I was waiting for people to unload on a flight in Dallas that had been delayed because of strong thunderstorms there. DFW is a major hub for American and many people were pushing to the front, anxious about making connections. The man in the row in front of me got irritated at a woman and her husband who had actually started walking up the aisle while the plane was still taxing to the gate.
"I guess you're more important than the rest of us", he said.
"It's the airlines fault," she retorted. "They're making us late. They can't do anything right."
Yeah, cuz American planned the thunderstorms that delayed the plane coming in to DC, which delayed our take off. And what were they thinking when they delayed us even further by taking a longer route to avoid flying through the thunderboomers?
My favorite line, though, took place in the DC airport when the delay was announced and someone actually said. "I don't see how weather can be delaying our flight - just look outside: it's perfect."
Expect more opportunities for people to display their lack of being embarrassed at their own ignorance today, especially for those unfortunate enough to be flying to Scandinavia or the U.K. (You can get a real time view of the air traffic here. France is really hopping today.) A quick look at the Heathrow arrivals board reflects the fact that everything flying into London today is canceled. I can just hear people now: "But the volcano was in Iceland. Why should that delay my travel within England?"
The volcano that erupted yesterday, called Eyjafjallajokull (I am afraid of injuring my tongue if I even try to pronounce that) in Southern Iceland is actually within a glacier. Its last eruption in 1812 prompted a "me-too" from nearby (and much more reasonably named) glacial volcano Katla. Katla is much larger and hasn't erupted since 1918. As if a volcano eruption weren't enough, because the volcano is within a glacier, an eruption also melts ice, and the primary reason for evacuation was because of fears of flooding. The eruption site has been named Fimmvörðuháls.
A volcano typically spews a lot material into the air when it erupts. Thanks to gravity, the largest pieces of rapidly solidified magma (a.k.a. "volcanic bombs") come down pretty quickly; however, the smallest pieces of ash can remain airborne for days to weeks.
When Mount St. Helens erupted in Washington in 1980, it took just 30 minutes for the ash plume to reach 90,000 ft and just 15 hours to travel 600 miles downwind. The ash plume from the Iceland volcano is mostly in the 6-11 kilometer range, which is about 20,000-36,000 feet. Nowhere near as high as Mt. St. Helens, but the ash cloud is well into the heights at which aircraft normally fly. The speed at which the cloud is moving and its trajectory will determine how long air travel is stalled. There are nine volcanic ash centers throughout the world tasked with tracking volcanic ash. The London office is posting regularly updated graphics of the path of the ash cloud.)
Ash is composed of particles on the order of 2 millimeters or less in diameter that are made of pulverized rock and/or glass (as shown at right). The pieces smaller than volcanic bombs (which are 64 cms and up) and larger than ash are called lapilli. Ash particles are entrained in the gas from the volcanic vent and sent high into the air, where they are carried away by prevailing winds. In this case, the winds are directing these tiny particles toward northern Europe and the U.K. and that is the reason for all the canceled flights.
What does an airplane have to fear from 2 mm particles? It's not like a duck or a goose flying into an engine, after all.
Well, we can start by noting that very small particles of glass and rock hitting something at high speed is basically sandblasting. Not very good for visibility and it can cause real damage to windows and paint jobs.
Next, we have to worry about airplane engines (cutaway courtesy of wikipedia.) The intake of an airplane engine (far left) brings air into the combustion chamber. The particles are small enough that they have no problem getting in past the intake and combustion stages (unlike birds). When the particles reach the combustion stage, however, they get hot enough to soften or melt and instead of going through the engine, they stick on the turbine blades. Once enough of the particles have accumulated, they block the flow of air through the engine, the engine stops generating thrust and the aircraft plunges. This has been documented in about 20 civil aviation craft, in which the plane was essentially a glider for 5-10 minutes. During that time of freefall, the engine usually cools, the particles agglomerates get pushed out and the engines can be restarted. But that's an awfully uncertain way to unclog an engine.
Compounding the problem is that standard radar can't detect the difference between water vapor clouds and volcanic ash clouds. Radar sends out radio waves - more recent radars use waves that have a wavelength on the order of a few centimeters. The radio waves run into things and bounce back to the source. The time it takes to return tells you how far off the object is. (In the more complicated case of Doppler radar, the shift in frequency is used to determine how fast the object is moving and in what direction, increasing the precision of the technique.)
Radar works because of the interaction of the outgoing waves with the object trying to be detected. Radio waves are electromagnetic in nature, so what changes their path is a change in the dielectric constant or diamagnetic constant. The dielectric constant measures how well a material concentrates electric flux lines. We chose vacuum to have a dielectric constant of 1, meaning that the electromagnetic waves travel through vacuum without change. Air has a dielectric constant pretty close to 1 - so close that we normally don't bother treating it as different from 1.
Materials with larger dielectric constants are polar - they have a decidedly positive end and a decidedly negative ends. When left alone, the directions of the position and negative ends are random. When an electric field is applied (right picture), the dipoles (the little things with positive and negative ends) align along the electric field lines. Diamagnetic constant is basically the same thing, but thing of the plusses and minuses as north and south poles instead.
The larger the change in dielectric constant, the more the electromagnetic waves scatter. We generally measure the dielectric constant as a relative number (relative to vacuum). Metals have huge dielectric constants because they have a lot of free electrons running around (and remember that the lack of a negative charge is a positive charge, so if all the electrons move to one side of the material, that leaves a positive charge on the other). Aluminum has a relative dielectric constant on the order of 1014, which is 100,000,000,000,000. (For the purists, that is at frequencies of about 1 kHz). Water has a relative dielectric constant around 80. Basically, the more conductive something is, the easier it is to see on radar. Also, the sharper the interface between air and the object, the easier it is to spot. That's why a stealth plane is all rounded curves - harder to detect.
Another way of preventing radar detection is to use non-electrically conductive materials. Teflon has a relative dielectric constant of around 2, for example. Volcanic ash is basically rock and glass, both of which have pretty low dielectric constants, plus, at a size of a few mm (compared to a few cm for the radar waves), they are difficult to detect unless they are in very large masses. Even then, they are indistinguishable from regular clouds.
There are some attempts to differentiate between volcanic ash clouds and water vapor clouds, mostly using other frequencies of electromagnetic radiation. There is a resonance in reflection near 11 and 12 micrometers from water. The volcanic ash particles and the clouds react very differently to the radiation, so a combination of radar and infrared radiation could allow pilots to identify volcanic ash in real time.How long is this going to go on? The original volcano started erupting on March 20th. The 1812 eruption lasted for slightly more than a year. Yesterday's ash cloud is the result of a particularly large burp. About the only thing more unpredictable than the airlines is volcanoes. Between more possible eruptions and uncertainties about the wind direction and speed, it is hard to predict how long the travel ban will last.
Looking on the positive side, Rayleigh scattering should be enhanced by the volcanic ash. This should result in a spectacular sunset. Plenty of people will have time to watch it: It doesn't look like they are going anywhere until tomorrow at the earliest.