This infrared image shows ash spewing out of Iceland's Eyjafjallajokull volcano. It was taken April 17 by a NASA spacecraft. Hotter areas, shown in red, can be seen at the ash plume's base. NASA/JPL/EO-1 Mission/GSFC/Ashley Davies hide caption

toggle caption NASA/JPL/EO-1 Mission/GSFC/Ashley Davies

Korean Air cargo planes sit idle on the tarmac at the Incheon International Airport in Incheon, west of Seoul, South Korea, on Monday. Every day, on average, 10,000 tons of goods are airfreighted between Asia and Europe. But none of that has moved for the past week after Iceland's volcanic eruption. Lee Jin-man/AP hide caption

toggle caption Lee Jin-man/AP

An empty runway at Edinburgh airport, where flights were canceled because of volcanic ash from Iceland. The situation is bad, but experts say it could be much worse. Jeff J Mitchell/Getty Images hide caption

toggle caption Jeff J Mitchell/Getty Images

Stranded airline passengers queue for information at El Prat international airport in Barcelona, Spain. All flights in and out of Barcelona's El Prat and 16 other Spanish airports have been grounded by the volcanic ash drifting across Europe. Jasper Juinen/Getty Images hide caption

toggle caption Jasper Juinen/Getty Images

The MODIS instrument on NASA's Terra satellite captured an ash plume from the Eyjafjallajokull volcano on Friday. Though satellite images can tell scientists where the ash is, they don't help forecasters determine how much ash is in the air — or at what point it becomes a hazard to airplanes. MODIS Rapid Response Team/NASA via Getty Images hide caption

toggle caption MODIS Rapid Response Team/NASA via Getty Images

Researchers Evgenia Ilyinskaya and Asgerdur Sigurdardottir sweep up volcanic ash from a small bridge just south of the Eyjafjallajokull volcano. The ash will be taken back to the University of Iceland for analysis. Joe Palca/NPR hide caption

toggle caption Joe Palca/NPR