LONDON — Ash clouds from Iceland's spewing volcano disrupted air traffic across Northern Europe on Thursday as authorities closed British and Nordic air space, shut down Europe's busiest airport at Heathrow and canceled hundreds of flights.Britain's Civil Aviation Authority said non-emergency flights would be banned until at least 6 p.m. (1700 GMT, 1 p.m. EDT). Irish authorities also closed their air space for eight hours.London Heathrow, Europe's business airport, handles upwards of 1,200 flights and 180,000 passengers per day. The closure also affected London's second- and third-largest airports, Gatwick and Stansted. It was not immediately clear when flights would resume.With the major trans-Atlantic hub at Heathrow closed, dozens of flights to the United States were on hold, and cancelations spread across the continent to major hubs at Brussels, Amsterdam, Geneva and Paris, where flights heading north were canceled until midnight.In Iceland, hundreds of people have fled rising floodwaters since the volcano under the Eyjafjallajokull (ay-yah-FYAH'-plah-yer-kuh-duhl) glacier erupted Wednesday for the second time in less than a month. As water gushed down the mountainside, rivers rose up to 10 feet (3 meters) by Wednesday night.The ash cloud has not disrupted operations at Iceland's Keflavik airport or caused problems in the capital of Reykjavik, but has affected the southeastern part of the island, said meteorologist Thorsteinn Jonsson. In one area, visibility was reduced to 150 yards (meters) this morning, he said, and farmers were advised to keep livestock indoors to protect them from eating ash particles as sharp as glass.The volcano was sending up smoke and ash that posed "a significant safety threat to aircraft," Britain's National Air Traffic Service said, as visibility is compromised and debris can get sucked into airplane engines.Emirates airline canceled 10 roundtrip flights between Dubai and Britain on Thursday because of the ash cloud."I think I might cry," said Ann Cochrane, 58, of Toronto, one of the passengers stranded in Glasgow. "I just wish I was on a beach in Mexico."In northern Sweden all air traffic was suspended, affecting the cities of Skelleftea, Lulea, Kiruna and Hemavan, the national aviation authority said.Air traffic in northern Finland was also halted.The U.S. Geological Survey said about 100 encounters of aircraft withvolcanic ash were documented from 1983 to 2000; in some cases enginesshut down briefly after sucking in volcanic debris, but there have beenno fatal incidents.In 1989, a KLM Royal Dutch Airlines Boeing 747 flew into an ash cloudfrom Alaska's Redoubt volcano and lost all power, dropping from 25,000feet to 12,000 feet (7,500 meters to 3,600) before the crew could getthe engines restarted. The plane landed safely.Volcanic ash is formed from explosive eruptions. Particles as hard as aknife blade range in size from as small as 0.001 millimeters (1/25,000inch) to 2 millimeters (1/12 inch), the Geological Survey says.Ash can melt in the heat of an aircraft engine and then solidify again, disrupting the mechanics, the agency says.