Every year, an average of around 1.5 million lightning strikes discharges over Germany. For an area of 357,042 km2 this corresponds to an average flash density of 4.2 lightning discharges per square kilometre and year. The actual flash density, however, depends to a large extent on geographic conditions. An initial overview can be obtained from the flash density map contained in Figure 22.214.171.124. The higher the sub-division of the flash density map, the more accurate the information it provides about the actual lightning frequency in the area under consideration.
Using the BUDS (lightning information service by Siemens) lightning detection system, it is now possible to locate lightning within 200 m in Germany. For this purpose, 145 measuring stations are spread throughout Europe. They are synchronised by means of the highly accurate time signal of the global positioning system (GPS). The measuring stations record the time the electromagnetic wave produced by the lightning discharge arrives at the receiver. The point of strike is calculated from the differences in the times of arrival of the electromagnetic wave recorded by the various receivers and the corresponding differences in the times it takes the electromagnetic wave to travel from the location of the lightning discharge to the receivers. The data determined in this way are filed centrally and made available to the user in form of various packages. Further information on this service can be obtained from www.siemens.de/blids (German website).
Thunderstorms come into existence when warm air masses containing sufficient moisture are transported to great altitudes. This transport can occur in a number of ways. In the case of heat thunderstorms, the ground is heated up locally by intense insolation. The layers of air near the ground heat up and rise. For frontal thunderstorms, the invasion of a cold air front causes cooler air to be pushed below the warm air, forcing it to rise. Orographic thunderstorms are caused when warm air near the ground is lifted up as it crosses rising ground. Additional physical effects further increase the vertical upsurge of the air masses. This forms updraught channels with vertical speeds of up to 100 km/h, which create towering cumulonimbus clouds with typical heights of 5 to 12 km and diameters of 5 to 10 km.
Electrostatic charge separation processes, e.g. friction and sputtering, are responsible for charging water droplets and particles of ice in the cloud.
Positively charged particles accumulate in the upper part and negatively charged particles in the lower part of the thundercloud. In addition, there is again a small positive charge centre at the bottom of the cloud. This originates from the corona discharge which emanates from sharp-pointed objects on the ground underneath the thundercloud (e.g. plants) and is transported upwards by the wind.
If the space charge densities, which happen to be present in a thundercloud, produce local field strengths of several 100 kV/m, leader discharges are formed which initiate a lightning discharge. Cloud-to-cloud flashes result in charge neutralisation between positive and negative cloud charge centres and do not directly strike objects on the ground in the process. The lightning electromagnetic impulses (LEMP) they radiate must be taken into consideration, however, because they endanger electrical and electronic systems.
Flashes to earth lead to a neutralisation of charge between the cloud charges and the electrostatic charges on the ground. We distinguish between two types of lightning flashes to earth:
- Downward flash (doud-to-earth flash)
- Upward flash (earth-to-cloud flash)
In case of downward flashes, leader discharges pointing towards the ground guide the lightning discharge from the cloud to the earth. Such discharges usually occur in flat terrain and near low buildings. Cloud-to-earth flashes can be recognised by the branching (Figure 2.1.1) which is directed to earth.
The most common type of lightning is a negative downward flash where a leader filled with negative cloud charge pushes its way from the thundercloud to earth (Figure 2.1.2). This leader propagates as a stepped leader with a speed of around 300 km/h in steps of a few 10 m. The interval between the jerks amounts to a few 10 ps. When the leader has drawn close to the earth (a few 100 m to a few 10 m), it causes the strength of the electric field of objects on the surface of the earth in the vicinity of the leader (e.g. trees, gable ends of buildings) to increase. The increase is great enough to exceed the dielectric strength of the air. These objects involved reach out to the leader by growing positive streamers which then meet up with the leader, initiating the main discharge.
Positive downward flashes can arise out of the lower, positively charged area of a thundercloud (Figure 2.1.3). The ratio of the polarities is around 90% negative lightning to 10% positive lightning.This ratio depends on the geographic location.
On very high, exposed objects (e.g. wind turbines, radio masts, telecommunication towers, steeples) or on the tops of mountains, upward flashes (earth-to-cloud flashes) can occur. It can be recognised by the upwards-reaching branches of the lightning discharge (Figure 2.1.4).
In case of upward flashes, the high electric field strength required to trigger a leader is not achieved in the cloud, but rather by the distortion of the electric field on the exposed object and the associated high strength of the electric field. From this location, the leader and its charge channel propagate towards the cloud. Upward flashes occur with both negative polarity (Figure 2.1.5) and with positive polarity (Figure 2.1.6).
Since, with upward flashes, the leaders propagate from the exposed object on the surface of the earth to the cloud, high objects can be struck several times by one lightning discharge during a thunderstorm. Depending on the type of flash, each lightning discharge consists of one or more partial lightning strikes. We distinguish between short strokes with a duration of less than 2 ms and long strokes with a duration of more than 2 ms. Further distinctive features of partial lightning strikes are their polarity (negative or positive) and their temporal position in the lightning discharge (first, subsequent or superimposed). The possible combinations of partial lightning strikes are shown in Figure 2.1.7 for downward flashes, and in Figure 2.1.8 for upward flashes. The lightning currents consisting of both short strokes and long strokes are impressed currents, i.e. the objects struck have no effect on the lightning currents. Four parameters which are important for lightning protection can be obtained from the lightning current curves shown in Figures 2.1.7 and 2.1.8:
- The peak value of the lightning current I
- The charge of the lightning current Qflash consisting of the charge of the short stroke Qshort and the charge of the long stroke Q,ong
- The specific energy W/R of the lightning current
- The steepness di/dt of the lightning current rise.
The following chapters show which of the individual parameters are responsible for which effects and how they influence the dimensioning of lightning protection systems.