As a child I remember a particularly stormy afternoon at summer camp. We were racing away from the large stream where we had been cooling off when a huge (to my little mind at the time) bolt of lightning struck in the field beside us. It was terrifying! It was also my first experience of how thrilling fear could be, the adrenaline pumping through you.
Fear of lightning is a good thing. There are 16 million lightning storms all over the Earth every year. One bolt of lightning can carry as much as 100 million volts. That’s 1.6 billion volts raining down on Earth every year. It kills an average of 90 people in the US every year and strikes approximately 400 people in the US every year. The lightning capital of the world is in the Republic of Congo in central Africa, and in the US it is in central Florida in an area called “lightning alley” between Tampa and Orlando. If you suffer from astraphobia (extreme fear of lightning), these are places best avoided.
There is still some debate about how lightning forms. Lightning is nothing more than a discharge of electricity. It is accompanied by thunder and a bright flash of light. The thunder is a product of the expansion of the rapidly-heated air around the lightning bolt. Light travels faster than sound so we see the lightning before we hear the thunder. What scientists do believe they know about the formation of lightning is that it likely involves ice crystals and the separation and attraction of negative and positive charged particles which somehow create a spark. Where those charged particles are in the clouds and surrounding air determines what kind of lightning the storm produces.
- Cloud-to-Ground Lightning
Cloud-to-ground lightning is the most commonly observed and the second most common form of lightning. Because the bolt (or stroke) passes between a cumulonimbus cloud and the ground, it is the type of lightning most responsible for deaths and property damage. Cloud-to-ground lightning is a discharge that is initiated by a downward-moving leader. There is sometimes a return stroke along the same path. There are three variations of cloud-to-ground lightning though none are kinds of lightning. Rather they are just visual effects created by the lightning stroke and return strokes.
Bead lightning is not technically a type of lightning but is more of a phase of the lightning stroke. Most cloud-to-ground lightning exhibits some kind of ‘beading’. The bead illusion is formed as the return stroke cools and the light of the entire path breaks up into fragments which look like beads on a string. Beading can usually only be detected when the observer or camera is very close to the stroke. Ribbon lightning is rarely seen by the naked eye but is captured on film when there is a strong wind. Ribbon lightning is actually a visual effect rather than a type of lightning. The effect is created when the wind blows hard enough to blow the return strokes slightly off the original channel. On film the effect looks like a ribbon. Forked lightning is a name given informally to a lightning stroke that branches. Besides those three, there is also staccato lightning which is a type of lightning and is nothing more than a leader stroke with a single return stroke. Staccato lightning tends to be highly branched as well as very short in duration and extremely bright.
- Ground-to-Cloud Lightning
This kind of lightning, also called upward-moving lightning, is usually initiated at the site of an object on the ground such as a tower. The lightning is a product of negatively charged ions, called the stepped leader, moving upwards toward the positively charged ions in a cumulonimbus cloud. The upward-moving stroke is usually followed by a return stroke moving from the cloud to the ground. Ground-to-cloud lightning is much more rare than cloud-to-ground lightning. Because this kind of lightning is usually initiated at a tower or other tall object on the ground, it is most commonly seen around tall towers, skyscrapers and tall trees.
- Cloud-to-Cloud Lightning
The most common type of lightning, cloud-to-cloud lightning can be inter-cloud lightning, occurring between two or more completely separate clouds, or intra-cloud lightning, occurring within the same storm cloud. Inter-cloud lightning is rather rare. Intra-cloud lightning occurs between sections of the storm cloud that have differing electrical charges, usually between the top anvil part of the cloud and the lower areas of the storm cloud.
Anyone who has witnessed a thunderstorm in the distance at night has seen heat lightning. This is lightning that appears to not produce thunder. In actuality, the thunderstorm is simply too far away to hear the thunder, but the intra-cloud lightning can still be seen. Sheet lightning is an informal name given to intra-cloud lightning. It seems to light up the bottom of the cloud in one large sheet with no apparent stroke, though the stroke is hidden in the cloud. Anvil crawlers are impressive to watch. An anvil crawler is cloud-to-cloud lightning that starts in the anvil part of the thunder cloud. The lightning branches and crawls across the upper levels of the cloud. Anvil crawlers are most often seen in well-developed thunderstorms.
Anvil crawlers can produce a very rare and unusual kind of stroke. Positive lightning, or “bolt from the blue”, develops on days with light cloud cover. The positive charges in the cloud spring forth horizontally forming a leader that can travel several miles before veering down to meet negative charges coming up from the ground. Positive lightning gets this name from those positive charges, and it’s named bolt from the blue because it seems to come out of blue skies. Positive lightning is very dangerous and is the kind of lightning that can strike up to ten miles away from a thunderstorm.
- Upper Atmospheric Lightning
The first report of upper atmospheric lightning dates back to 1886. Later, in the 1920s, Scottish physicist C.T.R. Wilson theorized that an electrical breakdown should occur high above thunderstorms. With the advent of airplanes, pilots reported seeing lightning above thunderstorms but those reports were discounted until the first visual evidence was documented in 1989. Since then, much research has been done on upper atmospheric lightning. This kind of lightning is an electrical phenomenon that occurs at altitudes much higher than normal lightning.
There are several kinds of upper atmospheric lightning –sprites, jets, and ELVES. Sprites are often called red sprites because of their reddish-orange color in their upper areas. They have greenish-blue tendrils that hang below. Though sprites are called a form of lightning, they are actually a cold plasma discharge rather than an electrical discharge. They appear to be triggered by positive lightning and occur 30 to 55 miles above the Earth’s surface. They are often preceded by a sprite halo. Like sprites, blue jets project from the top of a cumulonimbus thunderstorm cloud. Jets, however, are cone-shaped, only reach to about 25 to 30 miles and do not appear to be triggered by lightning. Blue jets that do not quite make it are called blue starters. Since 2002, scientists have also observed gigantic jets, blue jets that are twice as large as normal jets, reaching up to 45 miles. Gigantic jets branch at around 30 miles and look like trees or carrots. ELVES, which is an acronym for Emissions of Light and Very Low Frequency Perturbations from Electromagnetic Pulse Sources, are a halo-like glow, reddish in color, which appears around 60 miles above the ground and can be 250 miles in diameter. ELVES are sometimes confused with sprite halos.
- Other Kinds of Natural Lightning
Dry lightning happens when there is no precipitation present at the surface. It is the number one natural cause of wildfires. Ball lightning is a form whose existence is disputed. It is an electrical discharge that is round like a ball and may be inches to several feet in diameter. Accounts of its characteristics vary greatly, some witnesses say it constantly explodes while others say it splits into other balls. Some say it is attracted to buildings and people while other accounts report it being repelled by those same things. It is usually associated with thunderstorms but lasts much longer than the milliseconds that normal lightning lasts. St. Elmo’s Fire is not lightning, but is mentioned because it is often confused with ball lightning. St. Elmo’s Fire is a natural electrical phenomenon involving plasma rather than electricity. It occurs when the ground below a thunderstorm is highly charged. The air glows blue, often around objects such as the mast of a ship. In fact, St. Elmo’s Fire was first named by sailors who witnessed the glow at the tops of their ships’ masts and named it after St. Elmo the patron saint of sailors.
- Triggered Lightning
Lightning does not always occur with thunderstorms. Volcanic eruptions can trigger lightning. Volcanic lightning comes in three forms. Extremely large eruptions can trigger lightning. This phenomenon was documented by Pliny the Elder during the 79 AD eruption of Mt. Vesuvius. Volcanic lightning can occur around a volcano’s vents and is about two miles long. The third kind, spark-type lightning, is only around three feet long. Rockets and flying aircrafts can trigger lightning as well. Lightning rockets trigger lightning when they are launched into thunderstorms carrying spools of wire. As the rocket goes up, the wire unwinds giving the lightning a path. Scientists have also triggered lightning using lasers.
Lightning is an awe-inspiring, beautiful and deadly natural phenomenon that scientists still do not fully understand. While we watch it in the distance and marvel at its grace, elegance and power, scientists will continue to try to understand all the different kinds; and how lightning is formed.