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 immediately Wednesday but its eastern neighbor, Edinburgh, stayed open until midday. While Dublin was gridlocked, services at the Irish Republic's other international hub to the west, Shannon, didn't plan to stop until after 5 p.m. (1600 GMT). And Irish authorities said it appeared likely that the country's two most southwesterly airports in Cork and Kerry would miss the ash threat entirely.
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.
Market-leading airline Ryanair sought to discourage the passengers' dashing from airport to airport by announcing blanket closures through the rest of Wednesday at Glasgow and Edinburgh in Scotland; Northern Ireland's airports in Belfast and Londonderry; and both Dublin and Knock Airport in western Ireland.
Ryanair also warned customers planning to fly out of several airports in the west and north of England -- Bristol, Leeds, Liverpool, Manchester and Newcastle -- to check the company's Web site and remain alert for possible closure announcements.
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.
Iceland's Eyjafjallajokul volcano, about 900 miles (1,500 miles) northwest of Ireland, began disgorging tons of ash April 13 and has shown no signs of stopping. The glacier-capped volcano last erupted sporadically from 1821 to 1823.
Travel experts warned that Ireland was particularly vulnerable to summertime disruption if Eyjafjallajokul doesn't calm down. "If Iceland has the wrong kind of geology, Ireland has the wrong kind of geography. It's too close to Iceland and is dependent on air travel," said tourism industry analyst Simon Calder.
Last month European authorities canceled 100,000 flights affecting 10 million passengers as they sought to craft a plan for managing the ash menace. The groundings cost the aviation industry billions in lost business. EU rules also require the airlines to cover the hotel and food expenses of stranded passengers who stay put to wait out the delays.