What is Lightning

Lightning is one of the oldest observed natural phenomena on earth. Besides thunderstorms, lightning has been seen in extremely intense forest fires, volcanic eruptions, surface nuclear detonations, heavy snowstorms, and in hurricanes. At any given moment, there are about 1,800 thunderstorms happening across the Earth. It's estimated that 100 lightning flashes occur each second somewhere around the globe, totaling to about 8 million lightning flashes per day. Lightning is most common around the equatorial regions of the world, although it can potentially strike anywhere, and it appears in a variety of guises, depending on atmospheric conditions.

Lightning is a discharge of atmospheric electricity which is triggered by a buildup of differing charges within a cloud. The result is a sudden release of electricity which causes a distinctive bright flare, followed by a thunderclap.

Typically, the bottom of a cloud becomes negatively charged and it sends out what is known as a "stepped leader" which seeks a positive charge, either in another cloud or on the earth. As the leader approaches an area which is positively charged, a positively charged streamer emerges, meeting the stepped leader, sealing the connection, and generating a bolt of lightning. Air around this channel is heated to about 30,000°C (54,000°F), and the electrical discharge is about 1.5 million volts.

Types of Lightning

Lightning strikes exhibit particular characteristics. Scientists have given names to the various types of lightning. The lightning that is most-commonly observed is streak lightning. This is the 'return' stroke, the visible part of the lightning stroke. The majority of strokes occur inside a cloud so we do not see most of the individual return strokes during a thunderstorm. Also, observers see lightning differently depending on where they are situated.

Lightning can take place in several different areas of a thunderstorm. Most lightning (about 80%) occurs within a single cloud and is called cloud-to-cloud lightning. Most of the other 20% of lightning involves a stroke from the cloud to the ground. Damage is usually caused where the lightning strikes the ground. And sometimes lightning can jump from one cloud to another or to the surrounding air.

Cloud to Ground Lightning is the least common but most understood. It is also the most dangerous in that it causes the most damage.  This is the type of lightning that is commonly thought of when the word lightning is mentioned.

Ground to Cloud Lightning is lightning that starts on the ground and goes up to the cloud, usually a cumulonimbus cloud. This lightning comes in only one form-the strike just zaps up to the cloud.

Intra-Cloud lightning is the most common, and occurs within the cloud itself as opposite charged particles move within the cloud. Here, the cloud seems to light up within itself.

Inter-Cloud lightning This kind occurs between two different clouds when opposite charged particles are near the edges of neighboring clouds.

Blue Jets, Red Sprites, Green Elves and Trolls

High-altitude lightning or sometimes called 'megalightning' has been given other names such as "red sprites", "green elves", "blue jets" and "trolls". These types of lightning appear as brightly colored flashes, high above thunderstorms. These flashes shoot up above the thunderstorm about the same time as other lightning discharges inside the storm cloud.

Though first reported in 1886 as unidentified oddities, it was not until recently that scientists accepted their existence. Part of the reason for their slow acceptance is that these very short-lived phenomena appear above the clouds where they are usually hidden from ground-based observers. With the recent advent of manned spacecraft and regular high-altitude aircraft flights, reports of the phenomena increased creating an interest in their scientific study. The properties and underlying physics of megalightning are just starting to be discovered. There appear to be four distinct categories: the sprites and elves and the blue jets and gamma ray events. The latter two being extremely rare, and thus still poorly understood.

Sprites

Though first called Red Sprites, it was latter observed that sprites also contained faint tendril-like elements of blue and purple. As the catalogue of observations grows, we now see that sprites come in a menagerie of sizes and shapes described as giant red blobs, picket fences, upward branching carrots, or tentacled octopi. The luminous body of the sprite can extend as high as 95 km (60 miles) with peak brightness between 50 and 75 km (30 and 47 miles). Downward draping tendrils often drop below 30 km (19 miles) altitude but do not reach the thundercloud tops. Rather than forming a narrow channel like the cloud-to-ground lightning with which they are associated, sprites are estimated to be around 10 metres (30 ft) across and often appear as clusters that illuminate a large volume, perhaps thousands of cubic kilometres spreading out over 150 km (93 miles) from their origin.

Sprites emerge high above very large thunderstorm systems, appearing at intervals of up to several minutes and lasting several milliseconds. They seem to associate with cloud-to-ground lightning flashes of large positive polarity (most, but not all, lighting strokes are of negative polarity). A diffuse disk-shaped glow lasting about a millisecond precedes some sprites. These sprite halos are less than 100 km (62 miles) wide, and propagate downward in altitude from about 85 to 70 km (53 to 44 miles). Columnar sprites sometimes emerge from the lower portion of the sprite halo's concave disk.

Blue Jets

Blue Jets propagate from the cloud tops toward the ionosphere 20 to 50 km (12 to 30 miles) high and last from tenths to a full second. They are always blue and funnel-shaped: 1.6 to 3.2 km (1 to 2 miles) at their base and 8 to 10 km (5 or 6 miles) at the top. Simultaneous blue jets propagate slowly upward from the cloud tops, but extinguish simultaneously. The blue starter, a related phenomenon, may actually be a blue jet that fails to completely form. Blue jets appear to be very rare, but that may arise from the fact their faint blue light is quickly scattering by the surrounding air and thus difficult to see from the ground.

Elves

Elves appear as giant expanding disks of light between 65 and 95 km (40 and 60 miles) altitude. They are caused by the passage through the ionosphere of an electromagnetic pulse in the form of intense radio waves emitted from powerful lightning flashes. The radiating pulse excites the electrons in the nitrogen gas which then emits light by fluorescence. Though huge, sometimes expanding to more than 400 km (250 miles) in diameter, elves are so transient (less than one-thousandth of a second), it is unlikely the human eye could see them. The lightning that triggers elves can be as far as 50 km (30 miles) away from where they appear.

Trolls

Trolls are another addition to the menagerie similar to the blue jet but generally reddish in colour. Trolls occur following an especially vigorous sprite in which tendrils have extended downward to near cloud tops. The trolls exhibit a luminous head leaving a faint trail and ascend initially at around 150 km/sec (95 miles/sec), before gradually decelerating and then vanishing around 30 km (19 miles) altitude. It is still uncertain whether the preceding sprite tendrils actually extend to the physical cloud tops, or if the trolls emerge from the storm cloud. Researchers have also termed them embers and fingers, but troll has the advantage of being a plausible acronym (for Transient Red Optical Luminous Lineament).

v1.9.3 (1034)