About the volcanic eruption in Iceland that has brought a halt to air traffic over the North Atlantic and much of Europe, this morning's ten-minute set of links and tips.

- Is this a known issue in aviation weather, aviation safety, and so on?
  Yes, indeed. Among the list of weather-condition abbreviations that pilots are supposed to know in order to read METARs (don't ask, a readout of local weather conditions) , is "VA," for Volcanic Ash. A description of the oddity of METAR abbreviations  is here, including why "BR" means Mist (Americans are taught to remember, "Baby Rain," but that's not the reason) and "GR" means Hail.
  For the basic USGS background on the problem, see here and here; for a conference on the topic, here.

- Why does it matter in theory?
  The reasons laid out in newspaper stories worldwide in the past 24 hours are actually true! The volcanic ash particles are extremely fine bits of pumice with tremendous abrasive potential. Even in concentrations too low to be visible as a big threatening "plume," they can in theory cause big problems for: the turbine power plants (aka jet engines) of modern airplanes, operating at tremendous speeds and pressures with very fine tolerances among all the moving parts; the leading edges of the wings, whose precise contours affect the flow of air over the wing and therefore the plane's ability to fly; pitot tubes (for gauging air speed) and other external devices; the plane's paint job; and windows in the cockpit, conceivably making it impossible for the pilots to see.

- Why does it matter in practice?
  The best known early cases happened over Indonesia in 1982, when several airliners flying through what they later realized was an ash plume were damaged. In one famous case, a British Airways 747 lost power in all four engines and had to glide nearly all the way to a landing (it eventually got some of the engines restarted). The USGS description of a similar incident over Alaska says:

In 1989, a wide-body passenger jet destined for Anchorage airport flew into the volcanic ash cloud generated by Mount Redoubt, Alaska and lost thrust all 4 engines. The plane entered the ash cloud at 25,000 feet, accelerated, and then rapidly descended to 13,000 feet. The pilot was finally able to restart its engines. The Alaska Range in the area where the plane lost power has peaks from 7,000 to 11,000 feet, so this was an extremely close call. In 1992, the effects of volcanic eruptions on aviation were felt well beyond Alaska when a volcanic ash cloud from the Mount Spurr (Alaska) eruption drifted across the continental U.S. and Canada, shutting down airports in the Midwest and Northeast two days after the eruption. The Spurr cloud affected citizens who are normally not concerned about volcanoes. 

- Why is this one causing such widespread problems?
 Because the ash is drifting into such busy traffic lanes. Here is the latest chart from the British weather office showing the plume's predicted spread. (Click for larger. I think the red area indicates the spread at altitudes of about 20,000 feet and below; dotted green line, the spread between about 20,000 and 35,000 feet.)

Thumbnail image for AshMapp.png

- Is the mammoth flight-cancellation and attendant disruption a big overreaction?
  Really, no one can be sure right now. The charts above say, "Ash concentrations within indicated area are unknown." Odds are that it will look in retrospect like an overreaction, and it will be clear that more planes could have figured out a way to fly more routes sooner. But at the moment, what is the plausible alternative to precautionary overreaction? No government, airline, pilot, or passenger can afford to say, "Hey, I'm feeling lucky" for now. 

- What does this show about the press?
  Widely available reports have been accurate, informative, and non-alarmist. Who says journalism is headed straight down?

- What does it show about global warming?

 Topic for another time.