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Lightning Strokes
A basic principle how charging of clouds work
Lukas Neureiter
Abstract
The term lightning means an electrostatic discharge between two electrically charged regions. This
could happen between cloud and the earth surface, and between clouds. In this article I will discuss
how the process of electrical charging of Thunderclouds work. To do this I have to start from the
beginning of the process, the water cycle. To explain what the water cycle is, I need to talk about
evaporation and condensation. With this knowledge we are able to understand how clouds can form. I
discuss than different processes which can build up charge in clouds, which I divide into two groups.
The inductive and the non-inductive charging processes. But the process in which a lightning strike
can form is not as simply as it occurs on the first sight, and there are still some uncertainties. One of
this uncertainties is the initiation of a lightning strike, which will only occur at a specified level of
electric field. But the measured electric fields in clouds are too low to overcome the resistance of air.
The major processes that could be an explanation for this problem are the ionization of the air and the
so called runaway breakdown.
Lightning is one of the most beautiful spectaculars in nature, which almost everyone experienced in
their lifetime. But it’s not only beautiful, it could also be very dangerous. Approximately 24,000
people are killed, and 240,000 people are injured by lightning strikes every year on earth.[3] Despite
its power and beauty lightning is an complex process in nature which is one of its greatest mysteries.
So how does it work? It is common knowledge that lightning is based on charged thunderstorms, but
the exact process of cloud charging is still unknown. To understand the principle behind those cloud
charging we start from the very beginning of this phenomenon. It all starts with the water cycle. There
are two major processes in the water cycle. The first one is the evaporation process.[2] When
evaporation starts, a liquid absorbs heat and changes to a vapor. When heat is added to the liquid, the
water molecules move faster with increasing temperature. Some of this molecules move quickly
enough to escape the surface of the liquid and carry heat away in form of a vapor or gas. Once the
molecules are free, the vapor rise into the atmosphere. The second one process we need to understand
the water cycle is the process of condensation in which the vapor or gas loses again its heat and turns
back into a liquid. [2] This will happen, when the vapor rises to higher altitudes and lower
temperatures. The heat will be transferred to the colder surrounding air. When enough heat is lost, the
vapor will condense and return into a liquid. The water cycle starts when water or moisture on the
earth absorbs heat from the sun or other sources. As the evaporation takes place, the vapor will rise
into the atmosphere. If the temperatures in the surrounding air are low enough, the vapor can condense
and the freeze into snow or graupel. This is very important in building up electrical charging in cloud,
and will be described later on. In the process of water cycle more and more vapor rises and accumulate
in the atmosphere, which we can see as clouds. In this cycle droplets collide with other moisture. Also,
the rising moisture collide with ice particles or graupel particles. This collisions of various particles is
very important for building up the electrical charge of clouds. As the collisions take place, electrons
are knocked off of the rising moisture and creating a charge separation.
According to the classification of clouds by Luke Howard in 1802, there are four main groups of
clouds, called ‘cirrus’, ‘stratus’, ‘cumulus’ and ‘nimbus’. [1] These groups can be divided into four
main classes of groups. The cumulonimbus is the giant under these types, and is the primary source of
lightning. As not all cumulonimbus produce lightning, they can be called lightning-producing
cumulonimbus clouds. Two conditions must be fulfilled in order for an electrical discharge. First a
sufficiently high electric potential between two regions of space must exist. This charge separation and
aggregation in certain regions of the cloud, I will be talking later on, but the exact process is still not
fully understood. Second: a high-resistance medium must obstruct the free, unimpeded equalization of
the opposite charges. This electrical insulation that prevents free equalization between two charged
regions of opposite polarity is done by the atmosphere. If this electrical insulation is overcome, a
lightning flash occurs.
To understand how the charge separation mechanism in thunderstorms, I discuss two main routes to
electrify a cloud. This are 1) inductive and 2) non-inductive processes:
1) Inductive charging means that the processes are induced by an electric field.
The existence of a fair-weather field ensures that water particles suspended in the atmosphere
will become polarized. In a vertical, downward directed field such polarization will cause an excess of
positive charge to accumulate in the lower part of the particle, while negative charge will be preferably
located in the upper part. While the particle drops it will meet negatively and positively charged
particles. Since the lower part is positively charged, negatively charged particles are attracted by the
falling droplet, while positively charged particles are pushed away. As a result, the particle grows and
becomes more negatively charged. This leads to a cloud with positively charged particles at
the upper part, and negatively charged particles at the bottom. This effect is just of secondary
importance in later stages, when the electric field of the inner cloud becomes much stronger than the
fair-weather field.
2) Non-Inductive charging deals with charging processes which are not correlating to a presence
of an external electric field. There are two main mechanisms in non-inductive charging[1]:
i)
Convection mechanism:
In convection mechanism the cause of positive and negative charges are from external sources, such as
fair-weather space charge, natural radioactivity near the land surface and cosmic rays near the cloud
top. The warm air updrafts carry the positive charges near the ground to the top of the growing
cumulus. The negative charge produced by the cosmic rays are now attracted and attached to the
boundaries of the cloud. Through downdrafts caused by subsequent cooling and convective circulation
the negative charge is carried down the side of the cloud towards the cloud’s base. This, in its turn,
serves to produce a positive corona near the Earth’s surface, which leads to an additional positive
charge under the cloud. But the convection mechanism is not responsible for the overall cloud
electrification due to its not strong enough for generating all the electrification which is needed for the
several lightning discharges a thunderstorm produces during its lifetime.
ii)
Graupel-ice mechanism:
This basic principle of this mechanism are the collisions between graupel and small cloud-ice
particles. As in laboratory experiments proven (e.g., Jayaratne et al. 1983) there is a certain
temperature TR, above graupel particles acquire positive charge and in the other way acquire negative
charge when the temperature falls below TR. As a conclusion of this, the smaller ice crystals become
charged positively and get carried to the upper regions, while the larger graupel particles charge
negatively and decline relatively to the smaller particles after collision. This charge transfers leads to
polarity as in terrestrial clouds. Observations of thunderclouds shows a typical tripolar structure with a
small positively charge area at the bottom of the clouds.
Once the charge is build up, an electrical field is developed. To bridge the distance between the bottom
of the clouds and the Earth’s surface the electrical field needs to be of some considerable strength.
This strength is about 3*106 V/m. [5] This value is decreasing immense upon rising humidity, but it is
still too high for a discharge. Scientists took it as read, that before a discharge could take place, the air
becomes conductive by ionization. But there is another form of electrical discharge, which is called the
runaway breakdown. This was first introduced by Gurevich et al. (1992) and involves an avalanche of
relativistic electrons that are collimated by the electric field to form an electron beam[4][1]. Electrons
move with relatively low speed in conventionally discharges, because the air particles impeding them.
But when electrons move faster than 6×106 m/s, the frictional force experienced by electrons decreases
with increasing speed. Because of that, the electron gains more energy from the electric field between
collisions with air particles than it loses in a collision. Runaway electrons produced in this way
generate even more energetic electrons, achieved by powerful collisions with air molecules. These
generated electrons, in their turn collide again with molecules creating even more energetic electrons,
etc. The result of this process is an avalanche of highly energetic electrons, but can only be activated
by a highly energetic electron. The spring of this energetic electrons could be cosmic ray interactions,
which can penetrate to altitudes below the ionosphere and start the avalanche of energetic electrons.
With this knowledge, the problem could be solved. Observations of lightning strikes showed that a
lightning strike is formed by a so called “leader”, a channel of ionized air which is initiated from the
negatively charged bottom of the thundercloud, moving towards the earth, and an approaching upward
streamer, a positively charged ionic channel. When a downward leader connects with an upward
streamer, a low resistance path is formed and discharge could occur. First the neutralization of the
positive charges on the surface of the earth occurs. This is the most luminous part of the lightning
discharge and is called the “return stroke “. High speed cameras showed that most lightning flashes are
made up of 3 or more individual strokes, due to that other regions of the clouds discharge.
References and Bibliography:
[1] Poelman, Dieter R. On the Science of Lightning: An Overview. Publication scientifique et
technique No 56 (2010)
[2] Zavisa, John: ,,How Lightning Works’’. http://science.howstuffworks.com/nature/naturaldisasters/lightning.htm (09.06.2014)
[3] Ronald L. Holle Annual rates of lightning fatalities by country. 0th International Lightning
Detection Conference. 21–23 April 2008. Tucson, Arizona, USA.
[4] Alexander V. Gurevich and Kirill P. Zybin (2005). Runaway Breakdown and the Mysteries of
Lightning.
[5] Lightning. http://en.wikipedia.org/wiki/Lightning (09.06.2014)
[6] Commissariat, Tushna: ,,New insights into what triggers lightning’’.
http://physicsworld.com/cws/article/news/2013/may/07/new-insights-into-what-triggers-lightning
(09.06.2014)