Lightning is an atmospheric electrostatic discharge (spark) accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. From this discharge of atmospheric electricity, a leader of a bolt of lightning can travel at speeds of 220,000 km/h (140,000 mph), and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse silica sand into glass channels known as fulgurites which are normally hollow and can extend some distance into the ground. There are some 16 million lightning storms in the world every year. Lightning causes ionisation in the air through which it travels, leading to the formation of nitric oxide and ultimately, nitric acid, of benefit to plant life below.

Lightning can also occur within the ash clouds from volcanic eruptions, or can be caused by violent forest fires which generate sufficient dust to create a static charge.

How lightning initially forms is still a matter of debate: Scientists have studied root causes ranging from atmospheric perturbations (wind, humidity, friction, and atmospheric pressure) to the impact of solar wind and accumulation of charged solar particles. Ice inside a cloud is thought to be a key element in lightning development, and may cause a forcible separation of positive and negative charges within the cloud, thus assisting in the formation of lightning.

The irrational fear of lightning (and thunder) is astraphobia. The study or science of lightning is called fulminology, and someone who studies lightning is referred to as a fulminologist.

Lightning streaks across the sky as lava flows from a volcano
Lightning streaks across the sky as lava flows from a volcano in Eyjafjallajokul April 17, 2010. An Icelandic volcano that is spewing ash into the air and wreaking havoc on flights across Europe appeared to be easing up on Saturday but could continue to erupt for days or even months to come, officials said. REUTERS/Lucas Jackson
Lightning strikes just south of the McCarran International Airport air traffic control tower in Las Vegas
Lightning strikes just south of the McCarran International Airport air traffic control tower in Las Vegas, Nevada, during a powerful thunderstorm early September 2, 2003. Pictures of the month September 2003 REUTERS/Ethan Miller
Lightning flashes around the ash plume at above the Puyehue-Cordon Caulle volcano chain
Lightning flashes around the ash plume at above the Puyehue-Cordon Caulle volcano chain near Entrelagos June 5, 2011. The volcano in the Puyehue-Cordon Caulle chain, dormant for decades, erupted in south-central Chile on Saturday, belching ash over 6 miles (10 km) into the sky, as winds fanned it toward neighboring Argentina, and prompted the government to evacuate several thousand residents, authorities said. Picture taken June 5. REUTERS/Carlos Gutierrez
Lightning strikes over buildings during a thunderstorm in Belgrade
Lightning strikes over buildings during a thunderstorm in Belgrade June 8, 2011. REUTERS/Marko Djurica
Lightning lights up the night sky over Plainview, Texas
Lightning lights up the night sky over Plainview, Texas, late May 6, 2008, as more severe storms and possible tornados are forecasted for the next few days. REUTERS/Gene Blevins
The New York Times building is struck by lightning in New York June 10, 2008.
The New York Times building is struck by lightning in New York June 10, 2008. REUTERS/Nic Fulton
Volcanic lightning or a dirty thunderstorm
Volcanic lightning or a dirty thunderstorm is seen above Shinmoedake peak as it erupts, between Miyazaki and Kagoshima prefectures, in this photo taken from Kirishima city and released by Minami-Nippon Shimbun January 28, 2011. Ash and rocks fell across a wide swathe of southern Japan straddling the prefectures of Miyazaki and Kagoshima on Thursday, as one of Mount Kirishima's many calderas erupted, prompting authorities to raise alert levels and call on for an evacuation of all residents within a 2 km (1.2 miles) radius of the volcano. REUTERS/Minami-Nippon Shimbun/
Storm Storm Storm