Meteorologist have warned prevailing winds are blowing the ash cloud towards London's Heathrow airport, potentially closing airspace above Europe's busiest air hub. Officials say there is a strong possibility some Scandinavian airports may also be affected.  Among those changing travel plans have been U.S. President Barack Obama, who flew into London a day ahead of schedule, and Barcelona's soccer team, which is traveling to the UK capital early ahead of Saturday's Champion's League final clash against Manchester United.  How will the disruption differ from last year's? Ash from the eruption of Iceland's Eyjafjallajokull volcano in April 2010 prompted the closure of airspace across Europe, stranding millions of passengers and costing the airline industry more than $2 billion.  Authorities say there is uncertainty over how long the latest ash cloud, generated by Grimsvotn -- Iceland's most active volcano -- is likely to last, and how it will be affected by atmospheric changes. However, weather patterns indicate most mainland European airports will not be hit and airspace will remain open. Changes in flight safety regulations also mean fewer flights will be affected.  What are the safety regulation changes? Most significantly, aviation authorities have raised the threshold on what density of ash cloud is considered safe to fly through. Last year, 200 micrograms of ash per cubic meter was seen as the maximum, this year it has been raised to 4,000 micrograms per cubic meter.  Ash density is also being classified in terms of low, medium or high density. In theory, airlines can make a case to fly through all three provided they can prove they are taking adequate safety precautions.  Irish carrier Ryanair has, however, condemned these limited restrictions as pointless. It said it has successfully piloted a test flight into an area of high ash density, with no adverse affects.
Icelandic volcano ash is affecting flights in European airspace, but authorities say it is unlikely to cause the same levels of disruption triggered by a similar eruption last year.So what are the