U.S. meteorologists based in Maryland say huge ash cloud from Mount Pinatubo in the Philippines have passed over Vietnam and Thailand and have been tracked by satellite as far as the Bay of Bengal, 1,600 miles west southwest of the erupting volcano.
Aircraft in the area are being warned to avoid the highly abrasive volcanic clouds, which can cripple jet engines, obscure pilots' windshields and silence their radio and navigation gear.
The first two major eruptions this week sent the ash clouds as high as 60,000 feet - more than 11 miles up.
"The third of the eruptions probably went higher," said Jerome Heffter, a research meteorologist at the National Oceanographic and Atmospheric Administration's Air Resources Laboratory in Silver Spring.
Some news reports said the cloud reached as high as 80,000 feet, 15 miles up and well into the stratosphere.
Heavier particles are falling out of the cloud through lower altitudes and eventually reaching the surface.
Lighter ash below 35,000 feet has stayed over the South China Sea, Heffter said. But debris blown up beyond 35,000 feet have been swept up by faster winds and are drifting off toward the west.
James S. Lynch, meteorologist with the National Environmental Satellite and Information Service in Suitland, said the ash clouds have drifted west from the volcano in three distinct "puffs" since Tuesday. The most concentrated portions are each 150 miles long by 100 miles wide and clearly visible from space.
"The first plume ... from the very first eruption on Tuesday evening Eastern Daylight Time, has moved across Cambodia and is now moving over the Gulf of Thailand into the Bay of Bengal," Lynch said.
Ash from the second eruption, at 11 am [Wednesday] is now approaching the Vietnam coast, he said. And ash from the most recent eruption, late Wednesday, is over the South China Sea midway between the Philippines and Vietnam.
"And all three [clouds] are following the exact same track, west southwest toward the Indian Ocean," he said.
The clouds are being photographed twice a day using both visible and infrared light by the National Oceanographic and Atmospheric Administration's NOAA 11 satellite, a polar orbiter that passes over the Philippines twice a day.
Three other NOAA weather satellites were being rescheduled today (Friday) to transmit more pictures of the volcano's ash plume, adding six more daily photographs.
NOAA is also sharing images taken hourly by the Japanese Geostationary Meteorological Satellite.
Volcanic ash is made of a glassy silicate compound that may be invisible to pilots, but can cause catastrophic damage to aircraft.
Intensive monitoring of volcanic ash clouds was organized after ash from the Redoubt volcano in Alaska caused a Dutch KLM airliner to lose all four of its engines during a flight to Alaska in December 1989.
The Boeing 747 jumbo jet, with 245 people on board, plunged 30,000 feet before reigniting two of its four engines, said NOAA spokesman Frank Lepore. It pulled out of the dive at 5,000 feet and landed safely.
"When [the ash] hits a super-hot surface like a jet engine, it essentially coats the turbine blades with glass. If you get enough of that, it can stall the engine," Lepore said. The ash also gets drawn into the cooling systems of aircraft electronic gear, disabling it.
"The material is also very abrasive, and it would literally etch the glass on the forward windshield and obscure the pilots' vision," Lepore said.
Since Mount Pinatubo began erupting early this week, NOAA, the Federal Aviation Administration and Filipino authorities have all issued advisories to international aviation interests alerting them to the locations of the ash clouds.
The erupting Mount Unzen volcano in Japan so far has blown little of its ash higher than about 7,000 feet, Lynch said. It has therefore not become a hazard to international aviation and NOAA has not yet had to track its ash cloud.
If volcanic ash is blown high enough, it can travel extremely long distances, Heffter said, and NOAA must track the clouds for as long as they remain a hazard.
The Redoubt volcano had 29 significant eruptions over five months, he said, and NOAA accurately predicted ash clouds as far away as central Texas, 4,000 miles from the eruption itself.
Airborne volcanic ash can have spectacular effects on sunsets far from the eruption, and the most violent eruptions in history have lowered global temperatures.
Asked whether Mount Pinatubo's ash might eventually drift around the globe and reach the U.S., Heffter said, "I suspect it probably will ... I'm sure [scientists] will see evidence of this in the future if they're looking for particulates in the atmosphere."
He said it is impossible to predict now whether the ash might be detectable by anyone other than scientists.