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 either case, the bidirectional leader initiates between any two oppositely charge 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 [Coleman et al., 2003 and 2008]. 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 ends and the charge regions subsides, the leader stops growing and the flash stops. On average, 90% of lightning flashes are cloud flashes so a large majority do not connect with the ground.
Figure – cloud flashes between main charge region and screening layer
Cloud Flashes That Travel Outside The Thunderstorm
Cloud Flashes That Travel Above The Thunderstorm
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 crawler and spider flashes. In the case of horizontally extensive cloud flashes, one end of the bidirectional leader can travel through a charge layer in cloud while the other end of the leader can travel through a screening layer of charge along the cloud base. Recent research has shown that negative leaders tend to travel faster that positive leaders (sources), so frequently, you will observe a fast in cloud brightness (negative leaders) that propagates quickly across the sky followed by slower, visible leaders that trail behind “crawling” along the cloud base (positive leaders).The animation and high-speed video below illustrate this process.