Asked by: Frances Worthington, Somerset
Before a lightning strike, the strong electric field underneath a thundercloud causes the air to ionise as electrons are stripped away from air molecules. But this isn’t a neat or uniform process; fluctuations in the electric field and impurities in the air result in irregular paths of ionised air reaching down from the cloud towards the ground. These are called step leaders.
As they get closer to the surface, the electric field surrounding the step leaders causes objects on the ground to send out streamers of the opposite charge. Taller objects will start sending out streamers first and pointed or sharp edges are more likely to initiate streamers. The first streamer to reach a descending step leader completes the circuit and triggers the lightning strike. All of this means that tall, pointy objects standing alone in an open space are more likely to get struck, but it’s by no means a certainty. Sometimes the flat ground next to a tall tree can be hit.
A car or other enclosed metal structure is the safest place to be in a thunderstorm. Failing that, a ditch, trench or group of shrubs of uniform height is better than nothing. Keep away from boundary areas between dissimilar terrain (water and land; rock and earth; trees and fields). Also keep at least five metres away from metal objects or other people as lightning will often jump from one object to another.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.