Dublin ? Iceland’s clouds of volcanic ash are menacing European air traffic again, but transport chiefs insisted Tuesday they are learning from last month’s crisis and won’t let the hard-to-measure emissions ground their continent again.

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. today.

The British authority said Tuesday 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 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.