Lightning and Thunder

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Lightning is one of the oldest observed natural phenomena on earth. At the same time, it also is one of the least understood. While lightning is simply a gigantic spark of static electricity most often seen in thunderstorms, scientists do not have a complete grasp on how it works.

Lightning flashes globally

At any given moment, there can be as many as 2,000 thunderstorms occurring across the globe. This translates to more than 14.5 million storms each year. NASA satellite research indicated these storms produce lightning flashes about 40 times a second worldwide. Thus we live on an electrified planet.

How Lightning is Created?

Leading theories on origin of lightning focus around separation of electric charge and generation of an electric field within a thunderstorm. Recent studies also indicate that ice, hail, and semi-frozen water drops known as graupel are essential to lightning development. Storms that fail to produce large quantities of ice usually fail to produce lightning.

1 . Charge Separation

Thunderstorms have very turbulent environments. Strong updrafts and downdrafts occur with regularity and within close proximity to each other. The updrafts transport small liquid water droplets from the lower regions of the storm to heights between 35,000 and 70,000 feet, miles above the freezing level.

Meanwhile, downdrafts transport hail and ice from the frozen upper regions of the storm. When these collide, the water droplets freeze and release heat. This heat in turn keeps the surface of the hail and ice slightly warmer than their surrounding environment, and a “soft hail”, or “graupel” forms.

When this graupel collides with additional water droplets and ice particles, a critical phenomenon occurs: Electrons are sheared off of the ascending particles and collect on the descending particles. Because electrons carry a negative charge, the result is a storm cloud with a negatively charged base and a positively charged top.

2 . Field Generation

In the world of electricity, opposites attract and insulators inhibit. As positive and negative charges begin to separate within the cloud, an electric field is generated between its top and base. Further separation of these charges into pools of positive and negative regions results in a strengthening of the electric field.

However, the atmosphere is a very good insulator that inhibits electric flow, so a trmendous amount of charge has to build up before lightning can occur. When that charge threshold is reached, the strength of the electric field overpowers the atmosphere’s insulating properties, and lightning results.

The electric field within the storm is not the only one that develops. Below the negatively charged storm base, positive charge begins to pool within the surface of the earth .

This positive charge will shadow the storm wherever it goes, and is responsible for cloud-to-ground lightning. However, the electric field within the storm is much stronger than the one between the storm base and the earth’s surface, so most lightning (~75-80%) occurs within the storm cloud itself.

How Lightning Develops Between The Cloud And The Ground

A moving thunderstorm gathers another pool of positively charged particles along the ground that travel with the storm (image 1) .

As the differences in charges continue to increase, positively charged particles rise up taller objects such as trees, houses, and telephone poles.

A channel of negative charge will descend from the bottom of the storm toward the ground (image 2).

It is invisible to the human eye, and shoots to the ground in a series of rapid steps, each occurring in less time than it takes to blink our eye. As the negative charge approaches the ground, positive charge collects in the ground and in objects on the ground.

This positive charge “reaches” out to the approaching negative charge with its own channel (image 3).

When these channels connect, the resulting electrical transfer is what we see as lightning. After the initial lightning stroke, if enough charge is leftover, additional lightning strokes will use the same channel and will give the bolt its flickering appearance.

Tall objects commonly struck by lightning

Tall objects such as trees and skyscrapers are commonly struck by lightning. Mountains also make good targets. The reason for this is their tops are closer to the base of the storm cloud.

The atmosphere is a good electrical insulator. The less distance the lightning has to burn through, the easier it is for it to strike.

However, this does not always mean tall objects will be struck. It all depends on where the charges accumulate. Lightning can strike the ground in an open field even if the tree line is nearby.

Positive and Negative  Lightning
1 . Negative  Lightning

The previous section describes what is called “negative lightning”, because there is the transfer of negative charge from the cloud to the ground. However, not all lightning forms in the negatively charged region under the thunderstorm base.

2 . Positive Lightning

Some lightning originates in the cirrus anvil or upper parts near the top of the thunderstorm, where a high positive charge resides. Lightning that forms in this region follows the same scenario as previously described, but the descending stepped leader will carry a positive charge while its subsequent ground streamers will have a negative charge.

These bolts are known as “positive lightning” because there is a net transfer of positive charge from the cloud to the ground.

Positive lightning can be the dominant type of cloud-to-ground during the winter months and in the dissipating stage of a thunderstorm.

Positive lightning is more lethal

Positive lightning makes up less than 5% of all strikes. However, despite a significantly lower rate of occurrence, positive lightning is particularly dangerous for several reasons-

1 . Since it originates in the upper levels of a storm, the amount of air it must burn through to reach the ground is usually much greater. Therefore, electric fields associated with positive Cloud-to-Ground (CG) strikes are typically much stronger than those associated with negative strikes.

2 . The flash duration is also longer with peak charge and potential up to ten times greater as compared to negative CG strikes; as much as 300,000 amperes and one billion volts!

3 . Some positive strikes can occur within the parent thunderstorm and strike the ground beneath the cloud. However, many positive strikes occur near the edge of the cloud or from the thunderstorm’s anvil from which a strike can occur more than 25 miles away from precipitation.

4 . Also, positive flashes are believed to be responsible for a large percentage of forest fires and power line damage.

Thus, positive lightning is much more lethal and causes greater damage than negative lightning.

Bipolar lightning

Finally, there is bipolar lightning, lightning that actually changes its polarity (positive becoming negative or vice versa). It is no less dangerous than any other type of lightning but shows that we live on a complex planet with many aspects we do not fully understand.

Does lightning travel from the cloud to the ground, or from the ground to the cloud?

An entire lightning strike employs both upward and downward moving forces. However, the return stroke of a lightning bolt travels FROM THE GROUND INTO THE CLOUD and accounts for more than 99% of the luminosity of a lightning strike. What we see as lightning does indeed travel from the ground into the cloud.

Lightning Safety

Lightning is the most underrated weather hazard. On average, only floods kill more people. Lightning makes every single thunderstorm a potential killer. Lightning is the first thunderstorm hazard to arrive and the last to leave.

Lightning is one of the most capricious and unpredictable characteristics of a thunderstorm. Because of this, no one can guarantee an individual or group absolute protection from lightning. However, knowing and following proven lightning safety guidelines can greatly reduce the risk of injury or death.

Where to Go?

The safest location during a thunderstorm is inside a large enclosed structure with plumbing and electrical wiring. These include shopping centers, schools, office buildings, and private residences.

What to Do?

Once inside a sturdy building, stay away from electrical appliances and plumbing fixtures. As an added safety measure, stay in an interior room.

Thunder

Thunder is the acoustic shock wave resulting from the extreme heat generated by a lightning flash.

How the sound of thunder is produced?

Lightning can be as hot as 54,000°F (30,000°C), a temperature that is five times hotter than the surface of the sun! When lightning occurs, it heats the air surrounding its channel to that same incredible temperature in a fraction of a second.

Like all gases, when air molecules are heated, they expand. The faster they are heated, the faster their rate of expansion. But when air is heated to 54,000°F (30,000°C) in a fraction of a second, a phenomenon known as “explosive expansion” occurs. This is where air expands so rapidly that it compresses the air in front of it, forming a shock wave similar to a sonic boom. Exploding fireworks produce a similar result.