Dear Tom,
Why can you hear thunder so long after a lightning bolt? I have timed it to go on for
more than 40 seconds.
Dave Coleman, Naperville, Ill.
Dear Dave,
The phenomenal heating (about 50,000 degrees) brought to bear on a column of
air exposed to a lightning spark produces an explosive expansion and subsequent
contraction of the column—a process that sets up the shock waves we hear as thunder.
Rather than originating from a single point, these sound waves, propagating at about
1,100 feet per second, are generated along the entire length of a lightning stroke. That
length is often in excess of 10 miles. If a lightning bolt were oriented such that the
distance between its nearest and farthest points from you were 10 miles, the train of
sound waves thus generated would require 48 seconds to pass your ears. You would
hear thunder from that bolt for 48 seconds.
