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2.1 Lightning discharge and lightning ground underneath the thundercloud (e.g. plants) and is trans-
current curves ported upwards by the wind.
If the space charge densities, which happen to be present
Every year, an average of around 1.5 million lightning strikes in a thundercloud, produce local field strengths of several
2
discharges over Germany. For an area of 357,042 km this 100 kV/m, leader discharges are formed which initiate a light-
corresponds to an average flash density of 4.2 lightning dis- ning discharge. Cloud-to-cloud flashes result in charge neu-
charges per square kilometre and year. The actual flash density, tralisation between positive and negative cloud charge centres
however, depends to a large extent on geographic conditions. and do not directly strike objects on the ground in the process.
An initial overview can be obtained from the flash density map The lightning electromagnetic impulses (LEMP) they radiate
contained in Figure 3.2.3.1. The higher the sub-division of must be taken into consideration, however, because they en-
the flash density map, the more accurate the information it danger electrical and electronic systems.
provides about the actual lightning frequency in the area un- Flashes to earth lead to a neutralisation of charge between the
der consideration. cloud charges and the electrostatic charges on the ground. We
Using the BLIDS (lightning information service by Siemens) distinguish between two types of lightning flashes to earth:
lightning detection system, it is now possible to locate light- ¨ Downward flash (cloud-to-earth flash)
ning within 200 m in Germany. For this purpose, 145 measur- ¨ Upward flash (earth-to-cloud flash)
ing stations are spread throughout Europe. They are synchro-
nised by means of the highly accurate time signal of the global In case of downward flashes, leader discharges pointing to-
positioning system (GPS). The measuring stations record the wards the ground guide the lightning discharge from the cloud
time the electromagnetic wave produced by the lightning dis- to the earth. Such discharges usually occur in flat terrain and
charge arrives at the receiver. The point of strike is calculated near low buildings. Cloud-to-earth flashes can be recognised
from the differences in the times of arrival of the electromag- by the branching (Figure 2.1.1) which is directed to earth.
netic wave recorded by the various receivers and the corre- The most common type of lightning is a negative downward
sponding differences in the times it takes the electromagnetic flash where a leader filled with negative cloud charge pushes
wave to travel from the location of the lightning discharge to its way from the thundercloud to earth (Figure 2.1.2). This
the receivers. The data determined in this way are filed cen- leader propagates as a stepped leader with a speed of around
trally and made available to the user in form of various pack-
ages. 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 alti-
tudes. This transport can occur in a number of ways. In the case
of heat thunderstorms, the ground is heated up locally by in-
tense 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. Ad-
ditional 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 cumulonim-
bus 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 thunder-
cloud. 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 Figure 2.1.1 Downward flash (cloud-to-earth flash)

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