Of course, the biggest problem caused by the volcano has been the disruption of air travel. Some European airports are opening to some traffic today. You may have seen the satellite images that show the giant ash cloud that has gradually swept across much Europe. What those pictures don't show is just how much ash is in the air or at what point it poses a hazard to airplanes. NPR's Richard Harris has more.

RICHARD HARRIS: Weather services around the world are geared up to detect volcanic eruptions and to figure out where a potentially hazardous cloud of ash might spread. In most cases, volcanoes are in remote spots, like the Aleutian Islands, or they can be otherwise avoided.

So Mark Ruminski at the National Oceanic and Atmospheric Administration says drawing up a no-fly zone is not usually too tricky.

Mr. MARK RUMINSKI (National Oceanic and Atmospheric Administration): The fact there is going to be any ash is enough to issue the alert and to cut out airspace they want to, you know, fly around or not fly through.

HARRIS: But with an ash cloud drifting over large swaths of Europe, that abundance of caution is causing real headaches - to travelers and airlines alike. And it's casting a spotlight on how much forecasters actually know about the hazards.

The thickest part of the volcanic plume is clearly visible in satellite images. But that's not the whole story.

Mr. RUMINSKI: There's a forecast component to it, also, that we make out to 18 hours.

HARRIS: Those forecasts are just like weather predictions. Analysts watch how the winds are blowing, and from that, they can figure out where the cloud is going to move.

Dr. MARCUS BURSIK (State University of New York, Buffalo): The forecasts are usually quite good in terms of where the ash is going to be.

HARRIS: Marcus Bursik is at the State University of New York in Buffalo.

Dr. BURSIK: What we can't do very well yet, though, is forecast how much ash is actually up there.

HARRIS: And that's a huge problem right now. Forecasters know a lot of the air mass now over Europe had blown down from Iceland. A few research flights have sampled the air to look for ash. But, in fact, there's no really good way to measure how much is really up there, and whether the air is safe enough for jets.

Bursik says this isn't just an air sampling problem.

Dr. BURSIK: We don't even know what the concentration level is that well that affects jet engines.

HARRIS: And that problem is compounded, Bursik says, because airplanes and ash clouds often end up in the same layer of the atmosphere.

Dr. BURSIK: Ash actually gets sort of preferentially drawn into the jet stream, so you not only have a high concentration of ash in the jet stream, but it's also the place that's transporting the ash the fastest.

HARRIS: In fact, the jet stream is what carried a lot of the ash from Iceland to the rest of Europe. And, of course, the ash isn't just moving high up in the jet stream, it's drifting down to lower altitudes. Mark Ruminski from NOAA says the question now is how long it will persist in the air.

Mr. RUMINSKI: There's a couple of ways that you can get rid of the ash up there. One is that it just slowly settles down - basically, the gravitational forces. Or the rain basically brings it down, too.

HARRIS: So they should be hoping for rain in Europe, it sounds like.

Mr. RUMINSKI: Yeah, yeah, right. Exactly.

HARRIS: And, of course, they're hoping the volcano that started it all also goes back to sleep.

Richard Harris, NPR News.

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