RENEE MONTAGNE, Host:
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: So Mark Ruminski at the National Oceanic and Atmospheric Administration says drawing up a no-fly zone is not usually too tricky.
MARK RUMINSKI: 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: The thickest part of the volcanic plume is clearly visible in satellite images. But that's not the whole story.
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.
MARCUS BURSIK: 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.
BURSIK: What we can't do very well yet, though, is forecast how much ash is actually up there.
HARRIS: Bursik says this isn't just an air sampling problem.
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.
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.
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.
RUMINSKI: Yeah, yeah, right. Exactly.
HARRIS: Richard Harris, NPR News.
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