Airspace is gradually reopening throughout northern and central Europe, much to the relief of thousands of stranded passengers all over the world. But stronger eruptions from the Icelandic volcano are threatening to unravel those plans.
Europe's air traffic agency says 55 to 60 percent of flights over Europe are lifting off Tuesday.
"We should see progressively more planes start to fly and this is good news for Europe's stranded passengers, good news for airline industry," European Commissioner for Transport Siim Kallas said.
But not all airports are opening, including London's Heathrow.
Meanwhile, the infamous Icelandic volcano responsible for disrupting international air travel continues to fume and spit out ash.
Forecasters said it is actually intensifying again, spewing a new cloud of ash that is headed towards the United Kingdom. Another problem is that there are no international standards to determine how much ash in the air makes it too dangerous to fly.
"If we were given an indication as to what level of concentration constitutes danger then we could try and predict that and that might well give a different result," said scientist Derrick Ryall with the British Meteorological Office.
Ultimately, each airline must make its own decision as to whether to fly. And with airports like Heathrow closed for a sixth day in a row, many passengers are finding it difficult to remain optimistic.
"I have my boarding pass, but I don't believe I'm going to fly," one stranded airline passenger said.
The International Pilot's Federation is urging extreme caution. It says safety and not economics should drive any decision to resume flights. It also says final decisions should be left to the pilots themselves.
In the meantime, some stranded European travelers in the U.S. may get a taste of U.S. hospitality.
A Houston church is offering to take in dozens of stranded passengers. Some families have opened their homes to travelers after watching local news reports. And several amusement parks in Florida are offering free admission to customers with canceled flights.