At any given time, 20 volcanoes are erupting somewhere on the planet. Volcanic ash and jet engines do not mix well. This wouldn't normally be a problem, but on Unimak Island in Alaska there are six volcanoes, one of which, Shishaldin, has erupted at least 29 times since 1755. Unimak lies under one of the heavily traveled North Pacific air routes. Unfortunately for the pilots, the island's topography and lack of populated regions close to the volcano severely limit visual observations, forcing the pilots to rely on acoustic, seismic, and satellite data to understand Shishaldin's behavior and give early warning of plumes of dangerous ash.
Volcano eruptions can be Strombolian (characterized by lava) or Subplinian (characterized by plumes of ash); the trick is to figure out which kind of eruption is about to occur.
Seismologists with the Alaska Volcano Observatory check data on the 44 Alaskan volcanoes to pinpoint which ones are about to erupt and collect data to correlate sensor signals to volcanic behavior. In 1999, one of their tools was a Setra 239 pressure sensor mounted on the eastern side of the instrument hut in clear view of the summit of Shishaldin and used to detect the infrasonic oscillation generated whenever the volcano erupted. Jackie Caplan-Auerbach, a seismologist with the observatory, says, "Shishaldin exhibited an unusually wide range of eruptive behavior over a period of four days. Data collected by the pressure sensor provides us with new insights into this eruption, with its unique combination of lava flow and ash plume. . . . The Setra 239 was able to detect what was happening inside the volcano before the eruption occurred. It was able to pick up signals and record information that the seismometers could not."