Lightning consists of electric discharges, which lead to a transport of electric charges between areas of different space charges either within one cloud, between two clouds (cloud-to-cloud lightning) or between a cloud and the earth’s surface (cloud-to-ground-lightning). Cloud-to- cloud lightning occurs most frequently – and makes up to 90% of all lightning. In this case, no charge exchange with the surface takes place. In total, there are about one to two million lightning per year in Germany. The average lightning density is approximately 2.6 lightning per year and km². On a global scale, it is possible to observe on average up to 3.000 lightning at the same time.
In general, lightning is not a meteorological extreme event as it occurs very frequently. However, lightning strikes can result in significant damage to property due to fires and overvoltage. The process of the separation of charges and their discharging within cumulonimbus clouds has yet to be understood completely. Therefore, there are different approaches on how to explain them.
Charge separation in a thundercloud
The lightning activity in a thunderstorm system depends mainly on cloud microphysics and the dynamics within the cloud. In general, the charge separation in a cumulus cloud is caused by hydrometeors in different phases and with different sizes. Impacts, freezing or melting processes or coalescence (flowing together) trigger the separation process (Williams et al. 1991). After the charge separation by influence, small ice particles usually become positively charged, whereas larger hydrometeors, such as sleet, become negatively charged (Saunders et al. 2006). The former are transported by the updraft to the upper part of the cloud, whereas the latter are concentrated at lower levels due to their greater mass (or gravity).
Furthermore, a strong updraft leads to a higher collision rate between the particles, which leads to an increased exchange of charges between the particles. This may lead to a faster increase of the electric field (Price, 2013). Through this process, areas emerge with both positive as well as negative space charges. If a certain threshold is exceeded, charge exchange will take place in a flash. At an altitude of approximately 6 kilometers and a temperature of about –15°C, a strong negative space charge is very often in place. Discharges above this level are usually cloud-to-cloud lightning, below this level they are usually cloud-to-ground lightning.
If the difference in charge is large enough, a so called pre-discharge occurs at first. The ‘leader’, often also referred to as lightning channel, is formed through the impact ionization of air molecules. In the case of lightning between a cloud and the earth’s surface, the lightning channel expands from the cloud to an altitude of about 50 meters above the ground. Coming from the ground, a so called return stroke forms and fuses with the already existing lightning channel. The lightning channel consequently reaches from the cloud all the way to the earth and can also branch out. Through this lightning channel, the main discharge occurs which is visible in form of lightning (see the video link at the end of the article). This main lightning strike has a very high temperature of about 30.000°C and an average current of 20.000 Ampère. Many times, several main discharges termed to as flashes take place in the same lightning channel, which explains the often observed phenomena of “flickering” of a lightning. The temporal duration of the entire process is about 200 milliseconds (ms), of which the formation of the lightning channel takes about 10 ms, and one single main discharge about 4 ms (Krider, 1986)
Distribution of lightning density
The lightning density in Germany (lightning information service by Siemens AG, BLIDS) features a high temporal and spatial variability (see Figure 1). BLIDS is a member of the central-European collaboration among national lightning detecting networks EUCLID (EUropean Cooperation for LIghtning Detection); lightning is detected with a precision of up to 200 m. In Germany an explicit north-to-south gradient in thunderstorm activity can be observed. The number of lightning is highest in over the pre-Alps, near Swabian Jura as well as over the Ore Mountains (Erzgebirge). By contrast over the northern parts convective activity is limited and so is the number of lightning.
Influence on the ozone formation
The discharge of lightning also has an influence on the global formation of ozone. After a lightning discharge, nitrogen oxides (NOx) are released, which produce ozone in the upper troposphere. Additionally, strong updrafts in a thunderstorm can transport emissions form the ground (e.g., car exhaust gases) to the upper troposphere (suction effect). Measurements conducted by the Deutsches Zentrum für Luft- und Raumfahrt (DLR) show that thunderstorm events produce about five times as much nitrogen oxides as the global air traffic. This means that thunderstorms account for roughly 10% of the sources for nitrogen oxide in the upper troposphere (Huntrieser, 2012).
Huntrieser, H., Höller, H. & Grewe, V. (2012). Thunderstorms: Trace species generators. In U. Schumann (Hrsg.), Atmospheric Physics. Research Topics in Aerospace (S. 115-133). Berlin, Heidelberg: Springer. doi:10.1007/978-3-642-30183-4_8
Krider, E. P. (1986). Physics of Lightning. In National Research Council (Hrsg.), The Earth Electrical Environment (Studies in Geophysics) (S. 30-40). Washington, D.C.: National Academic Press. doi:10.17226/898
Price, C. G. (2013). Lightning applications in weather and climate research. Surveys in Geophysics, 34, 755-767. doi:10.1007/s10712-012-9218-7
Saunders, C. P. R., Bax-Norman, H., Emersic, C., Avila, E. E. & Castellano, E. (2006). Laboratory studies of the effect of cloud conditions on graupel/crystal charge transfer in thunderstorm electrification. Quartely Journal of the Royal Meteorological Society, 132(621), 2653-2673. doi:10.1256/qj.05.218