Rising volcanic activity spurred aviation authorities in Ireland, northwest Scotland and the Faeroe Islands to shut down services Tuesday after a two-week hiatus. Their airports reopened several hours later, once the densest ash clouds had passed over their airports and back over the Atlantic.
But soon a new wave of engine-damaging ash was approaching British airspace, forcing Britain's Civil Aviation Authority to announce that airports in Scotland and Northern Ireland had to cancel all services indefinitely, beginning at 7 a.m. (0600GMT) Wednesday.
The British authority said its forecasters had determined that ash in United Kingdom airspace "has increased in density." It said the prevailing winds would probably continue to push the threat southward, "potentially affecting airports in the northwest of England and North Wales tomorrow" -- but missing the key European air hubs in London.
Earlier, travelers and transport chiefs alike said Europe was learning to pinpoint the true nature of the threat versus last month's better-safe-than-sorry shutdown of air services for nearly a week in several countries. Airline and airport authorities branded that response overkill; it grounded 100,000 flights and 10 million passengers and cost the industry billions.
European Transport Commissioner Siim Kallas emphasized that, had last month's sweeping safeguards been imposed Tuesday, "a very large part" of Europe would have lost its air links again -- and for days, not hours.
Kallas and transport ministers from across the 27-nation European Union agreed Tuesday at an emergency meeting in Brussels to press ahead on plans to unify their divided air-traffic-control networks, research new ways to identify and measure radar-invisible ash clouds, and legally define safety standards for specific makes of jet engines and the airline industry as a whole.
"We want to give top priority to those measures which will accelerate the setting up of the single European sky," Kallas said.
But government and aviation officials from Ireland couldn't attend in person because their airports were shut. They warned that Iceland, some 900 miles (1,500 kilometers) to the northwest, could keep spewing untold tons of engine-destroying ash into air space indefinitely and could keep disrupting in Ireland, Britain and Scandinavia this summer.
"There's no doubt about it, we're probably facing a summer of uncertainty due to this ash cloud," said Eamonn Brennan, chief executive of the Irish Aviation Authority, who foresaw the potential for sporadic shutdowns dependent on the whims of prevailing winds.
Too often so far, the ash from Iceland's Eyjafjallajokul volcano has ended up traveling with unseasonal winds straight east or southeast into Europe rather than northeast to the uninhabited Arctic, the typical path in springtime.
Irish Transport Minister Noel Dempsey said Tuesday's closure of Irish airspace "emphasizes the need for a strong European response and action plan to deal with this situation as it continues to evolve."
Iceland's Institute of Earth Sciences said Eyjafjallajokul -- which erupted April 13 after a 177-year slumber -- has experienced increased seismic activity since Sunday and its ash plume has risen to nearly 5.5 kilometers (18,000 feet) in altitude. The last time it erupted, in 1821, its emissions ebbed and flowed for two years.
At Dublin Airport, passengers said they doubted whether aviation chieftains could effectively cork the Iceland ash threat soon. Some turned their anger on Irish airlines for allegedly taking advantage of their misfortunes to gouge them on emergency-rebooked flights.
"We only got married on Saturday and a wedding is a lot of stress, so this was the last thing we needed," said Maria Colgan, standing beside her husband Brian Halligan after they sped to Dublin Airport and paid euro600 ($790) to catch the last Aer Lingus flight Monday out of Dublin to London.
The couple, both 30, felt they had no choice but to shell out because their honeymoon in Barbados required them to make a Tuesday connection in London.
"The ash isn't our fault. Aer Lingus could work with people like us, but they aren't interested, charging us full whack for flights to London," she said.
But most passengers camped out Tuesday during the shutdown appeared resigned to a dawning reality of uncertain air bookings. Many applauded the authorities' more selective shutdown Tuesday as a sign of improving systems -- and literally applauded as the Dublin departures terminals began listing takeoff times again.
"Ireland's an island. We're kind of stuck with air travel, for better or worse," said Elaine McDermott, 23, who lost her early flight to Paris -- to attend a college friend's wedding -- but found herself boarding a replacement service eight hours later.
"I'll make it to the church on time," she said with a relieved smile once her backpack had been checked.
Weather forecasters and geologists agreed that the prognosis for Europe's harried air travelers should improve starting Thursday.
Irish meteorologist Evelyn Cusack said winds were expected to resume their typical northeasterly direction, pushing ash into the Arctic and away from Europe's airports.
And Brian Flynn, deputy director of operations at the Brussels air safety agency Eurocontrol, said the ash was not reaching altitudes that could threaten aircraft in mid-flight, only those that were climbing after takeoff or descending to land. This greatly limits the actual air corridors at risk, he said.
"This time the volcano is much less active" than during the April 14-20 shutdowns, Flynn said.
The Irish Aviation Authority said the risk of further shutdowns in the Republic of Ireland before midday Wednesday was "minimal."