Does lightning always have to strike the ground?

The most common way to draw lightning is a zigzag line from the cloud to the ground. However, this is not the only way lightning can strike. This relates to the rather complicated way in which lightning is produced in a storm cloud.

Even though it is made up of only water, a storm cloud is a huge structure, ranging from a few kilometres above the ground to 12 kilometres up. Some clouds can peak up to 20 kilometres above the ground! The top of the cloud reaches into the cold tropopause and is made up of ice chunks. Intense updrafts and downdrafts occur throughout the cloud. Droplets of water carried up by the upward flow collide with descending heavy chunks of ice. These are the nuclei of hail, known as graupel.

When they collide, charge is transferred by the triboelectric effect. The falling ice chunks carry a negative charge to the bottom of the cloud, while the rising droplets carry a positive charge upwards. Gradually, a large positive charge builds up at the top of the cloud, while a negative charge builds up at the bottom. Areas of separated charge can split into several other regions due to movements in the cloud.

A very strong electrical voltage is created between the charges. Because air is a poor conductor, it acts as an insulator and prevents electric current from flowing between areas of opposite charges. But when a really big charge builds up, the strength of the electric field overcomes the insulating power of the atmosphere and a lightning strike occurs.