How lightning works
Although there is still some debate in the scientific community about how the electrification of clouds actually occurs, it is agreed that the separation of positive and negative charges must occur within a cloud for lightning to take place. It is also generally agreed that ice must be present within a developing storm for it to eventually form lightning.
The turbulent wind environment of a thunderstorm with its updrafts and downdrafts is an ideal environment to separate electric charges: negative charges generally gather near the base of the cloud, while positive charges build in the upper reaches of the cloud. This allows electric fields to form and grow between the cloud and the ground and within the cloud itself - all necessary conditions for lightning to occur.
Since similar-charged objects repel each-other and opposite-charged objects attract each other, negative charges then begin to spread out near the base of the cloud. At the same time, positive charges start to build underneath the storm. This region of positive charges travels underneath the cloud, almost like a shadow. The positive charges tend to concentrate on tall objects, like trees, poles and buildings.
A cloud-to-ground lightning strike starts as a channel of negative charges makes its path towards the ground. This occurrence is known as a stepped leader. The stepped leader continues towards the ground in a series of steps that are each about 50 to 100 metres in length. This stepped leader can branch out in many directions.
In response to the discharge of negative charges coming from the cloud base, currents of positive charges start moving upward from the ground, usually along elevated objects; these are called streamers or upward leaders.
When the stepped leader and the upward leader meet, usually between 30 to 100 metres above the ground, the negative charges begin to flow downward. Almost instantaneously, a much larger and luminous electric current shoots up to the cloud, following the path taken by the stepped leader. This is known as the return stroke, and it is also what we see in the sky that is known as lightning. This whole process occurs so quickly (in less than one second!) that the lightning appears to travel from the cloud to the ground, when in fact, the opposite is true.
To see this process in action, you can watch the slow motion video which shows a single lightning strike. In the video you can see the many paths that the lightning wants to take, but it finds only one. The duration of the video is less than one second. No wonder the naked eye cannot see all the steps involved!
Examine above the photo of lightning hitting a tree. The stepped leader has obviously connected with the upward leader that came from the tree. But if you look closely, you can also see an upward leader coming from the tree that did not connect (indicated by the red arrow). There is also a very faint upward leader visible, coming up from a telephone pole just to the left of the tree (marked by the yellow arrow). These occur too fast and are often too faint for our eyes to see, other than when they are caught on film like this photographer managed. This photo also clearly demonstrates why it is not wise to seek shelter under a tree during a thunderstorm!
Cloud-to-ground lightning is the most dangerous and damaging form of lightning. Despite this, lightning within a cloud or that goes from cloud-to-cloud is the most common type of lightning. This type of lightning looks like a bright flicker in a cloud. For every cloud-to-ground strike, there can be three to five cloud-to-cloud strikes.
Another type of lightning occurs when some strokes actually emit from the ground into the sky. In this case, the stepped leader proceeds from the ground towards the cloud. The photo below shows this phenomenon well.
The Different Looks of Lightning
Lightning can look very different. For example, you can recognize forked lightning by its jagged or crooked lines. You can also often see several branches shooting from the cloud to the ground, within a cloud, from one cloud to another, or even from the cloud to the air.
Other types of lightning:
- The term “sheet lightning” is used to describe flashes that illuminate an entire cloud, but where the actual bolt of lightning is not seen.
- Heat lightning is a term used to describe distant lightning that you can see but not hear. In this case the visibilities are usually good and the sky is often clear overhead.
- The term “St. Elmo’s fire”, while not lightning, is used to describe still charged particles that appear as a blue or greenish glow above pointed objects such as trees, ship masts or airplane noses. It can sometimes be seen before a lightning strike.
- Lightning can also be seen at high altitudes, shooting upwards out of thunderstorms. In these cases lightning is given fanciful names such as elves, sprites and blue jets.
Negative versus Positive Lightning Strikes
The vast majority of lightning strikes are negative lightning strikes, meaning there is a net transfer of negative charges from the cloud to the ground. Negatives strikes make up about 95 per cent of cloud-to-ground strikes.
When there is a net transfer of positive energy from the cloud to the ground, the strikes are considered positive lightning strikes. These strikes originate from areas of the cloud that have areas of high positive charge, such as in the anvil or top of the cloud, or upper parts of the thunderstorm.
Even though only about five per cent of lightning strikes are positive, they are quite significant because they carry a higher charge and last longer than negative strikes. Due to this, they tend to cause more damage to power and electricity infrastructure, and also start more forest fires than negative strikes. Positive strikes are more common in severe thunderstorms and in thunderstorms that occur during the winter.
Lightning does not just happen in thunderstorms…
Lightning is also known to occur in dust storms, forest fires, and volcanic eruptions. Particles such as sand, smoke and ash, which exist in these environments, can also become electrically charged and create atmospheric conditions similar to that of a thunderstorm.
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