Runaway breakdown is a theory of lightning

Lightning

Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

 initiation proposed by Alex Gurevich

Alex Gurevich

Aleksandr Viktorovich Gurevich is a Soviet and Russian physicist. In 1992 he proposed the theory of lightning initiation known as the "runaway breakdown"....

 in 1992.

Electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s in air have a mean free path

Mean free path

In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...

 of ~1cm. Fast electrons which move at a large fraction of the speed of light

Speed of light

The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

 have a mean free path up to 100 times longer. Given the long free paths, an electric field

Electric field

In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 can accelerate these electrons to energies far higher than that of initially static electrons. If they strike air molecules, more relativistic

Theory of relativity

The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

 electrons will be released, creating an avalanche multiplication of "runaway" electrons. This process, relativistic runaway electron avalanche, has been hypothesized to lead to electrical breakdown

Electrical breakdown

The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....

 in thunderstorms, but only when a source of high-energy electrons from a cosmic ray

Cosmic ray

Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

 is present to start the "runaway" process.

The resulting conductive plasma

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

trail, many tens of meters long, is suggested to supply the "seed" which triggers a lightning flash.

External links

• How cosmic rays trigger lightning strikes
• Runaway Breakdown and the Mysteries of Lightning - Physics Today May 2005 Also available on-line at: http://www.phy.olemiss.edu/~jgladden/phys510/spring06/Gurevich.pdf .
• Nova Science Now segment on Lightning - Aired on PBS October 18, 2005