Description of How Lightning Occurs
By Dillon Sampson
Introduction
Lightning is a beautiful and powerful force of nature that occurs almost everywhere around the
world. This deadly phenomenon has bolt temperatures hotter than the sun (around 50,000
degrees Fahrenheit) and occurs about 40-50 times a second or around 1.4 billion flashes yearly.
Lightning is the occurrence of a massive electrostatic discharge either within a cloud or
between a cloud and the surface of a planet. Lightning has a short duration, extremely high
voltages, and is usually accompanied by a bright flash and thunder (a loud noise produced by a
lightning flash).
This document will take you through the process of lightning step by step and is designed for a
general audience. A lightning strike involves six steps: Charge Transfer in the Cloud, Charge
Separation at the Surface of the Planet, Stepped Leader, Connection of Channels, Return
Stroke, and Superheating.
Charge Transfer in the Cloud
Before lightning can flash there is a sequence of charge transfers
that begin with the clouds. Clouds contain millions of tiny water
droplets. These water droplets are in a continuous cycle of
condensing (the change from a gaseous state to a liquid form)
and evaporation (the change from a liquid state to a gaseous
state). As this cycle occurs, the water collides with other water
molecules that are rising and condensing. These collisions
generate electrons that create a charge separation in the cloud
(this separation can be seen in the figure to the right). A local
discharge occurs between the p-region (the small positively
charged region towards the bottom of the cloud) and the n-region (the larger negatively
charged region towards the top of the cloud). The negative electrons from the upper portion
move downwards to neutralize the n-region.
Figure 1 (right) shows the distribution of charges in a cloud.
Image from http://apollo.lsc.vsc.edu/classes/met130/notes/chapter14/charge_distribution.html
Charge Separation at the Surface of a Planet
As the cloud forms a negatively charged region at the base, there is a buildup of positive
charges underneath the cloud. Since positive and negative charges attract, the highly negative
region of the cloud pulls positive charges towards the surface of the planet, and inversely
pushes negative charges further from the surface. The positive charges form something similar
to a shadow that follows underneath the cloud on the ground. This positively charged shadow
concentrates at the highest points on the ground, usually tall objects (trees, buildings, poles,
etc.). The image above shows an example of positive charges moving towards the highest point
on the ground.
Formation of Channels
As the positive regions are formed at the ground, the electrons begin to
move downward from the cloud in what is called a “stepped leader”.
The stepped leader is a channel of ionized (charged) air that moves
towards the positively charged regions on the ground (seen in figure 2 to
the right and in the left image of figure 3). This channel initiates from a
repelling force of charges in the cloud (negative electrons pushing away
from one another) accompanied by an attractive force from the ground
(negative electrons moving towards the positive regions on the ground).
This channel can branch out into multiple smaller channels in a tree like
pattern. The stepped leader takes approximately one-hundredth of a
second to go from the cloud to the ground moving at around 200 miles
per hour.
Figure 2 (right) shows the stepped leader approaching the ground.
Image from http://apollo.lsc.vsc.edu/classes/met130/notes/chapter14/stroke2.html
Connection of Channels
As the stepped leader moves downward, an upward moving discharge (a channel of positive
charges) moves upwards to meet the negatively charged channel about 30-100 meters above
the ground. Positive charges are attracted to the approaching negative impulse and are pulled
towards the rapidly approaching negative channel. The upward discharge will determine where
the lightning will strike. When the two channels meet, a violent discharge of negative charge
moves rapidly downward towards the ground over the combined electric channel. Although
most people believe lightning strikes downwards, this step does not actually produce the visual
we call lightning.
Return Stroke
Almost instantaneously after the downward discharge of negative charge, a massive current of
positive charges moves upwards from the surface of the planet to the cloud in what is called a
“return stroke” (shown below). This movement of positive charges occurs to neutralize the
large imbalance of charge from the negative stepped leader. The upward flow of electrical
current reaches out to all branches of the stepped leader and follows the channel up to the
cloud. This upward surge happens at about half the speed of light and occurs within
microseconds.
Figure 3 (above) shows the stepped leader moving down (left image) and the return stroke that produces the flash
of lightning (right image).
Image from http://www.accuweather.com/en/weather-glossary/its-electric-lightning-safety/10822191
Superheating
The speed at which the return stroke occurs combined with the massive electrical current
rapidly superheats the completed leader channel. The channel turns into an electricallyconductive plasma channel reaching temperatures of 50,000 degrees Fahrenheit. This
temperature change produces the brilliant white-blue phenomenon we call a lightning strike.
In figure 3 above, you can see the visual difference between the movement downward and
upward. The stepped leader produces no light where the return stroke of positive charges
does. At the same time, the almost instantaneous heating of the air causes it to expand
explosively. This produces a shock wave of sound we call thunder. Once the current has
stopped flowing through the channel, the temperature will cool. The channel will then
dissipate in about 100 milliseconds.
Conclusion
Lightning is one of the most stunning and deadly forces found in nature. Understanding the
process of lightning proves vital in saving lives. An understanding of lightning strikes allows
humans to follow precautionary measures to prevent harmful incidents. Lightning occurs in six
steps: Charge Transfer in the Cloud, Charge Separation at the Surface of the Planet, Stepped
Leader, Connection of Channels, Return Stroke, and Superheating. Overall, the intense and
rapid electrostatic discharge serves as a way to create equilibrium of charges in the atmosphere
and on the surface of the planet.