Contrails over Buckingham Palace during the meeting of President Obama with the Royal family today provide visible signs that the ash from Iceland’s Grímsvötn volcano is proving far less disruptive than last year’s eruption of Eyjafjallajökull. Even so, hundreds of flights were canceled today leaving thousands of passengers in airline limbo.

To avoid the oncoming plume, last night President Obama left Ireland aboard Air Force One for London earlier than planned. He is scheduled to visit Poland and attend the G8 meeting in Deauville, France, before returning to the United States. On Sunday he will meet with residents of the tornado-struck city of Joplin, Mo.

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During last year’s volcanic crisis over 100,000 flights were grounded and over 10 million people affected. A no-risk policy at the time set a safety limit that banned flights if only 200 micrograms (0.0002 grams) of ash per cubic meter of air wafted over airports. Now the threshold for most airplanes is 4,000 micrograms (0.004 grams) per cubic meter, Philip Hammond, the English transport secretary, told the Guardian.

As of this morning, that threshold had spread over Scotland, but forecasts indicate the plume will scatter as winds shift it further south and east.

The change in policy marks an increased preparedness for ash emergencies, but the awareness of ash risk to planes is still under investigation. Some argue the strategy is creating more chaos than needed.

“At the moment, here in Europe, the ‘no fly’ zones are too big and much too cautious,” wrote atmospheric scientist Fred Prata of the Norwegian Institute for Air Research in an email to Discovery News. “Helicopters in northern Norway have been grounded because of a plume in the stratosphere that is composed entirely of SO2 [sulfur dioxide] gas.”

Following last year’s Eyjafjallajökull eruption, EasyJet airlines agreed to test on-board air sensors meant to warn pilots of approaching ash clouds. But the technology, which Prata created, is not yet in place.

In December 2010, the International Civil Aviation Organization (ICAO) identified the low (teal), medium (gray), and high (red) concentration rates used to map plume levels. As part of the end to the Eyjafjallajökull crisis last year, on April 21, 2010, flying through an atmosphere that contained less than 2,000 micrograms of ash per cubic meter of air was deemed safe.

Following Saturday’s eruption of Grímsvötn, the UK Civil Aviation Authority announced Monday that “any UK airline wishing to operate in areas of medium or high density ash, will need to have a safety case accepted by the CAA. Many airlines already have such safety cases in place and agreed for medium density. None has so far submitted a safety case to operate in high density ash.”

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Ash clouds are not soft powdery particles of burnt cinder floating in the atmosphere, but rather hard, sharp, and abrasive rock and mineral particles. At some point between no risk and some risk, a plane incurs damage that may require repairs or potentially lead to an earlier retirement of the plane down the road. Though it is difficult to judge as no engine has been built yet with a certified maximum ash tolerance level. The European Aviation Safety Agency is expected to formally request manufactures begin providing such data in 2012.

Currently, all airlines still consider flying through more than 4,000 micrograms of pulverized particles of stone and glass per cubic meter enough of a liability to require diverting or canceling flights.

But as EASA acknowledged in its April 12, 2011, proposal to place the burden of proof on the jet engine manufactures, establishing a one-size-fits-all threshold for atmospheric ash is unlikely anytime soon:

The European Aviation Safety Agency has been working on the issue of determining a threshold for volcanic ash since it was mandated to do so by the Transport Council conclusions in May 2010. The work to establish a single ash threshold is proving to be extremely challenging. It has become increasingly clear that in reality, every volcanic ash crisis may be different and the nature of the ash will be different. So establishing a “one size fits all” threshold is not viable in the short term. EASA will keep working on this issue, but it will certainly not be resolved in the short term. To note, this kind of threshold does not exist anywhere else in the world – even in areas like America, or Southeast Asia where there is a longer experience dealing with these issues – so it is not possible to adapt something from elsewhere in the world.

But while the safety threshold is still under debate, the danger threshold has long been known. In 1989 the amount of ash over Alaska’s Redoubt volcano was 2 grams (2 million micrograms) per cubic meter, Marianne Guffanti of the U.S. Geological Survey, told Discovery News. Considering that planes can travel about 15 km (8 nautical miles) per minute, that much atmospheric debris can quickly accumulate to dangerous levels inside an engine. Just 10 hours after the eruption, the Redoubt ash plume caused all four engines of a brand new 747 jetliner to flare out. After a five minute drop from 27,900 feet to 13,300 feet, the pilots managed to restart the engines and land in Anchorage. The cost of repairs came to $80 million, including the replacement of all four engines.

IMAGE 1: Queen Elizabeth II, US President Barack Obama, Camilla, Duchess of Cornwall, Prince Charles, Prince of Wales, First Lady Michelle Obama and Prince Philip, Duke of Edinburgh listen to the US National Anthem during a ceremonial welcome in the garden of Buckingham Palace on May 24, 2011 in London, England. The 44th President of the United States, Barack Obama, and his wife Michelle are in the UK for a two day State Visit at the invitation of HM Queen Elizabeth II. During the trip they will attend a state banquet at Buckingham Palace and the President will address both houses of parliament at Westminster Hall. (Photo by Christopher Furlong/Getty Images)

IMAGE 2: A graphic from the European Organisation for the Safety of Air Navigation (Eurocontrol) with data showing the concentration of the volcanic ash above England at a height of 6,000 meters (20,000 ft.) on May 24, 2011, at 6am UTC. The colored marking complies an ash concentration of 200-2000 micrograms (light blue), 2000-4000 micrograms (gray), and more than 4000 micrograms of ash per cubic meter of air. The volcanic ash of the Icelandic volcano Grímsvötn moves on and interferes the air traffic in Great Britain. (Eurocontrol/dpa/Corbis)

IMAGE 3: The eruption of the Grímsvötn continued on May 23, sending thousands of tonnes of volcanic ash into the sky above Iceland. (Photo by Jon Magnusson/Getty Images)