In addition to wind and heat, most storms produce electrical energy. This energy is generated by the formation and separation of charged particles within the clouds.
When chasing tornadoes, scientists like Tim Samaras look for
lightning in the storm. Lightning usually occurs ahead of the region in which a tornado is most likely to form. This knowledge helps Tim decide on the best locations for deploying his probes.
Although lightning is a common weather condition, scientists aren’t sure what causes clouds to become electrified. However, they do know that electrically charged clouds are unstable.
Whirled by winds, positive and negative charges separate and collect in different regions of the cloud. As the cloud’s store of electrical energy increases, this separation becomes increasingly unstable. When the separation of opposing electrical charges becomes too great for the cloud to maintain, it releases its stored energy. The release of charged particles races through the air and creates the brilliant flash that we see as lightning.
Lightning takes different forms. The most common discharge never reaches the ground. It doesn’t even leave the cloud in which it forms! These bolts transfer charges between regions of the same cloud and return the cloud to a more stable state.
The next most common lightning surges between clouds and the ground. As electrical charges within a cloud separate, the top of the cloud becomes positive. The bottom takes on a negative charge. This negative charge produces a strong electromagnetic field that upsets the electrical balance of the surrounding air. It also causes the ground beneath the cloud to assume a positive charge.
The unstable state becomes strong enough to overcome the insulating capacity of the air between the cloud and ground. The first stream of negative charge races from the cloud to the ground. Called a stepladder, this small stroke follows a zigzag path. As it nears the ground, a stream of positive charge rises up from the ground. When these two charge flows meet, the circuit is closed. Immediately, a massive stroke of electricity—a lightning bolt—moves between the cloud and ground.
On rare occasions, lightning can even form between clouds. As this major transfer of charge occurs, a bright flash of lightning can be seen in the sky.
Lightning superheats the air around it to temperatures that can exceed the temperatures observed on the surface of the sun. This sudden and extreme change causes the air to expand violently. The expansion produces a shockwave, and the resulting sonic boom we hear and feel is
thunder.