National Public Radio: Forecasters can see the volcanic ash cloud on satellite images, and they can forecast which way it’s going. But they can’t tell exactly how much ash is in the air — or at what point it poses a hazard to airplanes. That’s complicating efforts to decide how much of Europe’s airspace can be reopened to travel. 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 …