December 2014
Figure 1. Lake Calhoun Lightning with boats, dock and thunderstorm watchers, Minneapolis, Minnesota.
Photo courtesy of Bruce Challgren. © Bruce Challgren, PhotoPixels
(http://www.photopixels.com/lightning/index.html)
Look at the photograph above; what do you notice? Look again, at the contrast between the lightning and the buildings of Minneapolis, Minnesota. Lightning; usually happens on a large scale, and can both excite and intimidate us. Last time we investigated thunder, and had an introduction to lightning. Did you know the chance to be killed by lightning is 1 in 2,000,000? That is approximately the same chance of dying from falling out of bed!
In this installment, we investigate the cause of, and mechanism behind lightning strikes. As seen in the photograph above, lightning can be awe inspiring and beautiful. But, how does lightning form? This question is not a simple one. The topic of lightning is still being debated by scientists. Presently, the general understanding of lightning is categorized as: (1) static charge buildup in clouds, and (2) changes in the electric field surrounding a cloud.
Polarized (see def.), the positive and negative charges within a storm cloud, are a precursor to any lightning strike. The tops of storm clouds are known to acquire an excess of positive charge, and the
1 | P a g e

bottoms of the storm clouds acquire an excess of negative charge. Two mechanisms are known to be involved in the polarization process.
The first mechanism is a separation of charges by a process that resembles frictional charging, which is the same mechanism we use to shock people by rubbing our feet on the carpet. Inside of clouds are millions of water droplets and ice particles whirling about. When water from the ground evaporates and rises upward toward a cloud, clusters of droplets
Figure 2. Static charge formation in clouds taken.
(http://thunder.msfc.nasa.gov/primer/primer2.html) form. This upwardly rising moisture collides with water droplets within the clouds. In these collisions, electrons are ripped off the rising droplets, causing a separation of negative electrons from the positively charged water droplets or clusters of droplets.
The second mechanism contributing to polarization inside a storm cloud involves a freezing process.
Rising moisture encounters cooler temperatures at higher altitudes. These cooler temperatures cause the clusters of water droplets to begin to freeze. The freezing particles tend to cluster more tightly together and develop a separate, central region within the cluster of droplets. The frozen portion of a cluster of rising moisture becomes negatively charged and the outer droplets acquire a positive charge. Air currents within the clouds can rip the outer portions off the clusters and carry them upward toward the top of the clouds. The frozen portion of the droplets with their negative charge tends to gravitate toward the bottom of the storm clouds. Thus, the clouds become further polarized.
As negative charges gravitate to bottoms of clouds, electrons below on Earth’s outer surface (say for example those in trees, buildings, etc.) are repelled by the negatively charged bottom surface of a cloud. This creates an opposite (positive) charge on the Earth’s surface. Buildings, trees, and even people can experience a buildup of static charge as electrons are repelled by the cloud’s bottom. With the cloud polarized into opposites, a positive charge induced upon Earth's surface, and the static charge inside the cloud building, the next stage of formation of a lightning strike is about to begin.
Figure 3. Cloud-to-ground lightning over eastern
Colorado showing stepped leaders and multiple return strokes. Photograph courtesy of David
Blanchard Photography.
(http://www.dblanchard.net/blog/2010/07/lightnin g-images-return-strokes-and-stepped-leaders/)
Under normal circumstances, the air surrounding a cloud would serve as a fine insulator to prevent a discharge of electrons to Earth. But, with the cloud’s static charge polarized, the electric fields around the
Nature Physics: Lightning
December 2014
2 | P a g e

cloud become capable of ionizing (see def.) the surrounding air and making it more conductive. The ionization involves the shredding of electrons from the outer shells of the gas molecules that compose air, creating a conductive soup of positive ions and free electrons and making a charge transfer (lightning bolt) from the cloud to the ground (or even to other clouds) possible.
The bolt begins when excess electrons on the bottom of the cloud begin a zig zag, branching path to the ground at speeds up to 60 miles per second. The newly forming bolt is called a step leader. It is not the actual lightning strike; it merely provides the path to Earth that the actual lightning bolt will travel.
When the electrons of the step leader near the
Earth’s surface, the amount of positive charge on the surface is enhanced, migrating upward through nearly all things. The upward charge is called a streamer, and it approaches the step leader high in the air above the surface. Once contact is made between the streamer and the leader, a complete conductive pathway is forged and the lightning begins. This return stroke is shown in the photograph at the right. This was taken by David
Blanchard Photography in northern Arizona, in
2010. The initial strike is followed by several secondary strikes, which are spaced so closely in time that they may appear as a single strike. The tremendous charge along the pathway heats the surrounding air, causing it to expand violently, and create the shockwave we know as thunder.
Figure 4. Photograph courtesy of David
Blanchard Photography.
(http://www.dblanchard.net/blog/2010/07/lightning
-images-return-strokes-and-stepped-leaders/)
Lightning and the resulting thunder can occur in many different ways. Lightning can occur anywhere and in any direction that has a large enough charge differential (the difference between positive and negative charges). The different types of lightning are:
Intra-cloud
Fun Fact
At the moment when the pathway for a lightning bolt is mapped by the step leader and the streamer, the contact point between ground charge and cloud charge rapidly ascends upward at speeds as high as 50,000 miles per second. As many as a billion trillion electrons can transverse this path in less than a millisecond.
The most common form of lightning, occurs between two oppositely charged centers of the same cloud. This discharge causes the cloud to flicker and glow.
Cloud-to-Cloud
This form of lightning occurs between two oppositely charged centers of two different cloud formations. The lightning flash will bridge the gap between the two clouds.
Nature Physics: Lightning
December 2014
3 | P a g e

Cloud-to-Ground
This is the most damaging form of lightning. It is also the most dangerous. However, it is not the most common. This type of lightning forms toward the base of the cloud, and resolves upon contact with the
Earth. This is the type associated with lightning strikes.
Cloud-to-Stratosphere
Red Sprites, Blue Jets, and Elves are all forms of cloud to stratosphere lightning. This type of lightning originates in the upper levels of the clouds and travels upward into the stratosphere and has only recently being discovered with the first photographic images being captured by accident in 1989.
Fun Fact
Red Sprites
The temperature of a lightning bolt can be as high as 50,000°F. This is approximately 5 times the temperature of the surface of the sun!
Sprites are massive but weak luminous flashes that appear directly above an active thunderstorm system and are coincident with cloud-to-ground or intracloud lightning strokes. They extend from the cloud tops to altitudes up to about 95 km. Sprites rarely appear singly, usually occurring in clusters of two, three, or more (elf.gi.alaska.edu).
Current evidence strongly suggests that sprites preferentially occur in decaying portions of thunderstorms and are correlated with large positive cloud-to-ground lightning strikes.
Blue jet
Blue jets appear to emerge directly from the tops of clouds and shoot upward in narrow cones through the stratosphere. A blue jet was first captured on camera in 1994. The jets appear to spurt upward from speeds of 50–100 miles per second. They may reach a height of up to 25 miles. They last for only a very short period of time. Evidence suggests that they typically last for less than a quarter of a second. Blue
Jets differ from Red Sprites in that they do not appear to be connected to cloud-to-ground lightning discharges. They are also more likely to be seen near the highest parts of major thunderstorm cells.
During massive storms, lightning can brighten up the dark night sky like the mid-day sun. The light can be seen from miles away. To us, the local observer, the power and awe of lightning can feel intimidating, but provides a sense of grandeur. In our continued exploration of natural science, next time we will discuss lightning on a much smaller scale, static electricity.
Ionization.
To change into ions. An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge. Ions, in their gas-like state, are highly reactive, and do not occur in large amounts on Earth, except in flames, lightning, electrical sparks, and other plasmas.
Nature Physics: Lightning
December 2014
4 | P a g e

Polarized.
Separated or accumulated positive and negative electric charges in two distinct regions.
(http://www.photopixels.com/lightning/index.html) http://www.physicsclassroom.com/class/estatics/Lesson-4/Lightning http://www.geekphilosopher.com/GeekPhilosopher.com/gallery http://elf.gi.alaska.edu/ http://www.atmo.arizona.edu/students/courselinks/spring07/atmo589/articles/Lyons_et_al_BAMS_April_
2003.pdf
(http://www.dblanchard.net/blog/2010/07/lightning-images-return-strokes-and-stepped-leaders/)
Website: http://www.portageinc.com/community/physics.aspx
E-mail: Physics@portageinc.com
Nature Physics: Lightning
December 2014
5 | P a g e