Lightning

Lightning is a natural phenomenon consisting of electrostatic discharges. These discharges occur through the atmosphere between two electrically charged regions. One region may be located within the atmosphere, while the second region sometimes occurs on the ground. When lightning occurs, the two regions become partially or wholly electrically neutralized. This process involves a near-instantaneous release of energy. On average, a single flash releases between 200 megajoules and 7 gigajoules of energy.

The mechanism of lightning begins with charge separation. This often happens when warm air mixes with colder air masses. This mixing creates atmospheric disturbances that polarize the atmosphere. These disturbances result in storms, such as thunderstorms. Lightning also occurs in other energetic systems like volcanic eruptions or dust storms.

During a discharge, the air around the flash rapidly heats to extreme temperatures. This rapid heating causes the gases in the vicinity to experience a sudden increase in pressure. This pressure change creates a shock wave that we hear as thunder.

Scientists categorize lightning into three primary forms based on where they occur. Intra-cloud (IC) lightning happens within a single thundercloud. This is the most frequent type of lightning. Cloud-to-cloud (CC) lightning occurs between two separate clouds.

The third type is cloud-to-ground (CG) lightning, which is commonly called a lightning strike. While IC and CC lightning are more common, CG lightning poses the greatest threat to life and property. It terminates on a physical object on the ground.

Cloud-to-ground lightning follows a specific sequence of steps. It is initiated by a stepped leader moving down from the cloud. This leader is met by a streamer moving up from the ground. The overall discharge, or flash, involves several processes. These include preliminary breakdown, connecting leaders, and return strokes. The conductivity of the ground can also affect the discharge. This depends on whether the surface is soil, fresh water, or salt water.

Researchers distinguish between positive and negative lightning strikes. Most CG lightning is negative, meaning electrons flow downward to the ground. In a positive CG flash, electrons travel upward while a positive charge moves downward. Positive lightning is less common, making up less than 5% of all strikes. However, positive strikes are much more intense. An average negative bolt creates a current of 30,000 amperes. In contrast, large positive bolts can create up to 120 kA.

Positive lightning is significantly more dangerous than negative lightning. It produces higher peak currents and longer continuing currents. These factors allow the lightning to heat surfaces to much higher levels. This increased heat raises the likelihood of igniting a fire. Positive lightning can also propagate through clear air. This characteristic leads to the term "bolts from the blue." These strikes can occur during winter storms, known as thundersnow, or during intense tornadoes.

Lightning can also be triggered by human technology or extreme natural events. Intense forest fires can create pyrocumulonimbus clouds. These clouds are formed by rapidly rising hot air within a smoke plume. These systems can produce dry lightning, which may cause further forest fires. Even rocket exhaust plumes have been observed to provide a pathway for lightning. During the Apollo 12 takeoff, lightning was witnessed striking the rocket.

The study of lightning is a scientific field called fulminology. The World Meteorological Organization considers lightning an Essential Climate Variable. This means it is vital for understanding the global atmospheric electrical circuit. Lightning also influences atmospheric chemistry. By studying these discharges, scientists gain a deeper understanding of how energy moves through our planet's atmosphere.