LIGHTNING PHENOMENOLOGY
Each second, the earth is the seat of more than 2 000 storms with an extreme and exceptional violence...
They produce, every second, hundreds of flashes of tens of thousands of amperes.
1/3 of them touches the ground. Damages on properties and people can have important consequences.
Differents lightning strikes :
Negative descending lightning strikes, with a positive return arc
Negative descending lightning strikes without positive return arc
Positive ascending lightning strikes
From the 20th century, the lightning phenomenon becomes interesting.
We discover that the storms are necessary for life, that they maintain our planet electrical field .
The development of the energy transport networks, computer and electronic stimulated Research.
We have now a strong knowledge in Lightning phenomenon and means of protection.
STORM CREATION
Although we can observe flashes in various weather conditions
The main storm generator is the cumulo-nimbus.
- It is a cloud whose top has the characteristic shape of an anvil.
- its base is flat
- it can be situated between 500 and 3 000 metres from the ground.
- its top can reach an altitude of 15 000 meters in less than half an hour.
- This cloud can represent an area of several tens of km²
These huge masses are full of hundreds of thousands of tons of water...
Water droplets and ice crystals compete strongly as a result of violent turbulences caused by rising air currents.
Ice, snow and water particles compete each other with violence inside the cloud.
ALL THESE PERCUSSIONS LIBERATE THE ELECTRICAL CHARGES..
These shocks snatch electrons from the finest particles (ice crystals) that change their polarity and become electrically positive.
These electrons snatched are captured by heaviest particles (water droplets) which become electrically negative.
UNDER THE EFFECT OF THE GRAVITATION, THE NEGATIVE PARTICLES, HEAVY, ARE CONDUCED TOWARDS THE BASE OF THE CLOUD. ..
... The positive particles, light, are maintained at the top by rising air currents.
The violent shocks between all these particles produce alack of balance between the electrical charges électriques à l'intérieur du nuage.
A huge difference of potential of ens of millions of volts is established between the base and the top of the cloud.
MEANWHILE, ON THE GROUND...
UNDER THE INFLUENCE OF THE NEGATIVE CHARGES LOCALIZATED AT THE CLOUD BASE LEVEL...
- The polarity of the atmospheric electric field is reversed on the ground.
- Usually around -150 volts / meter in sunny weather, it can reach progressively values from 15 to 20 kV/meter.
- From 0 kV, dieelectrical characteristics of the air change. Electrical arcs can appear.
UNDER THE INFLUENCE OF THE NEGATIVE CHARGES SITUATED IN THE BOTTOM OF THE CLOUD...
The difference of potential between the cloud and the ground shows that a discharge is imminent.
UNDER THE EFFECT OF THE DIFFERENCE OF POTENTIAL ...
- The outside air to the space-ground base of the cloud is traversed by an electric current.
- Electrons charges negatively are pulled to the ground and rise to the top of the cloud....
By consequence, the ground is charged positively.
THESE ELECTRONS, GOING BACK TO THE CLOUD TOP, NEUTRALIZE THE POSITIVE CHARGES OF EQUAL VALUES...
- Then, a potential difference of several tens of millions of volts is established between the cloud base and the ground.
- For a given potential difference, there is the initialization of an electric arc called the downward leader.
The leader moves along a path apparently disordered.
LIGHTNING MECHANISM
FROM A CLOUD POINT FLASHES A LUMINOUS RAY WHICH PROGRESSES FASTLY ...
- The phenomenon is initiated by a series of pre-discharge (called "precursors") starting from the cloud to the ground.
- They are progressing by leaps and bounds about 50 m at a speed of about 50 000 km per second.
- A second ray leaves then from the same point, follows the precedent channel with a comparable speed
It exceeds the breakpoint of the first of about the same distance and disappears too.
THE PROCESS IS REPEATED AS...
- ... until the last head of the precursor reaches tens of meters or a few meters above the ground.
- The phenomenous does a break between each jump.
It is at this time that the pre-discharge bottom (tracers) are formed and are trying to reach the precursor
During the meeting moment with a precursor down, a conductive bridge is established between the cloud and the ground (ionized channel) and allows the passage of a current of high intensity.
The current is constitued by superficial charges of the ground which going up the ionized channel, neutralize les charges accumulated at the cloud base : it is the return arc .
The warm air produced by this current will cause the expansion and an explosion follows: thunder.
SOME INSTANTS LATER ...
- New precursors descendants and arcs back will take the same path
- They progress fastly.
- We count around an average of 4 go-returns on this same path.
- The human eye sees only one flash.
- We observe a minority in case of ascending lightning strikes. In this case, the leader develops from a large prominence of the ground for example, in the mountains or large buildings.
- ALSO THE DESCENDING AND ASCENDING LIGHTNING STRIKE ANOTHER CLASSIFICATION OPERATES ACCORDING TO THE POLARITY OF THE IMPACT OF LIGHTNING
- There are negative lightning strikes, from a negative charge from the cloud, and lightning strikes positive from a positive charge
- Descending and negative lightning strikes are the more frequent (more than 90 % in Europe).
- The average value of the intensity of a lightning strike current is : 35 000 amperes
- Total duration of a lightning strike : 0.2 to 1 second
- Number of discharges : 4 in average,
- Quantity of neutralized charges : 5 to 200 Coulombs,
- Local duration of a storm : 2 hours maximum,
- Scope of the spectrum: the spectrum covers a power range of a few MHz, this range can sometimes reach several GHz.
- in Lightning protection, the good knowledge of the phenomenon associated to the comprehension of the need of each customer allow reaching the best results.
