There are essentially only two types of lightning flashes. A cloud flash has a bidirectional leader develop between two charge regions and each end propagates toward and into the separate charge regions. A ground flash is the same as a cloud flash except that one end of the bidirectional leader connects with the surface of the Earth or an object attached to the Earth. For both types, the bidirectional leader initiates between two oppositely charged regions where the electric field generated between the two regions is the highest. Each end grows due to the electrical potential difference between leader tip and the charge region ahead of the leader. Like a stream of water filling an empty well, the leader will propagate throughout the charge region, which serves as a “potential well” for the leader. If the charge regions are unequal in size and magnitude, one end of the leader may continue to propagate (fill) the larger charge region (well), while the other end propagates beyond the smaller charge region (filled well). In that case, the leader will continue to propagate toward another charge region or possibly the ground, which is inductively charged. However, when the potential difference between the leader tips and the charge regions subside, the leader stops growing and the flash terminates. On average, 90% of lightning flashes are cloud flashes so a large majority do not connect with the ground.
Cloud Flashes That Travel Outside The Thunderstorm
Sometimes one end of the lightning leader travels outside the thunderstorm cloud and into clear air. This has been historically defined as cloud-to-air lightning. Essentially, this occurs when one end of the leader propagates beyond its oppositely charged potential well (cloud charge region) and continues toward additional charge regions such as the ground or screening charge layers that may reside on the boundary of the thunderstorm. The potential difference between the leader tip and region ahead of the leader remains high enough to promote continued propagation beyond the primary attractive charge region. Studies have shown using 3-dimensional lightning mapping that the other end of the leader likely continues to propagate in its associated charge region having yet to “fill” the potential well due to the charge region’s larger size.
The following high-speed video recorded at 10,000 ips shows a cloud flash in which the negative end of the bidirectional leader emerges from the upper part of the storm.
Cloud Flashes That Travel Above The Thunderstorm
On occasion, a cloud flash will have one end of the leader exit the upper extent of the thunderstorm and propagate toward the upper atmosphere. The rarified air and low pressure of the upper atmosphere leads to a visible transformation in the leader as the ionization travels upward. Close to the thunderstorm, the leader will initially resemble a typical plasma channel but tends to be brighter in the blue color tones. As the leader continues upward, it begins to spread or fan out and dim optically. It also emits blue light due to the excitation of molecular nitrogen below 50 km. Above 50 km, the excitation of nitrogen emits red light. The leader propagation above the thunderstorm has been defined as Blue Jets and Gigantic Jets.
The Gigantic Jet travels the highest, exceeding 50 km and exhibiting a red upper portion. Current understanding suggests that they form from a cloud flash between a large main lower negative charge region and smaller upper positive charge region without an overlaying negative screening layer. The negative leader propagating upward into the upper positive charge region fills the potential well and continues upward toward the ionosphere, while the downward propagating positive leader end continues to propagate in the lower negative charge region.
Below are two low-light standard speed video showing Gigantic Jets.
Below is a digital still image of a Gigantic Jet. Digital still images of Gigantic and Blue Jets are still rather rare, but more are being captured with increasing sensor sensitivity.
The Blue Jet is usually associated with a smaller cloud flash that occurs between the upper main positive charge region and an overlying negative screening layer. In this case, the upward propagating positive leader end travels beyond the small negative screening layer and continues toward the upper atmosphere. However, the relatively smaller magnitude typically associated with upper main positive charge region limits the extent of the downward propagating negative leader end and the bidirectional flash terminates before the positive end can reach 50 km. It therefore only results in the emission of blue light.
The Gigantic and Blue Jet are considered to be upper atmosphere discharges, however, they are in fact a component of a lower atmosphere cloud flash. As will be discussed in a later section, Sprites, Halos and Elves are upper atmospheric discharges that initiate and develop as a spatially separate discharge that is NOT directly part of a cloud or ground flash. However, they initiate due to the electric field change caused by a preceding ground flash.
Horizontally Extensive Cloud Flashes
When storms grow upscale and become large complexes, charge layers can become horizontally extensive and slope up or down in altitude over distances of 10s to 100s of kilometers. Flashes that have leaders propagating through these horizontally extensive charge layers can be spectacular as they can last multiple seconds and travel over 100 kilometers. When some of the leaders emerge below cloud and travel along the cloud base of the thunderstorm, they appear to crawl like spiders racing across the sky. Not surprisingly, these types of flashes have been called “crawlers” and “spider flashes.” In the case of horizontally extensive cloud flashes, one end of the bidirectional leader can travel through a charge layer above cloud base while the other end of the leader travels through a screening layer of charge along the cloud base. Recent research has shown that negative leaders tend to travel faster than positive leaders, so frequently, we observe a fast in cloud brightness (negative leaders) that propagates quickly across the sky followed by slower, visible positive leaders that trail behind “crawling” along the cloud base. The crawling appearance is due to recoil leader formation on decaying positive leader branches. The animation and high-speed video below illustrate this process.
Next we will discuss what happens when one end of the bidirectional leader connects with the ground.
Next section: Ground Flashes
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