During a recent ice storm, I was standing in my front doorway before dawn and the entire southern sky turned brilliant blue-green for about five seconds or more. What caused this effect? People who missed it tell me it was just a transformer “blowing up” but I’ve seen one blow up on our street and there is no comparison. The light I saw virtually filled the entire horizon.

You probably saw a sustained high-voltage arc between high-tension wires and/or the ground. I would guess that the ice pulled down one of the wires or caused a tree to fall across them. While transformer explosions often involve hundreds of kilowatts of electric power being turned into light and heat, most of that light is hidden from view inside the transformer. Such an explosion can be dramatic, with some nice sparks and flashes, but it’s usually not very bright. However, when a high-tension wire arcs, a significant fraction of the many megawatts of power flowing through the arc is converted directly into light. In effect, a high-pressure arc lamp forms right in the air and it looks like a camera flash that just keeps going until something stops the arc or the power is shut off. The blue-green color you saw comes from characteristics of the air and metal wires involved in the arc. As you saw, a couple of million watts of light are enough to light up the predawn sky quite effectively!

There is, however, an alternative explanation: you may have seen the “green flash” that occasionally appears just as the sun reaches the horizon at sunrise or sunset. This flash is a refraction effect in the atmosphere in which only blue-green light from the sun reaches the viewer’s eyes for a second or two while the sun is just below the horizon. However, this green flash should appear in the eastern sky just before dawn, not the southern sky.

With the amount of wind that’s produced by high-speed traffic on expressways, why don’t electric companies put wind-powered generators in the center lanes? Using this (wasted) wind to generate electricity would be cheaper, safer, and environmentally friendlier than the power plants that they are running now. — DJA

While wind generators are being used experimentally to charge batteries in roadway equipment that can’t be reached with power lines, there are at least three reasons why such generators aren’t in large scale use. First, wind generators that connect to the AC power grid work most efficiently when they turn at a steady rate—the generator itself must remain in synch with the cyclic alternating current in the electric power lines. The intermittent and sporadic winds produced by passing cars and trucks aren’t really suitable for such wind generators.

Second, to make efficient use of the wind created by traffic, hundreds of wind generators would have to be installed on each mile of expressway. Since wind generators are expensive, it’s much more cost effective to put them on windy ridges out in the country or by the seashore.

Third, the wind generators you propose would actually extract energy from the cars and trucks and reduce their gas mileages! That fact might surprise you, since it would seem that extracting energy from the wind wouldn’t have any effect on the cars and trucks that created that wind. But the wind and the vehicles continue to interact as they move along the expressway—each vehicle drags a pocket of air with it and interfering with this air pocket has the effect of interfering with the vehicle! The vehicle uses energy to maintain this moving air pocket and it burns additional fuel. An aerodynamically well-designed vehicle has a relatively small air pocket, but there is a limit to what can be done. To reduce the energy cost of maintaining the air pocket, the vehicle’s driver can steer it into the air pocket behind another vehicle so that the two vehicles share a single air pocket. The lead vehicle then provides most of the energy needed to keep the air pocket moving. This technique of sharing an air pocket is called “drafting” and is frequently used by bicycle racers. But while drafting makes it easier for many vehicles to keep their air pockets moving, the wind generators that you propose would make it harder—they would steal energy from the air pockets of every passing vehicle and make those vehicles fight harder to keep their air pockets moving.

A better way to save energy would be to encourage large-scale drafting in some safe way. Having chains of independent cars tailgate one another would be energy efficient, but would cause horrific accidents. However, assembling those cars into a tightly coupled “train” may someday become possible with advances in technology and computer controls.