Electric Sky: Lightning Types

Lightning Types Text written by Mark Seltzer, all rights reserved. [Image: Rough schematic of lightning types - note the G-C may be better positioned under the anvil] The above schematic demonstrates the seven most observed types of tropospheric lightning from a cumulonimbus cloud. Some thunderstorms I've observed produce nothing but I-C (sheet) lightning, whilst others are prolific C-G (cloud to ground) generators. Some produce all types throughout the lifecycle of the cloud. My theory, after decades of observations, is the abundant type of lightning is likely dependant on 3 main factors: vertical shear (winds at different heights dictating the cloud/electric field structure), vertical velocity (updraught downdraught speed and angle) and overall cloud structure (how many cells, where is the freezing level in relation to anvil top, how high is the tropopause, etc).
Cloud to Ground (C-G) aka Forked Lightning
T0108 18th July 2017 Exeter UK - Mid-Level MCS Spanish Plume		These well-known strikes are the most dangerous (after Positive Flash) to infrastructure, humans and animals during a thunderstorm, and prized in lightning photography. A lightning channel originating from the (traditionally) negative region in low-mid areas of a cumulonimbus cloud extends to a positively charged ground. These can vary in amplitude depending on the altitude of the negative region/freezing levels and other environmental conditions. The accepted theory of generation is via a faint "stepped leader" from the negative region, "feeling" it's way to a positively charged ground, launching a faint positive streamer upwards, connecting, then the current flows and you get the flash and the bang. In photos, you can often see downward-pointing branches coming off the main channel. These are a result of those "feeling" stepped leaders and are usually only visible milliseconds before the circuit is closed. *Likely location:* cloud-base, usually between a well-defined updraught and downdraught. In supercells, frequent negative ground strikes are often seen just northeast of the mesocyclone (when the steering flow is southwesterly). See T0077. *Requirements:* Strong negative region at middle altitudes, usually present during the development/mature stage of a cell. Look Left: The channel may have a few faint branches preceding the main channel as the stepped leader "feels" it's way to a primary discharge point. Sometimes these branches connect back into the cloud before finding the ground, resulting in a cloud to cloud.

Intra-Cloud (I-C) aka Sheet Lightning (resembling a sheet of light in the sky)
T0106 27th May 2017 Exeter UK - Severe Mid-Level MCS Spanish Plume		Lightning in tall cumulonimbus clouds mostly occurs within the cumulonimbus cloud itself, between the dipole region of an individual cell or between different cells. More often than not they are obscured to the observer, so the cloud lights up like a bulb. Typically severe "warm airmass" thunderstorms produce a lot of I-C in the upper regions of the cumulonimbus cloud, with apparently "quiet" distant rumbling thunder, and occasionally lower in the cloud in weaker thunderstorms which give a better rumble. If the cloud is well-sheared (leaning) then you're more likely to see these discharges as C-C. *Likely location:* Within the cloud inside the cumulonimbus tower. *Requirements:* Sustained negative and positive regions within the same cumulonimbus cloud, often needs to be un-sheared (on top of each other). Look Left: I-C can light up cumulonimbus towers like bulbs. Here are two intense cells next to each other in a linear multicell, and the flash is likely a hidden C-C between the two.

Cloud to Cloud (C-C) aka Spider / Streaked Lightning
T0108 18th July 2017 Exeter UK - Mid-Level MCS Spanish Plume		These non-ground connecting lightning streaks often occur between adjacent areas of charge in the horizontal along a precipitating cloud base (rather than the vertical like I-C). These are common in sheared thunderstorms (leaning vertically) where what may have been an I-C is now visible because the dipole is stretched laterally along an exposed downdraught underbelly, and also common in mid-level multicells where the charge distribution is chaotic and the cold regions of the storm are more visible due to the higher cloud base. They usually have fairly low luminosity and produce weak crackly thunder indicating low-amp nature. On the other hand some discharges may be large and chained together with other lightning channels throughout the whole storm cloud (much flickering and "crawling"), resulting in a more powerful, brighter discharge. Likely location: Widespread, usually between the base of updraught and downdraught regions, or between two or more individual cells. *Requirements:* Negative and positive regions side-by-side, preferably just outside or into to charged precipitation curtains. Multicells and mid-level AcCast thunderstorms flaunt C-C. Look Left: A large mid-level multicell of chaotic structure reflects this chaos in a seemingly aimless spray of lightning channels connecting different updraught points and precipitation curtains.

Anvil-Crawler (A-C) aka Spider / Streaked Lightning
T0102 15th May 2015 Oklahoma - Rearward sloping linear MCS		This powerful, photogenic but silent form of Cloud to Cloud thrives in large linear sheared thunderstorms (forward or rearward sloping) where large areas of positive anvil spreads quickly away from the negative towers. These mega-C-Cs are often seen visibly crawling along the underside of these large anvils, some 7000-35,000ft altitude, spreading the discharge across multiple channels for tens of miles, crawling and flickering as the parent channel encounters new areas of charge along the way. Due to the altitude and spread of charge, A-Cs are often relatively quiet. They can also occur in old storms, where a stray negative stepped leader finds fields of unused positive charge and discharges the whole lot at once, only for the storm to go completely silent thereafter. *Likely location:* Underneath the newest part of a widely advecting Anvil, connecting to the parent cumulonimbus tower. *Requirements:* Strong, well-sheared or linear rearward/forward sloping cumulonimbus structures, where fresh positively charged anvil-spread is dense, vast and quick, and efficiently sheared away from the negative regions in the main cumulonimbus towers (i.e. the Anvil has plenty of unused positive charge). Look Left: By far my best A-C capture of many. These are sometimes accompanied by C-G (often the C-G is the initial trigger), or extend from the tops of Ground to Cloud lightning (see G-C).

Positive-Flash (P-F) aka: Anvil Lightning / Bolt from the Blue / Positive C-G
T0102 15th May 2015 Oklahoma - Rearward sloping linear MCS		This type of lightning occurs when a C-G originates from a positive region, which is usually high-altitude in the upper level parts of a cumulonimbus tower or fresh anvil formation. The P-F extends all the way to the ground from such heights of 20,000-35,000ft (up to 8-10 times the length of a normal C-G). The electric field (potential difference) involved here is staggering, and therefore these are often the most powerful type of lightning you can get, resulting in explosive cannon-fire thunders. They can either occur outside the cumulonimbus tower ("bolt from the blue") or under a strongly charged anvil. Some can take an internal path and appear to the observer as high-amp base-C-Gs. Three key give-aways are the loudness and long duration of the thunder, and also sometimes a smooth or straight-line channel (less jagged and chaotic). *Likely location:* Out of newly-produced anvil cloud or top of a cumulonimbus tower. Often seen in tropical CBs, temperate winter CBs, and NE of the core of a mature supercell. *Requirements:* Strong fresh positively charged anvil-spread or cumulonimbus tower top. Look Left: This P-F originated from the anvil of a strong rearward-sloping linear storm, where Anvil Crawlers may have induced it. Note the channel cuts through low cloud, demonstrating it's true length.

Ground-Cloud (G-C) aka: Up-flash / Rocket Lightning
T0102 15th May 2015 Oklahoma - Rearward sloping linear MCS		These reverse-C-Gs have been seen to sprout from the tops of tall pointy objects. They happen traditionally when a negatively charged region on the ground connects to a large area of positive charge in the cloud above, usually the anvil. The top of the channel may convert into an A-C (Anvil Crawler) as shown left. The illusive characteristic of these discharges is that it appears as a progressive upward-moving Anvil-Crawler with reverse upward-pointing branches, which is where the slang names “up-flash” or “rocket lightning” come from. The leader channel often has long duration due to the progressive nature of the discharge. They are thought to be more common in the age of humans given most of these discharges are encouraged by tall man-made objects, although they have been photographed coming off pointy mountain tops such as volcanoes. *Likely location:* underneath strong positively charged clouds (often the underside of a large anvil) occurring from tall buildings, mountains, wind turbines or transmitter towers. *Requirements:* Strong fresh positively charged anvil-spread or other, and surface focus points. Look Left: This G-C was probably the 4th or 5th one to be launched from this distant transmitter tower. Note the direction of the branches indicating the direction of lightning travel, converting into an anvil-crawler downstream

Cloud-Air (C-A)
PS0001 12th September 1997 - Macclesfield - Cold Air Convergence Storm		As the name suggests, these are lightning channels travelling into clear air. Their classification is open to question, as subsidiary branches off the leader channel of CGs, for example, are commonly seen disappearing into clear air or into precipitation curtains. My interpretation is a true C-A should be seen to be terminating into clear air for some considerable distance outside of the cloud structure. These seem to happen more often near the tropopause (anvil height) whilst perhaps being accompanied with a P-F. For this type of lightning to be seen, the thunderstorm needs to be observed from a distance. The charge separation into thin air is not well understood, but could be caused by regions of charged air left behind from evaporated areas of previously charged cloud. Or perhaps there are some interactions with the stratosphere or ionosphere that are not fully understood. These are not quite the same as capturing stray unsuccessful stepped leaders on camera (which usually go everywhere), as they appear bright enough to suggest current flow is more definite, determined and final. *Likely location:* Tops of freshly developed cumulonimbus clouds near the tropopause, or branching off a P-F into blue/clear skies. *Requirements:* Strong fresh, usually positively charged anvil-spread, external P-F risk, and a good view. Look Left: One of my only captures of C-A. Several channels of lightning coming from the top of an cold-air low-topped cumulonimbus (PS0001). The video frame that followed this one showed one of these channels developing into a P-F.

Heat Lightning (H-L)
Ps0005 26th February 2002 - Macclesfield - Cold Air Storm		Heat lightning is the name given to distant flashes that can be seen on the horizon from distant thunderstorms. It gets its name from old folklore, when during hot summer months in temperate climates a thunderstorm is seen on the horizon lightning up the atmosphere and clouds immediately around it, but no thunder is heard. *Likely* *location: along thundery frontal systems or convergence zones lasting into the night at a distance. Requirements:* Well-forced MCS or linear thunderstorm in the distance, with continuous flickering, and a fair amount of clear sky around it. Look Left: A winter convergence thunderstorm lights up the atmosphere immediately around it at a distance. The blue hue was likely caused by a visible (outside the cloud) bolt on the far side of the cumulonimbus.

Crown Flash / Leaping Sundog (Confirmed Optical/Electrical Phenomenon)
One of the best Crown Flash examples on Youtube (Aaron Brigatti, Singapore).		Very exotic and rare, but becoming increasingly documented thanks to the abundance of camera phones. This is actually a daytime optical effect, caused by a thunderstorm's electric field. Under perfect situational conditions, it is hypothesised that the orientation of long thin ice crystals outside the cloud align with the intense electric field of the cumulonimbus, like iron filings. These orientated ice crystals then reflect the sunlight at a particular angle to the observer. It was first reported in 1885 by an explanation in the Monthly Weather Review journal. It can look like aurora or torch-light coming out of the top of a thunderstorm, and mechanically dances as the electric field changes within the storm. Looping can sometimes be seen during field reconnection events, akin to magnetic fields on the Sun. Only possible to see in daylight due to the requirement to reflect sunlight (perhaps moonlight?). Truly fascinating and one for the bucket list! http://amasci.com/amateur/sundog.html *Likely* *location: Above a freshly formed Cumulonimbus crown (before spreading to Anvil) if thin ice crystals are present above. Requirements:* Thin layer of ice crystals above a strong, freshly developed cumulonimbus, like a pileus or high humidity layer at the tropopause (translucent Cirrus), and sunshine nearby (perhaps ~22 degrees aside which is what Sundogs rely on). Look Left: Example of one witness's video from Youtube.


Ball Lightning (B-L) (Unconfirmed Electrical Phenomenon)
Most people have heard of the legendary myth of Ball Lightning, but no one has ever achieved any valid evidence of its existence, even with our smart phone/CCTV abundance in recent years. It is thought to be a plasma-ball phenomena occurring close to the ground during thunderstorms, and usually in urban areas. Some people have claimed to film or photograph orbs floating around cumulonimbus towers aloft, but are nearly always identified as aircraft or helicopters avoiding the thunderstorm by flying around it. *Likely location:* urban areas close to the surface.

ELECTRIC SKY LIGHTNING AMPLITUDE CLASSIFICATION High Amp – High current flow; resulting in high luminance & loud or explosive thunder. Low Amp – Low current flow; resulting in low luminance & soft or crackly thunder. Without measuring the electric field directly, these terms are devised from making 3 simple observations as follows: 1) Luminance It is acceptable to assume the brighter a lightning discharge, the higher the current that has flowed. This is a general rule and isn’t always the case - it depends on the environmental conditions and the size of the lightning channel. 2) Thunder Amplitude The sound of the thunder, taking into account the distance and air pressure in which the original discharge occurred (high altitude = low pressure, usually harder for sound to travel), can be a good indicator of the type of lightning channel that has occurred and also the strength. Excluding C-G for a moment, if the thunder sounds loud and defined with bangs and bass-booms, this often means it has occurred from a high-amp lightning channel. If the lightning channel has occurred at some distance and its thunder still sounds loud, then again this indicates a high-amp discharge. If the thunder is of a soft nature with soft rumbles or crackles, in particular C-C, then it is likely to have come from a low-amp lightning channel. Back to C-Gs; these tend to have a unique thunder signature due to them penetrating the high pressure boundary layer (layer of air closest to the surface) where detailed treble can travel efficiently or get trapped and bounce to the observer through terrain and stable atmospheric layers. As a result they usually behold clear high frequency bangs and cracks at close range. Assuming there are no dense rain curtains in between the lightning channel and the observer, there is also minimal "cloud/rain damping" (see "cloud damping" theory on thunder page). Therefore C-Gs have a higher chance of beholding a loud thunder. This makes it difficult to distinguish between a high-amp C-C from a low-amp C-G at close range. 3) Electromagnetic Pulses Lightning emits a large spectrum of electromagnetic radiation (almost all types from Radio to X-Ray). Notable electromagnetic pulses are usually associated with high-amp C-Gs and P-Fs. They can be detected on radio, TV, and what I found, video camera footage as a pop or a crack on the audio (rather like the occasional crack you hear when playing a vinyl record). If you are filming a thunderstorm and hear a snap in the audio track at the same time as a discharge, the discharge is likely to be of a high-amp nature.