Yes, we all know it has to do with the clouds, but let’s get deeper, what is the problem with the clouds after all?

The thunderstorm cloud:

First, let’s make clear that not every cloud generates a thunderstorm. The cloud has to be a Cumulonimbus cloud type, which is characterized for its huge anvil shape: a flat base that can extend up to 2 km, a very tall (up to 15km) towering body and a large circular chaotic top. The formation of a Cumulonimbus cloud requires moist air at the ground level to be warmed up fast (eventually through sun’s radiation) and ascend. In this processes, the moist air cools, condenses and starts building the thunderstorm cloud.

Inside a Cumulonimbus cloud:

Things get a bit crazy here. Associated with any storm, the atmosphere gets very unstable and dynamic. Inside the gigantic cloud there will be updrafts of warmer air carrying water droplets and downdrafts of cooler air carrying ice crystals. During their paths water droplets and ice crystals collide and rub each other generating static electrical charges. Particles in the ascending air become positively charged and particles in the descending air become negatively charged.

Out of this disorder, the top of the cloud becomes positively charged and the bottom becomes negatively charged. Furthermore, due to the huge negative charge accumulation at the bottom of the cloud, electrons at the ground level get repelled, making objects positively charged. All these events lead to the creation of a massive electric field with a huge potential difference.

Right before the lightning:

When charge differences in the cloud become large enough, the isolating capacity of the air breaks down. A ”stepped leader” is formed. Depending on its origin a stepped leader can be either positive (top of the cloud) or negative (bottom of the cloud) and they involve positive and negative charge transference respectively.

If the stepped leader is negative then it consists of a flow of electrons that leave the cloud and it is called so since the electrons travels in steps of 50m per 0,000001s doing a full stop between the next step for 50 microseconds. During the stop-gap, if there are any positive charged objects in reach (within 50m or less) the next step will hit it. This flow of electrons is not uniform during its path but instead it ramifies like a tree due to different conditions in the atmosphere, always trying to find the fastest route to an opposite charged end. As the negative stepped leader approaches a positive charged end it attracts a big amount of positive charges which will even start forming a channel towards it. This last flow of charges is called streamers. When a streamer and a stepped leader connects lightning takes place.

If the stepped leader is a positive leader the same process described before will occur with opposite charges involved.

a) stepped leader descending b) streamer forming and ascending c) connection moment

The lightning:

A lightning can occur between two clouds, inside the same cloud and cloud to ground (ground to cloud is also possible but it is extremely rare!). In addition it can be called positive or negative (depending on its stepped leader).

When the streamer and the stepped leader connects, a massive discharge takes place. Electrons (in a negative lightning), flow from the negative end to the positive end in an attempt to reduce the potential difference. This charge transference is now called a return stroke, and this is what we refer to as lightning. The return stroke starts with a flash at the connection point. That flash moves against the flow of electrons (from the connection point near the positive end to the negative end) as electrons accelerate towards it from the nearest to the farthest.

(Imagine a traffic light that changes his colour to green; The cars will accelerate in the direction of the traffic light from the front to the back of the row in the same way the electrons will accelerate towards the connection point from the nearest to the farthest)

Return stroke in cloud to ground negative lightning.

This current averages around 30 thousand amperes and 300 million volts!

When the ”return stroke” ceases, within the next instants, if there is enough charge remaining within the cloud, a new leader can follow (or not) the already opened channel between the two opposite charged ends and restart a discharge (this time the leader is called dart leader). Dart leaders confer the lightning its flickering appearance and they usually don’t ramify like a stepped leader. When they don’t follow the path created by the stepped leader the lightning will jump from one place to another almost instantly, this is known as ”forked lightning”.

Successive dart leaders become gradually weaker until they eventually cease (remember this is a very fast process). A ”leader” and its ”return stroke” constitute a single ”stroke”, furthermore all strokes that follow the same channel form a single flash (some flashes are made up with more than 40 strokes!).

Positive lightnings develop in the same way but with inverted charges. They are so rare that only account for 5% of the total lightning observations! However they are much more powerful than negative ones, being able to reach 1 billion volts and 300 thousand amperes!

positive lightning

A lightning bolt heats up the air around up to five times the sun’s surface, making the air expand so violently that it produces the explosive sound we hear associated to a lightning bolt.

Thunderstorm phenomena don’t end here. In fact, in the upper layers of the atmosphere there are more things going on like blue jets, Sprites and other short-lived electrical-breakdowns. These are a topic for a future post!

In order to easily explain this content I made up a search across the web (Nasa site, National geographic site) and join the information together in a summarized way.