Following is an updated factfile on Eyjafjoell, the volcano in southern Iceland that began to disrupt air traffic in Europe last Wednesday.
WHAT'S THE LATEST ON ITS ACTIVITY?
The volcano is emitting less ash, according to the World Meteorological Organisation (WMO) and Icelandic experts.
"We have received information that the eruptions had changed recently, with more molten lava flow and the remaining plume is now only reaching less than 3,000 metres" (10,000 feet), the WMO said in a press release issued mid- Tuesday.
"The whiteness of the plume furthermore suggested that it contains mainly steam and little ash."
"There is less ash. It's getting easier for Europe," said Gudrun Nina Petersen of the Icelandic Meteorological Office.
University of Iceland geophysicist Sigrun Hreinsdottir said a flight around the volcano on Monday saw magma splatter up from the cone. "What this means is that water is not getting as readily into contact with magma, which means... we should be seeing less ash in the air."
WHAT ABOUT THE ASH CLOUD?
The ash cloud remains a problem because it is being held in place by a high pressure (also called anti-cyclonic) system with weak winds, says the WMO. Because the ash is so fine, it takes a long time for the particles to descend, especially those above 6,000 metres (20,000 feet).
Things will change "towards the end of the week," when a low pressure system is likely to develop over Iceland, the agency says.
This will change the winds and push the ash towards the Arctic and also bring rains that will wash out ash at lower altitude levels.
HOW BIG ARE THE PARTICLES?
What is being called volcanic ash is in fact a mixture of small, jagged pieces of rocks, minerals and volcanic glass the size of sand and silt, less than two millimetres (one-twelfth of an inch) across. The smaller particles, which linger longest in the atmosphere, typically have a diameter of between one and 40 micros (about one-thousandth of an inch).
"Volcanic ash is hard, does not dissolve in water, is extremely abrasive and mildly corrosive," says the WMO.
WHAT ABOUT THE FUTURE?
If the past is a guide, explosive activity at Eyjafjoell could be followed by a "quieter, more effusive phase," suggested John Davidson, a professor at Britain's Durham University.
The last time the volcano erupted was over 13 months, from 1821-1823, but intermittently. Its worst activity was at the start, and followed by eruptions lasting a week, followed by pauses of two to three weeks, said Hreinsdottir.
"It's a volcano that hasn't erupted a lot so we have had limited experience with it," she cautioned.
The 1821-3 eruption was crowned by the eruption of a far bigger, neighbouring volcano, Katla.
However, there is no sign of any activity at Katla, and in any case ash from that volcano could easily be less fine (and thus less likely to travel long distance) compared with the present ash cloud, said Hreinsdottir.