Miscellaneous

These remarkable images show several "cloud to ground" flashes and also "cloud to cloud" or "intra-cloud" lightning which is more rarely observed. Man1+2.jpg illustrate the "Namargon" ("Lightning man") in Australia (see below).

In horizontally extended Cumulonimbus clouds charge separation occurs such that the top part of the cloud has a positive net charge. This is balanced by a negative charge at the cloud base. In these regions the total charge is in the order of 10 to 100 Coulomb. The charge is diffusely spread at the top of the cloud, but more compact in the middle. The so-called "charge zone" is located in a region between cloud base and cloud top at temperatures between -10 deg C and -25 deg C. Only there ice-particles, under-cooled water and hailstones might be able to coexist. On the earth's surface a positive mirror charge will be induced because of the negative cloud base.

The charge separation inside the cloud is driven by collisions of larger frozen hydrometeors, which carry a positive charge on their surface, with smaller ones. Thereby the small hydrometeors obtain a positive charge, leaving the larger ones negatively charged behind. If one of the collision partners is a hailstone, 1E-14 Coulomb is transferred in each event. At a hailstone-hailstone collision the transferred charge is larger. Whereas the larger hydrometeors are too heavy, small ones can be lifted upwards by the updraft flow. As a consequence positive-negative-positive charge zones are generated in the clouds. While this is the most important mechanism for charge separation in thunderstorm clouds, several other processes play also a role. However, it is still unknown how the charge separation exactly works during the particle collisions.

Potential differences of 10 Million Volts and more can be induced between the cloud base and the ground. If the electrical field strength (given in Volts per meter, calculated as the potential difference divided by the distance between the opposite charges) reaches the "breakdown value", lightning discharges occur. At first the electrical field exceeding the breakdown field strength drives the negative charge carriers from the cloud base to the ground with a velocity of 150 km/s. This generates the so-called "pre-flash". Because of the inhomogeneous spatial distribution of the electrical field, the spread of the pre-flash occurs in discrete steps of 2 to 50 m.

For the same reason the flash is not a straight line, but often branched. The pre-flash reaches the ground after approximately 20 ms. It generates a "charge channel" with a diameter up to 1 m which is occupied by electrical charge carriers.

Now a first discharge between the lower part of the cloud and the ground takes place and currents in the kilo-Ampere range occur. Inside the pre-flash a plasma channel with a diameter of several centimetres appears which will be heated up to 20 000 Kelvin by the heat of the electrical current. At these high temperatures NOx is also produced within this channel.

After this "short-circuit" is generated, the first "main flash" extends inside the charge channel bottom-up with a third of the speed of light. Thereby a current between 10 and 40 kA occurs. In rare cases even 100 kA might be reached. Positive charge carriers are transported from the ground into the lower part of the cloud. Typical amounts of transported charge are around 20 Coulomb with peak values around 200 Coulomb.

After the first lightning discharge more charge carriers enter the existing lightning channel. This time the transport happens continuously top-down with a velocity of 3000 km/s. As well as the pre-flash this "intermediate discharge" also glows only weakly.
When the intermediate discharge has reached the ground a new, weaker main flash ignites. This sequence is repeated until - as a consequence of charge balancing - the potential difference is too small to produce more flashes. The whole discharge procedure over pre-flash and several main and intermediate discharges takes place in about 10 ms. Therefore the observer mostly recognizes a flickering from longer-enduring "single" flashes. On average each observed lightning consists of three main flashes.
The thunder is created by the expansion of air in the lightning channel which is heated up by the flash. Inside the lightning channel with a diameter of 1 m a pressure around 10 MPa occurs. This pressure discontinuity spreads with sonic velocity. Thereby the pressure difference is quickly reduced to 8 hPa at a distance of 5 m and to 1 hPa at 300 m. In a distance of 25 km the pressure difference is so small that a person cannot "hear" them anymore. Nevertheless the human ear is capable to detect pressure differences down to 1E-3 hPa. If one is closer to a flash than 25 km, the high pressure differences will be noticed as a "bang". Since flashes have a certain length, that means some parts will be closer to the observer than others, a longer-enduring source of noise arises. The "rumbling" originates from several reflections of the bang at the ground, mountains and clouds and the superposition of all sound waves with different run times.

Globally 12 to 16 cloud-to-ground flashes reach the ground per second with a maximum value of 55 Hertz above the land in the north-hemispheric summer. 70% of the flashes occur between 30° southern latitude to 30° northern latitude. The lightning rate over the continents is ten times higher than those over the oceans. Lightning is the main source of the natural NOx production. The dissociation of atmospheric oxygen O2 and atmospheric nitrogen N2 takes place in the plasma channel at temperatures between 20 000 K and 30 000 K. The released N and O atoms can recombine to nitrogen oxide NO. Nitrogen oxides are decomposed by atmospheric chemical reactions. These processes become inefficient at temperatures below 3000 K so that maximum NO concentrations occur at around 3000 K.

According to current estimated the global NOx production caused by thunderstorms ranges between 2 and 20 Tg per year. In comparison the contribution of NOx emissions from civil air traffic in the upper troposphere is estimated to be 0.5-0.8 Tg. In order to understand how the anthropogenic NOx production influences the chemistry of the troposphere, it is necessary to know the natural sources and their magnitude at first.

The "Namargon" or "Lightning Man" can be found in several rock paintings which were created by the Aborigines in Arnhem Land in North Australia. The style of these two pictures shown here is named as "X-Ray painting". According to the mythology of the Australian native inhabitants the band surrounding the body is a flash. During the time of the pre-Monsoon, i.e. directly before the rainy season, the stone axes outstanding from the head, the elbows and the knees, are used to beat flashes from the clouds and to produce thunder. The "Namargon" has to watch and enforce the orders of the clan. The age of the rock paintings in the "Nourlangie Rock" region of the Kakadu National Park is estimated to approximately 20 000 years. In 1964 some of these paintings were repainted with new colours by the aborigine artist Najombolmi. (Informations from Roberts, D. A., A. Parker, Ancient ochres: The aboriginal rock paintings of Mount Borradaile, JB Books, Australia, 2003).

Lightning1-6.jpg: O. Hartmann, Mainz, Germany, 2001

Man1+2.jpg: S. Borrmann, Nourlangie Rock, Kakadu National Park, Northern Territory, Australia, 3 December 2005