths of European airspace as soon as they detected the presence of even a small amount of volcanic ash in the atmosphere. This year, they are trying a more sophisticated approach.
Aviation authorities will give airlines detailed information about the location and density of ash clouds. Any airline that wants to fly through the ash cloud can do so if it can convince its own national aviation regulators it is safe to do so.
The closures are already affecting travel plans across Europe. Officials at Spanish soccer team Barcelona, which plans to travel to London on Thursday for Saturday's Champions League final against Manchester United at Wembley Stadium, say they are monitoring the ash cloud disruption and could change their departure date.