Lightning is simply a transient, high-current discharge of electricity in the air. Usually, the lightning channel is composed of ionized gas, and its peak temperature is typically 30000 degrees Celsius. Interestingly, it is about five times higher than the temperature of the surface of the sun. The extreme heating causes the air to expand explosively, which results in a shock wave that becomes a booming sound wave called Thunder.
WHAT CAUSES THE SOUND OF THUNDER?
Since the speed of light is higher than sound, we will see light instantly. Sound travels 330 meters per second. So we will hear sound after the flash.
If the lightning is very close, we feel the sound of thunder, like a clap or crack, followed by a loud boom. If it is away, it often rumbles.
The reason for the rumbling is simple. It can be due to the sound emanating from different areas of the stroke, and the position of obstacles like high mountains, buildings, and hills may enhance this effect.
WHAT MAKE THUNDER SOUND INAUDIBLE?
Sometimes, you may see lightning without thunder. Does it mean the lightning doesn’t produce a sound?
Interestingly, the atmosphere is the reason. The atmosphere bends or refracts and attenuates the sound waves. So we will not hear the sound.
Moreover, sound travels faster in warm air than in cold air. The temperature normally drops in unstable atmospheric conditions.
These factors make sound inaudible to an observer who stands long away.
SOURCE OF LIGHTNING
Thunder-causing clouds or Cumulonimbus are special when compared with other clouds. Most of the time, it will appear in aggregates and be larger in dimension.
Even though there may be times we can notice isolated cumulonimbus clouds. These clouds have the shape of an enormous vertical tower with an anvil shape in the upper part. This upper part is called Anvil. It will appear at an altitude between 6 to 17 km.
This shape is due to the rising water vapour in the Cumulonimbus reaching the temperature inversion at the troposphere. Since it cannot rise further, it will form the Anvil.
The height of Cumulonimbus is related to the altitude of the tropopause, which varies according to latitude and season.
CLOUD ELECTRIFICATION PROCESS
GRAUPEL-ICE COLLISION MECHANISM
According to this theory, when soft hail or graupel and hailstones fall through a region of supercooled droplets and ice crystals. When both collide, latent heat is released, which results in a higher surface temperature for hailstones.
When this warm hailstone comes in contact with a colder ice crystal, there will be a net transfer of positive ions from the warm hailstone to cool ice crystals.
After the transfer, hailstones become negatively charged.
CONVECTION MECHANISM
Warm currents take positive fair weather change to the top of the growing cumulus. The positive charge absorbs the negative charge from the cosmic rays.
The cooling and convective circulation carry the negative charge down the sides of the cloud towards the cloud base. This produces a positive corona at the earth’s surface.
The convection mechanism results in a positive cloud charge dipole. However, it seems unlikely that the negative charge region formed by this mechanism would be in a similar temperature range for different types of thunderstorms.
In this model, precipitation does not play a role in forming the dipole charge structure.
WHAT ARE THE STEPS OF A LIGHTNING STRIKE?
When the localised electric potential gradient exceeds 3 million volts per meter along a path, it results in a discharge of electrons rushing towards the cloud base. Then, this discharge is directed towards the ground.
STEPPED LEADER
A discharge of 50 to 100 meters occurs and halts for about 50 millionths of a second. After the halt, this occurs again. This faint and invisible discharge is called a stepped leader.
The type of cloud-to-ground discharge depends on the polarity and direction of propagation of the stepped leader. Based on the stepped leader’s polarity and direction of propagation, it is classified into four types.
RETURN STROKE
When the tip of the steeped leader approaches the ground, the potential gradient increases and a positive charge current starts to move upward from the ground. When both meet, the result will be a bright return stroke.
The most common type is a downward negative leader. It is common in summer.
Downward positive leader is seen in winter and in some severe storms.
From the taller structure, we can see an upward positive leader and an upward negative leader. An upward negative leader is rare.
TYPES OF LIGHTNING
Lightning can happen in a variety of shapes and forms. Based on this, it is classified into different types.
BEAD LIGHTNING
It appears like a series of beads tied to a string and lasts longer than the usual discharge channel.
Bead lighting is rare, and its width varies.
The wider part cools slowly and remains visible longer, appearing as a string of beads.
FORKED LIGHTNING
In this case, it appears crooked or forked. In short, it exhibits a branching path.
This name is not used formally.
DRY LIGHTNING
Lightning that occurs with thunder does not produce rain. This is termed as Dry lightning. It can trigger wildfires and is more common in areas with wildfires.
When the higher levels of the atmosphere are cooler, and the surface is warmed to extreme temperatures due to wildfire, it results in convection that produces lightning.
The fire can create more dry lightning by developing more dry thunderstorms, which further enhances fires.
HEAT LIGHTNING
It is the name for lightning flash that we see without thunder. It occurs too far away, so we won’t hear the sound of thunder.
SHEET LIGHTNING
When a lightning flash occurs inside a cloud, intervening clouds mask the flash. So, the clouds appear as a flash. This is known as Sheet lightning.
POSITIVE LIGHTNING
Positive lightning or “bolts from the blue” is a type of positive lightning in which the positive charge is carried by the top of clouds rather than the ground.
It comes from apparently clear or only slightly cloudy skies.
It generates ELF and VLF radio waves. This type is more dangerous. Positive lightning is believed to be responsible for the crash of Pan Am Flight 214.
BALL LIGHTNING
Luminous sphere that appears to float in the air and moves for a few seconds or minutes.
Its size may vary from pea-size to several meters.
Ball lightning is believed to be produced by the ionization of air and dust by alpha particles during radon decay in the dust-laden atmosphere.
SPRITES
Sprites are red-cloured or greenish-blue-cloured lightning with hanging tendrils. It is created due to large-scale electrical discharges that occur high above a cumulonimbus cloud.
BLUE JETS
It is a brighter, narrow cone project from the top of the Cumulonimbus to the lowest levels of the ionosphere.
SAINT ELMO’S FIRE
In pointed objects like antennas, ships’ mast produces a luminous blue or green halo with continuous sparks. This is called Saint Elmo’s Fire. This is due to corona discharge into the atmosphere.
This phenomenon is named after the Saint Elmo, the patron saint of sailors. Sailors consider it as a protection sign by Saint Elmo.
It can also seen around powerlines. If Elmo’s fire is visible, thunderstorms or thunder may occur shortly.
PHYSICAL EFFECTS OF LIGHTNING
- It causes surges in the electrical lines due to galvanic or conduction coupling.
- During a lightning strike in the ground, due to resistance, local soil breaks down and can cause damage to buried electrical conductors.
- An increased temperature and consequent explosive channel expansion during lightning results in an acoustic shock wave with an initial propagation speed ten times higher than the speed of sound.
- Potential differences in an open loop induced during a lightning strike can produce sparks and ignite nearby flammable or explosive materials.
- It can cause over-voltages in relatively low-voltage distribution. The after-effect of such electromagnetic effects is a line outage.
- It strikes are dangerous to living creatures. It can result in burns, cataracts, hair burns, rupture of ear drum and death in extreme causes. Lightning victims can also have erythematous treelike discharges or Lichtenberg figures initiated by a leader circulating between clothes and skin. It will disappear after a couple of days.
- During a lightning strike, a ground current spreads through the soil in all directions. It will cause a high potential rise at the strike place, which can result in a steep voltage near the strike point. This steep voltage can result in electrocution in animals because of a potential gradient between the anterior and posterior legs.
