Is Lightning AC or DC ?

Electrical Technology

Is Lightning Alternating Current (AC) or Direct Current (DC)?

Lightning is a transient impulse, meaning it’s neither purely AC nor DC, but rather a combination of both. In other words, lightning consists of both AC and DC surges.

So, what exactly is lightning?

What is Lightning?

Lightning is a natural electrostatic discharge that occurs between two electrically charged neighboring clouds (known as a Direct Stroke) or between electrostatically charged clouds and the earth (known as an Electrostatic Induction Stroke), often accompanied by thunder. This discharge can release billions of joules of energy, generate extremely high temperatures (up to 30,000°C), produce plasma, and emit magnetic radiation and flashes of visible light due to the rapid movement of electrons. There are various types of lightning, including thundercloud lightning, head lightning, and dry lightning.

In essence, lightning is an electric discharge that appears as flashes and sparks within charged clouds. A single lightning flash may include multiple strokes, extending from 1 to 100 kilometers in length, and typically includes 4 to 40 strokes per flash. The time between individual strokes in a flash ranges from 20 to 700 milliseconds.

A lightning stroke can discharge up to 10⁶ volts and between 50,000 to 250,000 amperes in about 10⁶ seconds, making it more like an impulse signal than either AC or DC.

This demonstrates that lightning consists of strong, high-magnitude strokes that occur in rapid succession, lasting only a fraction of a second, hence, lightning is not AC or DC but a series of transient impulse signals.

Why Lightning Can’t be AC?

An AC (Alternating Current) signal continuously changes direction and magnitude over time, flowing in one direction during the first half of a cycle and in the opposite direction during the second half. In contrast, the waveform of lightning does not resemble the sinusoidal pattern of AC voltage and current as shown on below figure.

In fig 1 for Impulse wave shape parameters,

T₁ = Front time for switching impulses (Front time is the time taken for the wave to reach its Peak value.)
T₂ = Front time for lightning impulses
T₃ = Time to half value (tail time or time to half of the Peak value)

While:

The front time for a standard lightning impulses is 1.2 μs while its tail time is 50 μs.
Tolerance allowed in peak value is ±3%.
The tolerance allowed for front time is ±30% and that for tail time is ±20%.

Lightning is more like a sudden short circuit between clouds or between clouds and earth due to high charge density rather than a potential difference. Additionally, lightning acts like a capacitive discharge, with current flowing from positive to negative as in a capacitor, meaning the charge flows in one direction (unipolar), which is why lightning cannot be AC.

Related Post: Why Do Electronic Circuits Use DC Current instead of AC?

Why Lightning Can’t be DC?

A DC (Direct Current) signal has a constant magnitude and does not change polarity or direction over time. To be classified as DC, a signal typically has a ripple factor of less than 3%, with minor fluctuation in magnitude.

A signal can be DC if the ripple factor is less than 3% and the magnitude is ±3 during a time period.

However, lightning frequencies range from 1 kHz to 1 GHz, while DC has no frequency. Lightning also exhibits characteristics of both AC and DC components, with unidirectional, spiky unidirectional pulses that resemble irregular AC signals more than steady DC. That’s the reason why lightning can’t be DC.

The above discussion proves that lightning is a brief, intense phenomenon with waveforms that differ from standard AC or DC waves. Its signal and waveform characteristics resemble impulse signals rather than continuous AC or DC, which is why lightning is described as an impulse phenomenon.