Even though some say it's been a massive overreaction by badly prepared safety regulators. Hundreds of flights were canceled Tuesday as winds blew the cloud of ash from the Grimsvotn volcano over Scotland and other parts of Europe. Experts say that particles in the ash could stall jet engines and sandblast planes' windows.

The only comfort for frustrated passengers and airlines is that officials in Iceland said the amount of ash being released by the volcano is decreasing.

Travelers trying to go to or leave Scotland have been hit hardest, but the problem also began to affect Germany, where weather officials said it would not allow any takeoffs or landings at the northern Bremen and Hamburg airports early Wednesday due to increased levels of ash in the atmosphere.

Dozens of domestic and international flights were to be affected by the closure early Wednesday. Authorities said it may be necessary to halt all air traffic coming and going from Berlin's airports, as well as Hannover, depending on the winds.

In Sweden, 10 domestic flights were canceled Tuesday evening.

Although British authorities said concentrations of ash in the skies over Scotland were too high to fly.

"All the data we are receiving confirms our forecasts, that there is high-density ash over Scotland," said Barry Grommett, spokesman for Britain's weather agency.

While Irish budget airline Ryanair's chief executive Michael O'Leary disagreed. He challenged the results, saying it had sent its own airplane into Scottish airspace and found no ash in the atmosphere.