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The Earth is struck by lightning over 100 times every second.

The Earth is struck by lightning over 100 times every second.

Lightning is an example of high-energy physics you can witness from home.

Water cycles between the ground and the atmosphere. The Sun heats water on the surface of the Earth, and liquid water becomes water vapour which rises and enters the atmosphere.

As the water rises it starts to cool and condenses on small particles of dust and salt in the atmosphere to form accumulations of water droplets and ice particles, called clouds. This water is eventually returned to Earth as rain, hail, sleet or snow.

The tallest clouds – cumulonimbus clouds which produce thunderstorms – can generate the most spectacular lightning, occurring in cloud-to-ground strikes.

The bigger the cloud, the more the water droplets and ice crystals are subjected to updraughts, downdraughts and collisions between themselves and with dust particles.

In storm clouds the collisions can be violent, as warm rising droplets hit cold falling droplets. Electrons are dislodged from the rising water molecules, with the droplets becoming positively charged and accumulating at the top of the cloud.

The dynamics of freezing exacerbate this process, called charge-separation, resulting in a strongly negatively charged cloud base and a strongly positively charged cloud top. The negative charge at the base of the cloud can be so intense that it repels electrons in atoms on the Earth's surface, driving them deeper into the ground, leaving a positive charge on the surface.

In a mechanism that is not fully understood, channels of ionized air finger their way from the base of the cloud, like branches of a tree.

A negative ion is an atom with a surplus of electrons; a positive ion has a deficit of electrons. The electrons in the air molecules in a channel are attracted to the positively charged ground. Electrons accumulate at the tips of these downward branches, attracted by the positively charged ground. The channels move in a series of jerks and are called "negative step leaders".

Similar channels of ions branch their way up from the ground. However, their tips are positively charged because electrons in air atoms closer to the ground are attracted towards the positively charged Earth. In this case they leave positively charged air molecules in the vicinity of the ground which extend upwards – these are called "positive streamers".

The downwards-moving leaders "search out" the upward-snaking streamers. After a leader and streamer connect, the conductive path from cloud to earth is complete. A massive flow of electrons travels from the cloud to neutralise the positively charged ground.

This is extremely hot and rapidly expands the air in its vicinity causing it to explode and create a shock wave we call thunder. The accompanying electric discharge flashes with a blue-white light.

A shock wave moves with a speed greater than the local speed of sound. The sound waves emanate from this region, cause pressure to build and announce their release with a load crack.

Lightning discharges commonly occur within a cloud and between clouds. In both these cases the high electric-charge differences can lead to a luminous electrical discharge.

Some volcanic eruptions are also accompanied by bolts of lightning. Particles of magma, volcanic ash and dust start out as electrically neutral, but violent collisions inside the volcano disrupt the electrical-charge distribution; this initiates the separation between positively and negatively charged ions.

As the detritus exits the volcanic cone, the electric charges segregate, forming clouds of oppositely charged particles. As in the thundercloud lightning, a point is reached where the charge difference and intensity becomes so great that current flows between the accumulations of oppositely charged masses and a lightning bolt appears.

The massive Tarawera eruption of 1886 was accompanied by violent lightning flashes. Witnessing lightning from such a volcanic eruption must have been terrifying. In February 2016, the eruption of Sakurajima in Japan was also accompanied by an impressive display of volcanic lightning.

According to the National Severe Storms Laboratory in the United States, about 100 cloud-to-ground lightning strikes occur every second in the world. One of the most lightning prone regions in the world is Northern Australia.

Global warming is introducing more energy into the atmosphere, powering more severe storms with the possibility of more frequent and more intense lightning strikes.


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