The latest threat from Iceland's Eyjafjallajokul volcano caused two days of runway shutdowns and flight cancellations in the Republic of Ireland, Northern Ireland and Scotland, inconveniencing an estimated 100,000 travelers. Irish and British airlines launched extra services Thursday to help get them on their way.
The volcano has regularly belched out ash since its eruption began April 13, and European air authorities initially reacted April 14-20 by shutting down all air services in several countries to the east, stopping 100,000 flights and 10 million passengers.
This time, newly negotiated European safety rules restricted the aircraft grounding to Britain and Ireland and involved more precise closure orders based on how close the densest ash clouds were to airports' landing and takeoff paths.
Until Eyjafjallajokul stops its emissions, the key to the future course of Europe's ash crisis will be the prevailing winds that guide the ash clouds at altitudes of 10,000 to 20,000 feet (3 to 6 kilometers).
When the Atlantic winds blow to the northeast toward the unpopulated Arctic — the typical pattern in springtime — the danger to aircraft is minimized. But when they shift southward, as happened this week and in mid-April, airlines' ability to land and depart safely is jeopardized.
Ireland's major airlines largely resumed normal schedules by 9 a.m. (0800 GMT) Thursday, although Aer Lingus canceled 34 flights and Ryanair just four, chiefly involving airports in southwest Ireland where the ash was last to leave. Scotland's airports reopened earlier.
"For now Irish airports are open and will remain open for the remainder of the day," said Donie Mooney, operations manager at the Irish Aviation Authority.
Authorities maintained a cautious note because of the uncertainty of the winds. Forecasters said they expected the winds to shift to the northeast again Thursday and stay that way through the weekend — before they turn south again next week, putting the security of airports' landing and takeoff paths in doubt again.
The volcano, about 900 miles (1,500 miles) northwest of Ireland, has shown no signs of stopping since it began belching ash April 13. The glacier-capped volcano last erupted sporadically from 1821 to 1823.