As we know that Lightning is an electric discharge in the form of a spark or flash originating in a charged cloud. Lightning is a very long electrical spark, ‘very long meaning greater than about 1 kilometer. Most lightning is generated in thunderstorms and is characterized by a length of 5 to 10 km, at the extreme about 100 km.
What appears as a single flash of lightning usually consist of a number of successive strokes, following the same track in space, at intervals of a few hundredths of a second. The average number of strokes in a multiple stroke is four, but may be as many as 40. The time interval between strokes ranges from 20 to 700 ms, but is most frequently 40-50 ms. The average duration of a complete flash being about 250 ms. Refer Understanding Lightning Phenomena for detail.
Thus we see that Lightningis a phenomenon which lasts for very few seconds and consists of many strokes of high magnitude. Therefore, Lightning is neither DC nor AC. It is more like an impulse signal or a series of occurrences of impulse signals.
A DC signal should have constant magnitude throughout the entire time. Mathematically its magnitude must be same in -∞<t<∞ .
Practically any signal whose ripple factor is less than three percent is taken as DC signal and its magnitude must be within the limits of ± 3 % during the usage period.
Thus Lightening cannot be a DC.
Lightning cannot be AC, since the wave shape of Lightning current is not sine-wave shaped voltage. A typical lightning current waveform looks something like this as shown below. This complete event (A, B and C) is a single strike.
Therefore, Lightning is neither DC nor AC. It is more like an impulse signal or a series of occurrences of impulse signals.