How lightning works

There is still some debate in the scientific community about how the electrification of clouds occurs. However, scientists agree that positive and negative charges must separate within a cloud for lightning to take place. Scientists also agree that a developing storm must produce ice for it to form lightning.

The turbulent wind environment of a thunderstorm with its updrafts and downdrafts is an ideal environment to separate electric charges. Negative charges gather near the base of the cloud, while positive charges build in the top 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 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. The image below shows positive charges as red plus symbols and negative charges as blue minus symbols.

An overlay of positive and negative charges added to a photo of a mature prairie thunderstorm as it builds near Yellow Grass, Saskatchewan. Photo credit Dwight Clarke.
Photo: © Dwight Clarke 2004

A cloud-to-ground lightning strike starts as a channel of negative charges called a stepped leader makes its path towards the ground. The stepped leader continues towards the ground in a series of steps that are each about 50 to 100 metres in length. The 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 called streamers or upward leaders start moving upward from the ground, usually along elevated objects.

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 immediately, a much larger and luminous electric current shoots up to the cloud, following the path taken by the stepped leader. This is called the return stroke and is what we see in the sky as lightning. This entire process occurs so quickly (in less than one second) that lightning appears to travel from the cloud to the ground, when the opposite is true.

Slow-motion view of a lightning strike

In the video above, you can see many paths that the lightning wants to take, but it finds only one. The video at normal speed lasts less than one second, faster than the naked eye can follow all the steps involved. 

A lightning strike, or lightning flash, usually comprises more than one stroke of lightning. After the initial strongest stroke, weaker strokes can follow which usually, but not always, follow the same path as the initial stroke.  When the lightning follows the same path, it appears to flicker. If it takes a different path, it can appear to dance. There are usually three to four subsequent strokes on average, but the number can be as high as over twenty.

a photo of lightning hitting a tree at night, with a yellow and a red arrow indicating the direction of two branches of the lightning bolt. Photo credit Johnny Autery, 1984.
Photo: © Johnny Autery 1984

In the above photo of lightning hitting a tree, the stepped leader has connected with the upward leader that came from the tree. 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 (indicated 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. This photo also clearly shows 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 inside a cloud or that goes 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 moves from the ground towards the cloud, such as in the photo below.

A photo of ground-to-cloud lightning.  Photo Credit: James Syme, 2014
Photo: © James Syme 2014

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: 

  • Sheet lightning - flashes that illuminate an entire cloud, but where the actual bolt of lightning is not visible
  • Heat lightning -  distant lightning that you can see but not hear. In this case, visibility is usually good and the sky is often clear overhead
  • “St. Elmo’s fire” - still-charged particles that appear as a blue or greenish glow above pointed objects such as trees, ship masts or airplane noses. While not lightning, it is sometimes seen before a lightning strike
  • Lightning seen at high altitudes, shooting upwards out of thunderstorms, is given fanciful names such as elves, sprites and blue jets

Negative versus positive lightning strikes

Most lightning strikes are negative lightning strikes, meaning there is a net transfer of negative charges from the cloud to the ground. Negative 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 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 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 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 can occur in dust storms, forest fires, and volcanic eruptions. Particles such as sand, smoke and ash, which exist in these environments, can become electrically charged and create atmospheric conditions similar to that of a thunderstorm.

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