[MarinTeams] Lightning discussion

[Bill Smith]

Regarding comments and the recent interest in the CME event, the subject of lightning came up at our last HWA meeting.  Those of you who are curious about the subject might enjoy the following discussion.  Comments are from the solarcycle24 website blog:

What causes lightning? Nobody really knows. It's one of the really big open questions in physics. Everything you were told in high is probably wrong, i.e. it's not clouds rubbing together to produce static electricity.

The key phrase in the NSSL article (see below) is
" ...but there is still debate about exactly how lightning forms."

Some of this debate is on whether the graupel collisions generate sufficient static electricity to account for observed phenomena. For example the longest lightning bolt recorded (in Dallas Texas) was 190km long. This would be roughly equivalent to having ten-foot long sparks jumping off your body when you shuffle your feet on a carpet. 


>From the NSSL lab in Oklahoma:
What causes lightning?

The creation of lightning is a complicated process. We generally know what conditions are needed to produce lightning, but there is still debate about exactly how lightning forms.The exact way a cloud builds up the electrical charges that lead to lightning is not completely understood. Precipitation and convection theories both attempt to explain the electrical structure within clouds. Precipitation theorists suppose that different size raindrops, hail, and graupel get their positive or negative charge as they collide, with heavier particles carrying negative charge to the cloud bottom. Convection theorists believe that updrafts transport positive charges near the ground upward through the cloud while downdrafts carry negative charges downward. What follows is a summary of what we know.

Thunderstorms have very turbulent environments - strong updrafts and downdrafts occur often and close together. The updrafts carry small liquid water droplets from the lower regions of the storm to heights between 35,000 and 70,000 feet - miles above the freezing level. At the same time, downdrafts are transporting hail and ice from the frozen upper parts of the storm. When these particles collide, the water droplets freeze and release heat. This heat keeps the surface of the hail and ice slightly warmer than its surrounding environment, and a soft hail, or graupel forms.

When this graupel collides with additional water droplets and ice particles, a key process occurs involving electrical charge: negatively charged electrons are sheared off the rising particles and collect on the falling particles. The result is a storm cloud that is negatively charged at its base, and positively charged at the top.

Opposite charges attract one another. As the positive and negative areas grow more distinct within the cloud, an electric field is created between the oppositely-charged thunderstorm base and its top. The farther apart these regions are, the stronger the field and the stronger the attraction between the charges. But we cannot forget that the atmosphere is a very good insulator that inhibits electric flow. So, a HUGE amount of charge has to build up before the strength of the electric field overpowers the atmosphere's insulating properties. A current of electricity forces a path through the air until it encounters something that makes a good connection. The current is discharged as a stroke of lightning.

While all this is happening inside the storm, beneath the storm, positive charge begins to pool within the surface of the earth. This positive charge will shadow the storm wherever it goes, and is responsible for cloud-to-ground lightning. However, the electric field within the storm is much stronger than the one between the storm base and the earth 's surface, so about 75-80% of lightning occurs within the storm cloud.



GROUND FLASHES

There are two categories of ground flashes: natural (those that occur because of normal electrification in the environment), and artificially initiated or triggered. Artificially initiated lightning includes strikes to very tall structures, airplanes, rockets and towers on mountains. Triggered lightning goes from ground to cloud, while "natural" lightning is cloud to ground.

Terms used to describe ground flashes include forked lightning, which shows branching to the ground from a nearly vertical channel; ribbon lightning, when the horizontal displacement of the channel by the wind appears as a series of ribbons; and bead lightning, when the decaying channel of a ground flash will sometimes break into a series of bright and dark spots. Ball lightning is a luminous sphere whose physics is not well understood.

Cloud-to-ground lightning (CG's)

A channel of negative charge, called a step leader, will zigzag downward in roughly 50-yard segments in a forked pattern. This step leader is invisible to the human eye, and shoots to the ground in less time than it takes to blink. As it nears the ground, the negatively charged step leader is attracted to a channel of positive charge reaching up, a streamer, normally through something tall, such as a tree, house, or telephone pole. When the oppositely-charged leader and streamer connect, a powerful electrical current begins flowing. A return stroke of bright luminosity travels about 60,000 miles per second back towards the cloud. A flash consists of one or perhaps as many as 20 return strokes. We see lightning flicker when the process rapidly repeats itself several times along the same path. The actual diameter of a lightning channel is one-to two inches.

A typical cloud-to-ground flash is a negative stepped leader that travels downward through the cloud, followed by an upward traveling return stroke. The net effect of this flash is to lower negative charge from the cloud to the ground. Less common, a downward traveling positive leader followed by an upward return stroke will lower positive charge to earth. These positive ground flashes now appear to be linked to certain severe storms and are the focus of intense research by scientists.