Lightning discharge phenomenon, Types of lightning will be explained

Technical information

Lightning discharge phenomenon

Lightning discharge phenomenon

In cumulonimbus clouds with heavy rain, lightning, and snow, the positive and negative charges are separated by the collision of hail※1 and hailstones※2inside the cloud. In cumulonimbus clouds with lightning (thunderclouds), the separation of charges is particularly strong, so the insulation of the atmosphere is destroyed, and an electrical discharge occurs within the cloud and between the cloud and the ground surface. This is lightning.
The charging mechanism inside a cumulonimbus cloud is still not strictly understood. The most popular theory is that the collision of hail and ice crystals※3 causes the hail and ice crystals to be charged with different polarities. Hail is negatively charged and ice crystals are positively charged at a temperature of -10℃. Lighter ice crystals are located upward due to updrafts, while heavier hail is located downward, resulting in the accumulation of both positive and negative charges in the cloud. At the earth’s surface, charges of opposite polarity corresponding to the charges at the bottom of the cloud collect by electrostatic induction, and as they accumulate, the voltage between the cloud and the earth increases. Eventually, the charges are neutralized by breaking the insulation of the atmosphere and electrically bonding, resulting in a ground discharge (lightning strike). This is the main discharge of a lightning strike.

Breakdown
The breakdown of an insulator (in this case, air) that separates a conductor from another conductor (in this case, the thundercloud and the earth), so that the insulating state is no longer maintained and an electric current flows.
Charge
A physical or microscopic form of electricity is called electric charge.
  1. HailIce particles less than 5mm in diameter falling from clouds.
  2. HailstoneIce blocks of 5 mm or more in diameter that fall from cumulonimbus clouds.
  3. Ice crystalsIce crystals with a diameter of a few micrometers to 100 micrometers. When ice crystals grow to a large size, they become hail or hailstones.

Basic data

Discharge time
approx. 0.001 s to 1 s
Temperature of the surrounding area
about 30,000°C
Total length of discharge
several kilometers
Number of lightning events observed worldwide
50 per second
Voltage
approx. 100 million V
Current
3000A to 200,000A

Summer and winter thunderstorms

Lightning can be broadly divided into two types, summer lightning and winter lightning, due to differences in seasonality and electrical properties.
Summer thunderstorms occur when the atmosphere is unstable due to heat from solar radiation and cold air in the air masses covered by the Pacific High in summer. On the other hand, winter thunderstorms are caused by strong cold air blowing out from the Siberian air mass, and the cold air passes over the relatively warm sea of Japan, causing atmospheric instability, especially along the coast of the Sea of Japan from Tohoku to Hokuriku regions.
Figure 1 schematically shows summer and winter thunderstorms. Both summer and winter lightning basically have a structure with a positive charge distribution area at the top and a negative charge distribution area at the bottom. The negative charge in winter lightning and the positive charge in the lower part of the thundercloud disappear in a short time (from a few minutes to a few tens of minutes), so the short-lived charge distribution is shown by (+) and (-). Compared to winter thunderstorms, summer thunderstorms have a higher altitude at the cloud base and cloud top, and the charge distribution area within the cloud is distributed at an altitude of about 5 km or more above the ground surface, which is characterized by a higher number of lightning strikes. On the other hand, in winter lightning, the altitude of the cloud base and cloud top is lower than that of summer lightning, and the electric charge distribution area in the clouds is distributed at an altitude of about 2 km or more above the ground surface. Although the number of lightning strikes is small, it is characterized by the fact that the amount of neutralized charge (energy of lightning strike) of each lightning strike is often several 10 to 100 times larger than that of summer lightning.

Figure 2 : Summer and winter thunderstorms
Figure 2 : Pattern diagram of summer and winter lightning

Types of lightning strikes

Lightning strikes are classified into four types according to the difference in polarity and the direction of the discharge, as shown in Figure 3. When the positive charge in the cloud is neutralized during a lightning strike, it is called a positive polarity lightning strike, and when the negative charge is neutralized, it is called a negative polarity lightning strike. When the direction of the leader preceding the main discharge (intense lightning) of lightning strikes is from the ground to the thundercloud, it is called upward, and when it is from the thundercloud to the ground, it is called downward. More than 90% of summer lightning strikes are downward negative polarity lightning strikes (a), in which the branching of the lightning discharge path spreads downward, and this phenomenon is often seen in summer lightning strikes. On the other hand, positive polarity lightning (b) and (d) account for about half of winter lightning. This is because the updraft in the cloud is weaker than in summer lightning, so the negative and positive charges in the lower layers of the thundercloud fall with the rain and snow, while the positive charges in the upper layers remain, which is thought to result in more positive polarity lightning strikes. In addition, because the cloud base is close to the ground surface, the frequency of upward lightning strikes (c) and (d) from high structures is high.

Figure 3 : Types of lightning strikes
Figure 3 : Types of lightning strikes

The process of lightning strike

Here is an example of a multiple lightning mechanism for a common downward negative polarity lightning strike.

  1. Stepped leader goes extension about 50 meters and stops for 30 to 90 microseconds (1 microsecond = 0.000001 second) again and again. And it approaches the ground gradually. Its average extension speed is about 150km/s, and if the height of the thundercloud is about 3000m from the ground, it will take about 0.02 seconds for the stepped leader to approach the ground.
  2. When the tip of stepped leader approaches the ground, several upward welcoming discharges are generated from the ground, and one of them connects with the stepped leader to form a lightning discharge path.
  3. Return strokes (return lightning strikes) occur through the formed lightning discharge path toward the thundercloud. The time of the return stroke is estimated to be tens to hundreds of microseconds, and the average extension speed is 100,000 km/s, which is 1/3 the speed of light. This neutralizes the electric charge on stepped leader and part of the electric charge on the thundercloud. The above is a series of lightning discharge phenomena.
  4. A few tens of microseconds after the return lightning strike, a second preceding discharge (dart leader; all subsequent discharges are called dart leaders) extends along the same path from the thundercloud to the earth. Dirt leaders do not have a pause period like stepped leader, but are a continuous discharge, so their extension rate is tens of times faster than that of stepped leader.
  5. When the dart leader reaches the ground, a second return lightning strike (subsequent return lightning strike) is extended. Normal lightning strikes are often accompanied by multiple subsequent return lightning strikes, which is called multiple lightning. Multiple lightning strikes occur 60 to 70 percent of the time. In about half of the lightning strikes, a current of about several hundred amperes flows in the discharge path for about 0.001 to 1 second following the return lightning strike, and this is called continuous current. The amount of neutralized charge ranges from several C (coulombs: amount of charge) to several hundred C, which is larger than the neutralized charge of the return lightning strike, and the destructive force of the lightning strike is also larger.
Coulomb
A unit of electric charge; the amount of electric charge carried by one ampere of current in one second is one coulomb.
Figure 4 : Lightning process (example)
Figure 4 : Lightning process (example)

Types and characteristics of lightning

Typical Summer Lightning – “Thermal Lightning”

Most of the evening thunderstorms in summer are caused by “Thermal Lightning”. Strong solar radiation heats up the earth’s surface, causing updraft, which destabilize the atmosphere and generate cumulonimbus clouds (thunderhead). “Thermal Lightning” is generated by these cumulonimbus clouds.

Typical Summer Lightning – “Thermal Lightning”

Seasonal lightning – “Boundary Lightning”

The term “Boundary Lightning” is often used to describe lightning that occurs at the turn of the seasons. When two different air masses, one warm and one cold, come into contact with each other, they do not immediately mix, but instead form a cold front and a warm front. “Boundary Lightning” is generated close to both a cold and a warm fronts.

Seasonal lightning – “Boundary Lightning”

Lightning caused by low pressure or typhoon – “Vortex Lightning”

“Vortex Lightning” occurs in low pressure or near the center of typhoon when more updraft blowing by air flowing from surrounding area. When the temperature is high, “Vortex Lightning” remains strong for a long time and affects a wide area because of its fast moving speed.

Lightning caused by low pressure or typhoon – “Vortex Lightning”

Comparison of lightning characteristics

  Summer thunder Winter lightning Induced lightning
Voltage of thundercloud※1 Tens of millions to hundreds of millions of V Tens of millions to hundreds of millions of V
Height of thundercloud(cloud base) 1200~2000m 300m~
Height of thundercloud(height) 7000~16000m 4000~7000m
Lightning current(peak value) Thousands of A to 300 kA Thousands of A to 300 kA
Duration time(front wave shape) 1 to several tens of μs 1 to several tens of μs 1 to several tens of μs
Duration time(tail wave shape) tens to hundreds of µs Sometimes exceeding tens of ms Several to several tens of µs
Voltage induced in distribution lines(peak value) Up to several hundreds kV※2
Energy Large Extremely large(several hundred times larger than summer lightning) Small

The Institute of Electrical and Electronics Engineers : Lightning and the Advanced Information Society (1999)

  • The voltage of thunderclouds is an estimated value.
  • More than 90% of the current value flowing through arrester which is installed to 6kV distribution line is less than 2000A.