Ask A Scientist General Science Archive

Author:     john blaylock
Text:       Does lightning strike from the ground up or from the clouds down?

Response #:  1 of 1
Author:      Robert Topper
Text:        A Most Excellent Question! I actually did not know the answer to 
this right away, but I have done a little reading up on the subject.  Lightning 
happens when a cloud builds up a separation of charge. The bottom of the cloud 
(facing the earth) has a negative charge (extra electrons) while the top of 
the cloud expels positive charges up and out into the upper atmosphere.  So 
the cloud has all this extra electrical energy on the side facing the earth, 
and eventually it dumps the energy down towards the earth, which tends to have 
a positive charge (for complex reasons) compared to the cloud's underbelly.  
When the stroke begins, a series of little pulses of electricity come down 
from the cloud towards the earth at really high speeds (about 1/6 the speed of 
light!).  These pulses are called steps, and the trail they pulse down along 
is called the step leader. (not ladder).  The air gets ionized along the step 
leader and so this column of air becomes a good conductor of electricity. So 
when the step leader (which is so faint that it is practically invisible) hits 
the earth, a "conducting wire" of air is set up from the cloud to the earth.  
Now charge can flow, but the first charges to move are the ones at the bottom
of the wire (near the earth).  So, the stroke (which is now visible) starts at 
the bottom (the earth) and runs UPWARDS from the ground! This return stroke 
produces the bright light and a lot of heat, which causes the air to expand 
suddenly, which causes a thunderclap.  This sequence often repeats, which the 
charge going up and down the leader.  -Thanks to Feynman.  There is a really 
neat article in National Geographic on this subject!  Check it out!