Last month an Icelandic volcano erupted, sending up a giant ash cloud that forced airports across the continent to close for days. Airlines estimated that they lost more than 2 billion pounds in the melee, which stranded thousands of passengers.

Now, intensifying activity from the Eyjafjallajokull volcano has sent up a new cloud, which briefly closed airports in the UK and Ireland before they reopened Thursday. On Saturday, Spain announced it was closing 19 airports as the ash cloud moved south into western Europe. Airports are also expected to close in northern Portugal and southern France.

Most flights between North America and Europe are being rerouted around the 1,200-mile long cloud.

The clouds are created when ice and meltwater fall into the erupting volcano, with the temperature difference between the water and lava creating explosions that shoot ash more than 35,000 feet into the air. Ash clouds are comprised of ultra-fine particles that can erode metal and clog a plane’s fuel system. Flight instruments, air supply, wings and lights can also be affected.

Data from a series of test flights allowed European aviation authorities to alter the guidelines regarding flying through volcanic ash. Scientists and engineers have agreed to a concentration threshold of 0.002g per cubic meter of air. It’s still unclear whether the first ash cloud – which was larger than the new one – ever actually reached this threshold.

“Previous to this, the regulations were, if you see ash, you fly 100 miles away from it. But now we have this blanket over the biggest airports in northern Europe. It’s an unprecedented situation,” Dr. Colin Brown of the UK’s Institution of Mechanical Engineers told the BBC.

The episode has renewed calls for new satellite equipment to monitor volcanic ash. The instruments make it easier for scientists to figure out the concentration of ash in the air, which is the golden number airlines abide by when canceling flights.

“Of the present suite of satellite instruments, none were developed for the volcanic ash problem. Therefore, they are sub-optimal for detecting and determining quantities of volcanic ash,” remote-sensing expert Dr. Fred Prata told the BBC.

Fears of climate change being wrought by the ash clouds have largely been allayed by experts. Volcanic ash can have a cooling effect by reflecting heat from the sun back into space during the day and acting like a blanket at night, keeping cool air closer to the Earth’s surface. But most experts are saying that the clouds from Iceland are not concentrated enough to have this effect.