Volcanic ash sifted down on parts of northern Europe on Friday and thousands of planes stayed on the tarmac to avoid the hazardous cloud. Travel chaos engulfed major European cities and the U.N. warned of possible health risks from falling ash.

Eurocontrol, the European air traffic agency, said the travel disruptions that reverberated throughout the world on Thursday were even worse on Friday, with about 11,000 flights expected to operate in Europe instead of the usual 28,000. It said delays will continue well into Saturday as the massive yet invisible ash cloud moves slowly south and east.

“There will be significant disruption of air traffic tomorrow,” spokesman Brian Flynn said, adding the agency would hold a meeting Monday of aviation officials from all 40 Euro-control countries.

Polish officials fretted that the ash cloud could threaten the arrival of world leaders for Sunday’s state funeral of President Lech Kaczynski and his wife, Maria, in the southern city of Krakow. Kaczynski’s family insisted on Friday they wanted the funeral to go forward as planned.

So far, President Barack Obama, Russian President Dmitry Medvedev and German Chancellor Angela Merkel are among those coming and no one has cancelled.

Ms. Merkel, however, could not even get back to Berlin after a visit to Washington. She was diverted to Madrid and looking for a hotel room.

Train stations, hotels and car rental agencies were jammed in key European cities by people scrambling to make alternative plans. Extra long-distance trains were put on in Amsterdam and lines to buy train tickets were so long the train company was handing out free coffee.

The high-speed Thalys trains, a joint venture of the French, Belgian and German rail companies, allowed passengers to buy tickets even if there were not enough seats.

“We think we can help a lot of passengers get closer to their final destinations,” said Thalys spokeswoman Patricia Baars.

Aviation experts said it was among the worst disruptions Europe has ever seen.

“We don’t have many volcanoes in Europe,” said David Learmount of Flight International, an editor at the aviation publication. “But the wind was blowing in the wrong direction.”

Ice chunks the size of houses tumbled down from a volcano beneath Iceland’s Eyjafjallajokull (ay—yah—FYAH’—plah—yer—kuh—duhl) glacier on Thursday as hot gases melted the ice. The volcano began erupting on Wednesday for the second time in less than a month.

As torrents of water roared down the steep slopes of the volcano, flash floods washed away chunks of Iceland’s main ring road. More floods are expected as long as the volcano keeps erupting, which scientists said it was continuing to do in daily pulses.

The cloud of basalt, drifting between 20,000 to 30,000 feet (6,000 to 9,000 meters) high and invisible from the ground, at first blocked the main air flight path between the U.S. east coast and Europe. On Friday, the British Meteorological Office said the cloud’s trajectory was taking it over northern France and Austria and into eastern and central Russia at about 25 mph (40 kpm).

Fearing that microscopic particles of highly abrasive ash could endanger passengers by causing aircraft engines to fail, authorities shut down air space over Britain, Ireland, France, Denmark, Norway, Sweden, Finland and Belgium. That halted flights at Europe’s two busiest airports - Heathrow in London and Charles de Gaulle airport in Paris - as well as dozens of other airports, 25 in France alone.

A Finish F-18 Hornet jet had a scare, overheating even on a short flight as the ash blocked its cooling ducts. Air Force spokesman Joni Malkamaki says the Hornet “flew for about an hour” on a regular training flight in clear weather and the pilot saw no signs of any volcanic cloud.

As the cloud moved east, flights were halted Friday at Frankfurt airport, Europe’s third-busiest terminal, and at 10 other German airports including Duesseldorf, Berlin, Hamburg and Cologne. No flights were allowed either at the Ramstein Air Base, a key U.S. military hub in south-western Germany.

Only about 120 trans-Atlantic flights reached European airports on Friday morning, compared to 300 on a normal day, said Euro-control. About 60 flights between Asia and Europe were cancelled.

Air space restrictions were lifted or imposed or extended as the cloud moved east and south. Aviation authorities in Ireland reopened airports in Dublin and Cork and France allowed some planes to land at Paris’ three airports on Friday afternoon.

Sweden and Norway declared skies in the far north to be safe again even as flights in both capitals - Stockholm and Oslo - were still on a lockdown. Norwegian Prime Minister Jens Stoltenberg managed to get a flight to Madrid from New York but was still not sure when or how he would get home.

Switzerland, Croatia and Slovakia closed their airspaces and Poland expanded its no-fly zone on Friday to most of the country, excluding the southern cities of Krakow and Rzeszow.

Britain and Belgium extended flight restrictions until Saturday morning, but Britain allowed some flights out of Northern Ireland and western Scotland.

Britain’s Met Office said the wind was expected to blow from the north, which would bring further ash across parts of Britain. Small amounts of ash settled in Iceland, northern Scotland and Norway.

Professor Jon Davidson of the Department of Earth Sciences at Durham University in England said the dispersal of the ash cloud depended on the weather - and if the volcano still erupted.

The World Health Organization in Geneva said the ash cloud mostly remained high in the atmosphere on Friday, but it could pose a health risk if particles reached the ground. It advised Europeans to try to stay indoors if the ash fell, because inhaling the particles can cause respiratory problems, especially for those suffering from asthma and respiratory diseases.

The volcano caused ministers and officials from at least 12 countries to miss the start of a European Union finance ministers meeting in Madrid.

Iceland, a nation of 320,000 people, sits on a large volcanic hot spot in the Atlantic’s mid-oceanic ridge, and has a history of devastating eruptions. One of the worst was the 1783 eruption of the Laki volcano, which spewed a toxic cloud over Europe with devastating consequences.

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