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# Is Lightning AC or DC?3

## Is Lightning AC or DC?

(OP)
Can anyone confirm the nature of LIGHNING? Is it alternating or direct current?

Am I right to assume that the current discharged during a lightning is a DC (applying the electrostatic principle of capacitors) because I cannot find a reason how it could be alternating (as some I know says).

Thanks.

### RE: Is Lightning AC or DC?

I'm with you. I can't see any reason for ac to be associated with lightning. As you point out the basic mechamism is capacitive discharge.

### RE: Is Lightning AC or DC?

Lighning discharges are transient current pulses rather than DC or AC currents. They last a very short time, increase very quickly up to a maximum and decay to zero. In most cases, their sign do not change. Some times, and due to the presence of capacitances, the effect of one lightning dischage in some circuit element may behave as several consecutive transients looking like AC.

Julian

### RE: Is Lightning AC or DC?

Suggestion: Reference:
1. Fink D. G., Beaty H. W., "Standard Handbook for Electrical Engineers," 14th Edition, McGraw-Hill, 2000
Reference 1 Section 27.3 Mechanisms and Characteristics of Lighning includes Figures 27-7, 27-8, 27-9 and 27-10, which depict clouds charged with + and - charges. The lightning is usually shown from - charges to the earth. This suggest that the lightning is DC. (I wish there were other areas so clearly and well covered as lightning in Reference 1.)

### RE: Is Lightning AC or DC?

As lightning is a transient, would it be possible to do a Fourier Transform of a "typical" pulse and get a frequency distribution to predict its AC like effects?

### RE: Is Lightning AC or DC?

Guest062601,
If you call AC as the pure sine wave or periodically changing from positive to negative, then lightning is DC. If you call DC as the constant value of current or charge flow, then it is AC. As rightly pointed out by 230842, it is transient current pulses.
However when the charge flows downward from the cloud, in the end period there is upward flow of the opposite charge.

### RE: Is Lightning AC or DC?

Butjela,
The problem with trying a fourier transform is that there is no typical lightning strike.  usually lightning goes frm the earth to the clouds or meet somewhere in between, however, cloud to cloud lightning occurs.  Either way, if you want to try a fourier transform analysis, try using an impules function as the model.

From the highlands, MacCloed

### RE: Is Lightning AC or DC?

RE: Lightning :

What is the typical time duration of the lightning ?

Anyone knows if it generates X-rays ? How much ? can

### RE: Is Lightning AC or DC?

Suggestion: DC may have superimposed variations; however, it is still an overall DC voltage or current since it does not meet a definition of AC voltage or current. There are DC transients and AC transients. They are not supposed to be mixed up.

### RE: Is Lightning AC or DC?

I'm with jbartos here. I also have the book "Standard Handbook for Electrical Engineers" 13th Edition by Donald G. Fink and H. Wayne Beaty. I went to these pages and the illustrations shows clearly the charges.

It is discussed here (in the handbook) that lightning stroke is characterized by polarity.

AC and DC currents decay in time as a characteristic of a short-circuit. However, as with transients (as the speed of lightning is said to be 10 to 50% the speed of light), multiple strokes were recorded to last a longer period of almost 1 second. Maybe this is where some think that it is AC because of the change in the origin of discharge: from earth to cloud or somewhere in between and from cloud to earth or somewhere in between - which may happen during a multiple stroke on an already stablished conductive channel (dielectric strength rapture of the "material" between the cloud and earth or cloud to cloud).

### RE: Is Lightning AC or DC?

hi
just to see if I can do it again. Consider lightning with reference to ground. the earth potential would not change. (lets call it negative for the exercise) Then the lightning event will be positive to earth. while pulsing etc may occur then the static potential will always be above the earths ==>> dc power supply? (ok so really shitty quality but still dc I would imagine)

any other thoughts
regards
Don

### RE: Is Lightning AC or DC?

Suggestion to the previous posting:
1. A good approach when it comes to the Earth absolute ground and absolute end of lightning strokes
2. If lightning is between two clouds, then it is still the DC
3. If one considers more clouds and a string of lightnings, then one can possibly create the AC as a mathematical function of several lightnings to have it mathematically modeled and functionally depicted around some axis, nicely.

### RE: Is Lightning AC or DC?

For the attention of jbartos:

Correct me if I'm wrong but do I perceive your Jul. 2 post as: AC can be modelled out of a "string" of lightnings - (just) for the purpose of showing an AC wave?

You personally don't mean to say that lightning itself maybe alternating to the effect that a single stroke of lightning originating from a single pair of "anode" and "cathode" may manifest such. Am I right? That is, from your point of view, the "string of lightning" is a result of multiple pairs of positive and negative "plates" simultaneously striking on a common path. If that is the case, I somewhat agree with you. However, the resulting wave would be somewhat like that from a seismograph or a polygraph machine.

Still, I believe, that by the nature of how a lightning is developed during cloudy times, the stroke is pure DC-considering a single stroke from a single pair of "plates". I cannot see a reason for a single stroke to be alternating. "Stroke" is analogous to the term "discharge" - to discharge a capacitor/battery via a resistor/ground - of which no one can "charge" using AC.

### RE: Is Lightning AC or DC?

Waxing philosophical...
We made up the terms "AC" and "DC" to describe flows of electrons because the machines we use generally require flows of two types.  The term "lightning" does not pertain to the use of such a flow in a machine.  If you were to construct a machine (or anything - even a lightning rod) based on the idea that lightning is "AC" or "DC" you would most likely run into problems.  It would be much wiser to take the raw characteristics of lightning into account during your design rather than use the charateristics of a made up category that it barely fits into.  I think that's a common mistake in every field.

In a nutshell, I see us trying to fit a violent and messy natural phenomenon into one of two man-made categories.  But it's kind of fun, so let's carry on...

I have a question about Earth to cloud lightning:  If the speed at which the electrons move through the atmosphere is sometimes faster than their speed through the ground, isn't it possible that the location at wihch the lightning struck receives (or loses?) more electrons than it needed to to equalize the potentials, and couldn't that lead to another strike along the same path (now "greased" by the previous strike) with opposite polarity?  Just to clarify, the "greasing" of the path ("dielectric strength rapture of the 'material' between the cloud and earth") is due to the ionizing of molecules in the lightning's path, right?

### RE: Is Lightning AC or DC?

hi
this is getting very philisophical but very interesting.

Lets have a look at the clouds~~ yeh why not ac as some sort of over all function
considering movement back and forth from locations around thors throne.

impact with (or from ) earth maybe we could say that there would be an excess of particles here. May be it would look something like the discharge of your cars ignition coil with a kettering system (points and condenser type thingo))

So would this be dc I think so

Don

### RE: Is Lightning AC or DC?

Suggestion marked ///\\\to:
butelja (Mechanical) Jun 26, 2001
As lightning is a transient, would it be possible to do a Fourier Transform of a "typical" pulse and get a frequency distribution to predict its AC like effects?
///Yes, however, this is mathematical modeling of a wave or response in general. E.g. a DC Heaviside's Unit Step has its Fourier Transform in the frequency domain good for all frequencies, i.e. 1/j2pif or 1/s by Laplace transform.
Also, as I posted earlier, the various DC levels can be viewed as AC by an appropriate location of mathematical axis. This hower represent then AC variations in time segments of essentially DC current or voltage.\\\

### RE: Is Lightning AC or DC?

A lightning wave is defined by a t90%(time to 90% of peak) and t50% (time to 50% of peak). Typical lightning impulses are denoted as 8/20 microseconds or 1.2/50 microseconds where the first digit is the time to 90% and the second digit is the time to 50% of peak. After which the waveform, with suitable surge protection should revert to normal operating charcteristics. The lightning wave is therefore essentially an impulse waveform.

### RE: Is Lightning AC or DC?

Any waveform which is not pure dc , has ac content in it.
Lightning is an impulse and hence essentially contains ac waveforms of  high frequencies along with dc.

### RE: Is Lightning AC or DC?

Natural surges waveshape are unidirectional (DC) with concave wave front. Most lightning have multiple strokes ranging from 2 to 42 (3 to 4 in average) and radiate electromagnetic frequencies ranging from extremely low radio frequency to X-ray frequencies. Typical duration to crest value from fraction of microsconds up o several microseconds with multiple strokes. The probability density function adopted by IEEE use 31 kA as median for crest value with a mathematical simplified model of  I(P)=1/[1+(I/31)^2.6]. The CIGRE model is similar to the IEEE.

The standard lab impulse waveshape 1.2x50, remotely resemble the natural surge stroke but, still useful to determine the dielectric strength of electrical components and apparatus. Few basis differences are as follow:
- The breakdown of gap in a lab is achieved increasing steadily the voltage while the natural surge the voltage is virtually constant and the “gap” is steadily decreased until the breakdown occur.
- Lab impulse waveshape are the same with small tolerance. Natural surges are all different in shape and duration. Their impedance of stroke is a function of height and velocity of return stroke that typically varies from 900 to 2000 Ohms producing current from 50 kA to 10 kA.

http://www.glatmos.com/lightinfo/science.html

http://thunder.msfc.nasa.gov/bookshelf/images/white_paper_large_images/big_flash_summary.gif

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