10ms before contact: The negative descending stepped leader breaks through the cloud base moving toward the earth at a speed of approximately 200km/s and its direct influence is just beginning to be felt in a fairly small zone on the ground below the leader.
2.5ms before contact: The descending leader is now only 500m above the ground. Building corners, edges and extended protruding objects like flagpoles and antennas begin to experience significant discharge activity. Some of these very same objects had periodically been producing electric discharges (St. Elmo's fire, onset streamers in the centimeter range) under the ambient electric field that existed prior to the descending leader's appearance. But now, this is a whole new level of activity, much more intense. The streamers are growing fast and in voluminous bunches.
1.5ms before contact: A number of positive upward leaders, forming from the various locations of intense streamer activity, rise up toward the descending leader tip. A race is on, only one of these upward leaders will connect with the descending negative leader. The stepping action, which characterized the descending leader''s motion, has now stopped and it moves along its own trajectory continuously.
0.5ms before contact: An upward connecting leader originating from the corner of a building some 60m in height will win the race. Beyond a critical length, the upward leader continuously accelerates and alters its trajectory to meet the negative descending leader tip. The streamer zone at the head of the upward leader tip grows longer as it approaches the negative descending leader. The upward leader has now grown to a length of 70m and is about to encounter the downward leader in a process that is termed "the final jump". This will occur more than 100m above the ground and at a horizontal distance of 80m from the 60m high building that launched the upward connecting leader.
Right before contact, the descending leader may alter its path slightly toward the upward connecting leader. When the two leader tips are about 40m apart, the electric field between them reaches critical values that result in a very rapid breakdown of the air insulation, known as a streamer breakdown. The last 40m are spanned entirely by streamers, extending like tentacles from either leader tip toward the other.
The return stroke
At the instant that the final jump was completed, the localized volume of charge in the clouds that originally produced the negative descending stepped leader had a direct conducting connection to ground.
First the negative charges at the bottom of the descending leader move quickly through the zone of positive charge created by the upward connecting leader path. Then all the negative charges produced by the negative descending leader and which sheathed its path, suddenly begin to rush toward the earth, starting from the bottom up to the top, lighting up every branch and every aborted path. Like a column of ice that is suddenly melting from the bottom to the top, although all the water will fall to the earth, we would observe a "zone of melting" move upward. The remaining charges in the localized volume of charge in the clouds, which started this process, now flow to ground. This was the first return stroke and as per above, the peak current reached was 12kA, a value deemed sufficiently high to cause damage to buildings.
Any other nearby localized volumes of charge in the clouds now have a viable path to ground. As they pass through the conducting link provided by the attachment process, they create new current peaks referred to as subsequent return strokes. A typical lightning strike will produce three to five return strokes.
The channel temperature reaches 30,000 K. Huge bursts of electromagnetic waves radiate out from the leader channel. After the last return stroke passes, the process comes to an end and the conducting connection with the clouds is lost, the lightning strike is over. The entire process lasted some 200-300ms. The distinctive audible noise echoes for many kilometers.
-- continued in Part 2
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