10.4: Lightning and Thunder

Lightning

Lightning is caused by a discharge of electricity due to charge separation that typically occurs in mature thunderstorms. The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward rapidly (updraft) and temperatures range from -15 to -25 Celsius, as illustrated in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Strong updrafts brush against precipitating particles and result in charge separation in the centre of the cumulonimbus tower. (Public Domain; Understanding Lightning via NOAA/NWS)

At that place, the combination of temperature and rapid upward air movement produces a mixture of supercooled cloud droplets (small water droplets below freezing), small ice crystals, and soft hail (graupel). The updraft transports supercooled cloud droplets and very small ice crystals upward. At the same time, the graupel, which is considerably larger and denser, tends to fall or be suspended in the rising air. Differences in precipitation movement cause collisions. When the rising ice crystals collide with graupel, the ice crystals become positively charged, and the graupel becomes negatively charged.

The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended within the thunderstorm cloud or falls toward the lower part of the storm. The result is that the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged. While this is the main charging process for the thunderstorm cloud, some of these charges can be redistributed by air movements within the storm (updrafts and downdrafts).

The buildup of charge influences the charge on the ground beneath the clouds. Normally, the ground has a slight negative charge. However, when a thunderstorm is directly overhead, the large negative charge near the base of the storm cloud repels negatively charged particles on the ground beneath it. This causes the ground and any objects (or people) directly underneath the storm to become positively charged, as depicted in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): People's hair often stands on end during thunderstorms because positive charge enters our body from the ground. The positive charge on the hair is attracted to the negative charges in the cloud. (Public Domain; Understanding Lightning via NOAA/NWS)

As the negative charge in the cloud increases, the ground becomes more positively charged. As the charge difference increases, positively charged particles are repelled by taller objects such as trees, houses, and telephone poles. A channel of negative charge, called a "stepped leader", will descend from the bottom of the storm toward the ground. It is invisible to the human eye and shoots to the ground in a series of rapid steps, each occurring in less time than it takes to blink your eye. As the negative leader approaches the ground, a positive charge accumulates on the ground and on objects on it.

This positive charge "reaches" toward the approaching negative charge via its own channel, called a "streamer". When these channels connect, the resulting electrical transfer is what we see as lightning. These steps are illustrated in Figure \(\PageIndex{3}\). After the initial lightning strike, if enough charge is left over, additional lightning strokes will use the same channel and will give the bolt its flickering appearance.

Figure \(\PageIndex{3}\): The three-step process of a cloud-to-ground lightning strike. In step 1, a positive charge accumulates on the ground near a negatively charged base of a thunderstorm. In step 2, a stepped leader descends from the bottom of the storm toward the ground, while the positive charge from the ground reaches out to meet the leader. When negatively and positively charged particles come into contact, a lightning strike occurs. (Public Domain; How lightning develops via NOAA/NWS)

Tall objects such as trees and skyscrapers are commonly struck by lightning. Mountains also make good targets. This is because their tops are closer to the base of the storm cloud. However, this does not always mean tall objects will be struck. It all depends on where the charges accumulate. Lightning can strike the ground in an open field even if the tree line is close by. Lightning can also strike within the same cloud, from one cloud to another, or from a cloud to open air. Only about 20% of lightning strikes are cloud-to-ground strikes, while the majority of strikes occur within the cloud.

As a lightning strike travels through the atmosphere, it heats the surrounding air to a temperature of approximately 30,000°C, which is roughly five times hotter than the surface of the Sun. This intense heating causes a sudden, explosive expansion of the air, generating shock waves radiating in all directions from the flash. This expands outward as a sound wave called thunder. Due to the speed at which light travels, we see lightning instantly as it happens. Thunder travels much more slowly at about 330 m·s^-1, so it takes much longer to reach our ears. You can estimate how distant a lightning stroke is by counting the seconds after you see the flash until you hear the thunder. Sound travels about 5 seconds to travel one mile. If you hear thunder roughly 10 seconds after you see the lightning flash, then the strike occurred about 2 miles away. However, if the lightning is too far, you may not hear any thunder, because sound waves attenuate and bend as they travel through the atmosphere, especially through layers of differing air temperature.

Figure \(\PageIndex{4}\): Rapidly expanding air within a lightning strike creates a shockwave in all directions. As the shockwaves from multiple locations along a lightning arc converge, the sound of thunder becomes a rumble. (Public Domain; The Sound of Thunder via NOAA/NWS)

When lightning strikes, a shock wave is generated at each point along the path of the lightning bolt. In the presence of nearby lightning, thunder will sound like a loud bang, crack, or snap, and its duration will be very short. As the shock wave propagates away from the strike centre, it stretches, attenuates, and elongates. Subsequently, shock waves from more distant locations reach the listener. To the listener, the combination of shock waves gives thunder its continuous rumble.