Ireland's key hub, Dublin Airport, admitted defeat for the day and canceled all flights until midnight. More than a dozen other airports throughout the Republic of Ireland, Northern Ireland and Scotland announced shorter closure periods as unseasonal winds pushed the engine-wrecking ash southwest back towards the Atlantic rather than northeast into the unpopulated Arctic.
The renewed volcanic-ash threat in the skies of Britain and Ireland this week, following a two-week lull, has tested the more precise safety rules adopted by European aviation authorities following the unprecedented April 14-20 closure of most northern European airspace.
Britain's Civil Aviation Authority said Wednesday's ash threat might reach northwestern England and Wales but would miss the four major airports of London.
Authorities are seeking to stop flights only when the ash reaches certain density levels and gets within 60 miles (100 kilometers) of an airport's path for landings and takeoffs — a stark contrast to last month's closures of air services throughout several countries.
In Scotland, Glasgow Airport shut Wednesday but its eastern neighbor, Edinburgh, planned to stay open until the afternoon. Similarly in Northern Ireland, Belfast's two airports kept operating but the main airport to the west, Londonderry, was closed. And while Dublin was gridlocked, operations continued normally at Ireland's other two main airports to the west in Shannon and Cork. All those currently open expected to shut later in the day, but others expected to reopen by the evening.
The rapidly changing situation obliged would-be fliers to hop on trains, buses and taxis to reach nearby airports. Virgin Trains also said it was offering extra services Wednesday between Scotland and London.
Aviation chiefs in Ireland and Britain said they were updating their closures and reopenings within minutes of receiving updated ash maps every six hours from the Volcanic Ash Advisory Center in England.
Donie Mooney, operations director at the Irish Aviation Authority, said the volcano's emissions changed over the past few days and caught forecasters off guard, forcing Ireland to abandon its hopes of staying open Wednesday.
"The ash plume has been going higher and the ash is of a coarser nature. That threw our projected opening times into some disarray," he said.
Still, the ash clouds are remaining below 20,000 feet (6 kilometers), far lower than the cruising altitude of passenger jets, therefore they pose a danger only to ascending or descending aircraft.
In Iceland, authorities said poor weather Wednesday was obscuring their view of the volcano and preventing coast guard aircraft from flying over the volcano. Civil Protection Coordination Office official Agust Gunnar Gylfason said the volcano's seismic activity has been unchanged in recent days.