BRUSSELS— A plume of volcanic ash snaked its way through southern France, Switzerland and northern Italy Sunday, shutting down airports and disrupting flights across Europe.


Weather forecasts said the ash cloud will gradually weaken as it spreads to southern parts of Germany, the Czech Republic and Austria by tonight. The ash, stretching from the surface up to 20,000 feet, has forced the closure of airports throughout much of northern Italy.


Separately, a finger of the main ash cloud – centered in the mid-Atlantic at altitudes of up to 35,000 feet – was still touching on parts of Portugal and Spain, affecting airports at Porto, La Coruna, Vigo, and Santiago.


The Irish Aviation Authority described the cloud as 2,100 miles long and 1,400 miles wide. It ordered Ireland’s five westernmost airports to close this afternoon. However Ireland’s three biggest airports in Dublin, Shannon and Cork were expected to stay open because the cloud is remaining off Ireland’s Atlantic coast.


Irish airline Aer Lingus apologized to its customers for a string of flight cancelations since Tuesday, when the ash threat returned to Irish air space after a two-week break. Its trans-Atlantic services to Boston and New York were operating today subject to delays.


“When the plume impacts on our air space, our first focus is to plot a different flight path to avoid canceling flights. However this is often unavoidable. When airports are closed for business, or flight paths are not available, we must unfortunately cancel flights,” Aer Lingus chief executive Christoph Mueller said in a statement on the airline’s website.


The disruptions to air traffic appeared minor compared to the five-day closure of European airspace last month, which forced the cancellation of over 100,000 flights, stranded passengers around the world and caused airlines direct losses of more than one billion euros.


Eurocontrol, the Brussels-based agency that coordinates air traffic control centers throughout the continent, said trans-Atlantic flights will continue to be diverted northward over Greenland to avoid the cloud stretching from Iceland to the Azores Islands.


It warned airlines to plan on taking on more fuel for the longer flight around the oceanic no-fly zone.
“Flights are required to make significant rerouting to avoid the area of ash cloud coverage,” a midday advisory said. “This is leading to some delays. However significant numbers of cancellations have not occurred.”


Eurocontrol said there would be approximately 24,500 flights within the European area, about 500 below average for a Sunday at this time of year. It said the ash cloud was expected to dissipate and that most of the closed airports were likely to reopen later today.


Brussels airport listed at least six flights bound for North America as either delayed or canceled, with only one flight – an American Airlines flight to New York – having departed.


In Geneva, dozens of flights to Portugal, Spain, France, Ireland, Britain and Hungary were canceled, while Zurich airport listed flights to Washington, Dublin and Porto as canceled.


Geneva airport is one of the main hubs for budget carrier easyJet. The airline warned passengersto expect further disruption to flights operating to and from Switzerland, southern and central France, northern Italy and northern Portugal.


Meteorologists say that until Eyjafjallajokul, the volcano in southern Iceland, stops erupting, the future course of Europe’s ash crisis will depend heavily on the prevailing winds. The eruption of the glacier-capped volcano has shown no signs of stopping since it began belching ash April 13. It last erupted from 1821 to 1823.