Lighting explained
Since the dawn of humanity, man is fascinated by lightning. He attributed the phenomenon then stormy mythical dimension which reflected the anger of the gods.
Since the twentieth century that the study of lightning has become a science. It was then discovered that thunderstorms are essential to life, they maintain the electric fields of our planet.
Since the mid-twentieth century to today, the development of transport and distribution of energy, information technology and electronics motivates serious research on protection against lightning and thus the phenomenon it own.
Lightning Facts and figures:
- An average of 1 000 000 lightning strikes per year (source METEORAGE)
- – The annual cost of damage caused by lightning are estimated in billions of francs
- Between 8 and 15 deaths per year (source Association. Protection. Lightning)
- 20 000 animals foudroyés
- 20 000 claims due to lightning fires, including 15 000
- 50 000 electricity meters destroyed
- 250 bells destroyed
Physical phenomenon
Although one could observe lightning in storms of sand, snow or in the pyroclastic flows of volcanic eruptions, the main generator is the cumulonimbus storm clouds.
This cloud is growing in altitude to over 10 km and its base, which is about 2 to 3 kilometers above the ground, occupies several tens of square kilometers. It is home to very strong winds ascending and descending, causing the collision of ice particles, water surfondue involved. This is the friction between the particles resulting in electrifying the cloud and the separation of charges. The heavier particles (water drops) are negatively charged at the bottom of the cloud, while the lighter particles (ice crystals) are positively charged at the top of the cloud. Sometimes an island of positive charges is contained in the negative ground.
Under the influence of negative charges is located at the bottom of the cloud, the atmospheric electric field on the ground usually around 100 volts / meter reverses and reaches values of about -15 to -20 kV / m: The potential difference between the cloud and the ground is such that the discharge is imminent.
There are two types of lightning, the intra (or inter-cloud) that the discharge occurs within the cloud (or between two clouds) and cloud-ground, or thunderbolt, which the discharge occurs between the cloud and the ground. The latter are responsible for numerous losses and damage caused to the environment, buildings and men.
In most cases, fall in love (or cloud-to-ground) is a descendant.The phenomenon of the discharge is initiated by a succession of pre-discharge from the cloud to the ground and progressing by leaps (precursor for bonds).
Once one of the peaks of the precursor approaches the ground, pre-discharge bottom (tracers) form and join the forerunner down. It was then a driver bridge between the cloud and the ground (ion channel) that will allow the passage of a current of high intensity. This current is the expense of surface soil back channel ion neutralizes charges from the cloud: the arched back. The warming of the air caused by the current will lead to its expansion and an explosion will follow: the thunder.
The first discharge leaves a conductor connecting the cloud to the ground.
Moments later (10 to 70 ms) of new precursors and descendants arches will take back that track moving much faster. On average, 4 to go back on the same track. The human eye, because of the persistence of vision, usually does not receive a single flash.
There is a minority of cases of lightning strikes ancestors, in this case the tracer develops from a large prominence on the ground. On observe le phénomène en montagne ou sur des grands immeubles. There is the phenomenon in the mountains or tall buildings.
In addition to the thunderbolt descending and ascending, a classification based on the polarity of the impact of lightning. There are lightning strikes that is negative from a negative charge from the cloud and lightning strikes positive from a positive charge.