Lightning can be defined as a transient, high current electrical discharge whose path length is at least a kilometer in length.
The most common producer of lightning is the cumulonimbus cloud.
Pre-Lightning Charge Distribution
pocket of positive charge forms near the base of the cloud
Preliminary Breakdown
negative current from the charging region will balance, and overcome, the positive pocket releasing negative ions that begin the stepped leader process
total elapsed time : 1 millisecond (ms) (Note: 1 ms = 1/1000 second)
brightest, most prominent component of the entire process
luminosity actually propagates upward
channel illumination appears to be simultaneous because the process happens so quickly, takes about 100 $\mu$s
the stroke channel remains positively charged in response to the main charge center in the cloud
total elapsed time : 20.1 ms
Dart Leader
electrons supplied to the top of the channel (in the cloud) by "K" or "J" streamers
a sufficient number must collect within 100 $\mu$s for the process to continue
if so, a negatively charged dart leader will, most likely, follow the same path as the first return stroke, and deliver negative charge to the surface
followed by another return stroke that delivers positive charge to the cloud
most ground flashes contain 3 or 4 strokes, each separated by 40-60 ms, which causes the flickering effect
total elapsed time : 20.1 ms + n $\times$ (20 + 50) ms, where n is the number of subsequent dart leader - return stroke pairs that occur after the first
Additional Facts
Lightning can occur with both positive and negative polarity.
An average bolt of negative lightning carries an electric current of 30,000 amperes ("amps") - 30 "kiloamps" (kA), and transfers fifteen coulombs of electric charge and 500 megajoules of energy. (Note: a typical house will consume 95,000 megajoules in a year.)
An average bolt of positive lightning carries an electric current of about 300 kA - about 10 times that of negative lightning.
Lightning also occurs in snowstorms, sandstorms, clouds associated with erupting volcanoes, and even in "clear" air (giving rise to the phase "bolt from the blue").
cloud-to-cloud, or cloud flashes, are not as complicated - made up of a single stroke
cloud flashes are more common in the tropics - ratio of 10 CF for each GF
the ratio is 1:1 in the mid-latitudes
estimated to be 30 million GF per year in the northern hemisphere
maximum rate is 55 per second over land during the NH summer
70% of all lightning occurs between 30 degrees south and 30 degrees north latitude
return stroke raises the channel temperature to 20,000 - 30,000 K (temperature of the sun : 6000 K)
rapid increase in temperature leaves little time for the air parcels to expand, so pressure increases 10 to 100 times the normal value
initially, a shock wave is created as the high pressure channel expands into the surrounding air (no sound yet)
eventually, the shock wave slows to the speed of a sound wave, and thunder is heard
the cone of silence: waves (shock or sound) travel faster in warmer air and denser air, so the "silent" shock wave moves further near ground before becoming a sound wave