Lightning arises when a large potential difference breaks the dielectric strength of atmospheric air.
Electric discharges that propagate between clouds and the ground due to a large electric field are called lightning and can have two polarities : positive and negative. About 90% of the rays are of negative polarity , carrying electrons from the cloud to the ground. The other 10% are much rarer positive polarity rays . In these rays, electrons are transported from the ground to the cloud .
The first process is that of preliminary stiffness breaking . In this process, small electrical discharges are formed inside the cloud between its negative and positive regions. Then a generally invisible electrical discharge , called a staggered leader , leaves the cloud towards the ground – its speed can reach up to 400,000 km/h ! Altogether, this initial discharge can deposit up to 10 C of charge on the ground.
When the stepped leader reaches a few tens of meters from the ground, the electrical potential produced exceeds 100 million volts. Such a potential difference causes the breakdown of the dielectric strength of the air, which becomes a conductor . All this energy generates new ascending rays (that go up), called connecting leaders . When the ascending and descending rays meet (which descend), the greatest amount of light from the rays (lightning) is produced. The electric current that forms towards the ground can reach values of more than 200 thousand Amperes.
The rays can produce plasma, X-rays and gamma rays
While the surface temperature of the Sun is approximately 5800 K , the temperature of the air that conducts the rays can easily reach 30,000 º C. The great difference in potential between clouds and the ground can generate a great acceleration in electrical charges, producing electromagnetic waves that range from visible radiation to X-rays and even gamma rays , the radiation with the highest penetration power known.
Because of the tropical climate and its large territorial extension , Brazil is the country with the highest incidence of lightning in the world. In all, more than 57 million lightning strikes are recorded every year. However, the chance of being struck by lightning in Brazil is very low: around 1 in 1.5 million , more than thirty times easier than hitting the six dozen in the Mega-Sena da Virada .
Even with such low odds, Roy Sullivan, the ranger of a US National Park, entered the Book of Records after being struck by lightning no less than seven times, on separate occasions. Despite being wounded, he survived them all.
Rays and Mythology
Lightning has always been present in the myths of several ancient civilizations . They were, in most cases, interpreted as divine manifestations . To the ancient Greeks , thunderbolts were spears produced by the Cyclopes so that Zeus could punish humanity for its failures. The laurel , a shrub widely used on the head of Roman generals and emperors, was seen as an amulet of protection against lightning. Even in Medieval Europe it was believed that the tolling of church bells functioned as divine protection . against the rays.