The presence of unstable air masses, warm moist results in the formation of storm clouds:cumulonimbus : cumulonimbus.
This type of cloud is very developed, both horizontally (about 10 km diameter) and vertically (up 15 km). Its shape, very characteristic, is often compared to an anvil profile he has the lower and upper horizontal planes. The existence in a cumulonimbus very high temperature gradients (the temperature can drop to - 65 ° C top), resulting in very fast air currents upward; it follows an electrifying water particles.
In a typical storm cloud, the upper part, consisting of ice crystals, is generally positively charged, while the lower part, consisting of water droplets, is negatively charged. By induction, the bottom of the cloud leads to the development of oppositely charged (ie positive on the part of the ground that is nearby).
Electric exchanges occur within the cloud, but also between different clouds similar. We then see lightning.
However, the cumulonimbus also behaves like a giant capacitor cloud to ground level, the intermediate distance often reaches to 1 2 km.
The electro-atmospheric field on the ground, which is of the order of hundreds of volts per meter (weather permitting), is then reversed and can reach absolute 15 20 in kV / m when a discharge is imminent ground .
This discharge when it occurs is called Thunderbolt.
Observation without specific hardware can not discern the phases of the formation of a thunderbolt.
Yet the following phenomena occur in most lightning strikes :
- A falling bright tracer from a cloud point and the lead of about 50 m at a speed of 50 000 km / s.
- A second downward leader from the same point, following the previous path with comparable speed, beyond the stopping point of the first of similar distance and disappears too.
The process is thus repeated until the head of the last tracer reaches tens of meters from the ground.
As soon as the head of a downward leader approaches the ground, a cloud-to-ground link is established with electrostatic corona (corona) from the floor. This produces an upward leader from the ground to the cloud: this is the boot phenomenon.
Note: The "greatness" of the boot is inversely related to the distance to the storm cloud. That is why a high-rise building will be "easily" hit by lightning.
The meeting of two phenomena (downward leader, bottom) is the main discharge (lightning), which may be followed by a series of secondary discharges browsing the channel ionised by the main discharge of a single tracer.
In an average negative lightning stroke, the value of the current intensity is close to 25 000 amps.
The characteristics phases of the evolution of a sudden negative lightning :
1. Plotter downhill by leaps and bounds.
2. Initiation of the ascending tracer.
3. Meeting between an upward leader and the descending stepped leader and flow of arc current in return.
3 RUE DU GOLF