Lightning

What is lightning?

By definition, a lightning is a very bright glow, an intense light, forming a sinuous line, translating the discharge of atmospheric electricity, at the time of a storm.

Lightning is a meteorological phenomenon related to lightning and thunder. More precisely, lightning is the term for the phenomenon of electrical discharge and therefore includes lightning and thunder. Lightning is the visible part of lightning. The thunder is the sound part and comes from the rapid heating of the air related to the lightning.

How is lightning formed?

Lightning is formed from a storm cloud, the cumulonimbus. This cloud contains billions of ice crystals and water drops. When they collide, they exchange electrons, it is the friction of the hot and cold air currents that causes the electrification of the cloud.

The drops of water (the heaviest) which are negatively charged, will fall in the lower part of the cloud, a large quantity of positive charge thus remains at the top. This phenomenon creates a potential difference in the cloud. Since similarly charged objects repel each other and oppositely charged objects attract each other, it is the air, a very good insulator, that keeps them apart.

However, when the friction and the potential difference become too important, the air decreases its insulating capacity and the electric currents will make a passage in this central zone to join. It is at this moment that a lightning bolt is formed between the two charge packets, in the cumulonimbus.

Characteristics

Lightning can be divided into four types according to their spatial course.

• Intra-cloud lightning: it corresponds to the aerial electric discharge which propagates inside the cloud (diagram above). It is about 80% of lightning.

• The extra-cloud lightning: it corresponds to the aerial intra-cloud discharge (as seen previously) which escapes from the cumulonimbus and dies in the air or ends in a lightning strike. It is outside the cloud.

• Inter-cloud lightning: this type of lightning very often comes after an intra-cloud lightning and works on the same principle. The only difference is that the electrons are not exchanged within the same cloud but through two distinct clouds.

• The cloud-to-ground lightning: it is also known as lightning strike and represents the electric discharge developing from the cloud to the ground (said descending/negative), they represent about 20% of lightning. Conversely, we speak of positive clouds (ascending) when the discharge develops from the ground to the cloud. Negative flashes are more frequent, they represent 95% of cloud-to-ground flashes. Although only about 5% of lightning is positive, it is very important, since it carries a much higher charge, lasts longer and tends to cause more damage. A cloud-to-ground lightning strike begins when negative charges move to the ground, attracted by positive charges that are concentrated on high objects such as trees, poles and buildings. An electric current is created; this is the phenomenon we see in the sky and call lightning.

All lightning does not occur only during thunderstorms. Indeed, they can occur during forest fires, volcanic eruptions but also during sand or dust storms. In these cases, it is the particles of sand or ash, for example, that become electrically charged and create atmospheric conditions similar to those of a thunderstorm.

Color

The color of a lightning bolt depends on the current density, the distance between the observer and the storm, and the particles present in the air.

• The lightning will be red when there is rain in the air
• Blue when there is hail
• Yellow when there is a large amount of dust in the air
• White if the air is very dry

Shape

• Sinuous : when it has many apparent lines and segments
• Globular : a very rare phenomenon where a luminous sphere with a diameter between 5 and 30 cm appears
• Ramified : when it divides into several branches
• Fulminant : when it is curved

Size and speed

Concerning their characteristics, they can measure from 100 m to 20 km long for a thickness of 3 cm and move at the speed of light or 300 000 km/s. Several situations for you, if :

• You hear the thunder 30 seconds after viewing the lightning: the storm is then far enough, you are safe.
• You hear it 15 seconds after the lightning: you are only 5 km away from the storm.
• You are at risk if you hear the thunder after only 5 seconds.

The records

The Megaflash, the longest lightning in terms of distance :

The Megaflash, the longest lightning in terms of distance
On April 29, 2020, the new record of a mega lightning was recorded by the WMO (World Meteorological Organization). It is considered as the longest lightning, which extended over Mississippi, Louisiana and Texas, that is to say over 768 km. This is the distance from London to Hamburg.

The longest lightning in terms of duration :

The WMO also recorded a new record, this time concerning the duration of a lightning. 17.1 seconds, on June 18, 2020 over Uruguay and northern Argentina.

The most deadly lightning :

By direct impact: it was recorded in 1975 in Zimbabwe where 21 people lost their lives by a single lightning strike, while they were sheltered under a hut.
By indirect impact: in 1994, 469 people lost their lives in Dronka in Egypt, by a lightning strike on a fuel depot, which then caught fire and spread in the city.