A huge yet invisible cloud of volcanic ash from Iceland spread southward over Europe and eastward into Russia on Friday at the speed of a car in city traffic, meteorologists said.
How far the volcanic ash spreads and when the microscopic dust will begin to disperse or settle on the ground depends entirely on two unpredictable events: How long the volcano beneath Iceland's Eyjafjallajokull (ay-yah-FYAH'-plah-yer-kuh-duhl) glacier keeps pumping tons of dust into the air and what wind patterns do.
The British Met Office, the country's weather agency, said the trajectory was taking the cloud at least over northern France and Austria and into eastern and central Russia.
"The current forecast is that the dust cloud will spread over quite a wide area by the early hours of tomorrow morning," said spokeswoman Helen Chivers.
Harry Geurts of the Dutch meteorological office KNMI said the cloud was moving southeast around 25 miles (40 kilometers) per hour from the North Sea, but normal cloud cover was making it difficult to track.
He said the direction and speed of the cloud is hard to predict because wind speeds vary at different altitudes.
Chivers said no major changes in the weather or wind speed were predicted for the next 24 hours.
The volcano, which erupted Wednesday for the second time in a month, was continuing to erupt daily in pulses rather than in a continuous stream of ash and smoke, she said. The ash cloud drifted between 20,000 to 30,000 feet (6,000 to 9,000 meters) high but was not a solid band of ash and particles.
It was invisible from the ground, and most of the dust will dilute and dissipate over time, but some ash was beginning to settle in parts of Scotland and England, lightly coating some of the Met's instruments, Chivers said.
Ash was also reported in rural Iceland, parts of Norway and the Faroe Islands.
A 2008 study by Italian scientists looked at the dispersal of ash from the Etna eruption in 1998 -- and highlighted the uncertainty of any predictions.
It said the trajectory of an ash cloud "is highly variable and can change within a few hours" in response to changes in the wind at various heights. It also depends on the size of the particles being thrust into the air. The smaller the grains of ash, the more concentrated it will remain in the air rather than fall to Earth.
Scientists are also looking for possible effects of the Icelandic eruption on climate change, the gradual warming of the Earth's average temperature caused by greenhouse gas emissions, mainly carbon dioxide from industry and tailpipes.
Major volcanic eruptions in the past were found to have had a temporary cooling effect on the planet. Scientists say an eruption in Peru in 1601 and a series of volcanos in the South Pacific later that century sent temperatures plunging.
But Chivers said the Icelandic eruption so far did not appear as powerful as those historical events, and Dutch meteorological professor Bert Holtslag said the ash cloud likely was too low.
"I expect that the impact of this volcano on global temperatures so far will be small because most of the material is apparently emitted and transported in the troposphere (below 6 miles or 10 kilometers)," said Holtslag of Wageningen University's Center for Water and Climate.
If the Icelandic volcano continues erupting and if the dust forms a layer in the stratosphere "” which is 6 to 30 miles (10 to 50 kilometers) high "” the "implications may be more significant," he wrote in an e-mail.