Illumination

 

It’s 2:00 a.m. this morning, and the light through the Venetian blinds is brighter than noon on a cloudless day.  Nature is putting on a light show of amazing proportions.  I watched and listened for a while; probably got back to sleep around 3:00.

Writing in  Dream Pool Essays, back in 1088 AD, the Song Dynasty  Chinese scientist Shen Kuo wrote in a discussion of the phenomenon of lightning: “Most people can only judge of things by the experiences of ordinary life, but phenomena outside the scope of this are really quite numerous. How insecure it is to investigate natural principles using only the light of common knowledge, and subjective ideas.”

A bolt of lightning can travel at a speed of 45 km/second (100,000 mph) and  can reach temperatures approaching 28,000 °C (50,000 °F), hot enough to fuse soil or sand into glass.
An average bolt of lightning carries a negative electric current of 40 kA, although some bolts can be up to 120 kA, and it transfers enough energy to power a 100 watt lightbulb for just under two months. 

A local TV station has added lightning strikes per hour to their weather reports, and the graphic includes percentages for negative and positive strikes.  In our area, it appears that the ratio of negative to positive is about 95 to 5 or 19:1.

The voltage depends on the length of the bolt: with the dielectric breakdown of air being three million volts per meter, this works out at about one billion volts for a 1,000 foot long  lightning bolt. With an electric current of 100 kA, this gives a power of 100 trillion watts.

As Kuo noted almost a thousand years ago, we still do not fully understand the phenomenon and several theories try to explain lightning generation.  The most common goes something like this:

As a thundercloud moves over the Earth's surface, an equal but opposite charge is induced in the Earth below, and the induced ground charge follows the movement of the cloud.
An initial bipolar discharge, or path of ionized air, starts from a negatively charged mixed water and ice region in the thundercloud. The discharge ionized channels are called leaders. The negative charged leaders, called  "stepped leaders", proceed generally downward in a number of quick jumps, each up to 50 meters long.

Along the way, the stepped leader may branch into a number of paths as it continues to descend. The progression of stepped leaders takes a comparatively long time (hundreds of milliseconds) to approach the ground. This initial phase involves a relatively small electric current (tens or hundreds of amperes), and the leader is almost invisible compared to the subsequent lightning channel.
When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the electric field. The electric field is highest on trees and tall buildings. If the electric field is strong enough, a conductive discharge (called a positive streamer) can develop from these points.

As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. It is also possible for many streamers to develop from many different objects simultaneously, with only one connecting with the leader and forming the main discharge path. Photographs have been taken on which non-connected streamers are clearly visible.

When the two leaders meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud with a "return stroke" that is the most luminous part of the lightning discharge. This electrical discharge rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder.

Of course there are other theories;  native Americans have the tradition of the Thunderbird, and the little girl at Sunday School explained that it was caused by
Arc-Angels.