From the clouds to a nearby tree or roof, a lightning bolt takes
only a few thousandths of a second to split through the air. The
loud thunder that follows the lightning bolt is commonly said to
come from the bolt itself. However, the grumbles and growls we
hear in thunderstorms actually come from the rapid expansion of
the air surrounding the lightning bolt.
As lightning connects to the ground from the clouds, a second stroke
of lightning will return from the ground to the clouds, following
the same channel as the
first strike. The heat from the electricity of this return stroke raises the
temperature of the surrounding air to around 27,000 C° (48,632 F°). Since
the lightning takes so little time to go from point A to point B, the heated
air has no time to expand. The heated air is compressed, raising the air from
10 to 100 times the normal atmospheric pressure. The compressed air explodes
outward from the channel, forming a shock wave of compressed particles in every
direction. Like an explosion, the rapidly expanding waves of compressed air create
a loud, booming burst of noise.
Because electricity follows the shortest route, most lightning
bolts are close to vertical. The shock waves nearer to the
ground reach your ear first, followed
by the crashing of the shock waves from higher up. Vertical lightning is often
heard in one long rumble. However, if a lightning bolt is forked, the sounds
change. The shock waves from the different forks of lightning bounce off each
other, the low hanging clouds, and nearby hills to create a series of lower,
continuous grumbles of thunder.
Thunder Fun Facts:
- To judge how close lightning is, count the seconds between
the flash and the thunderclap. Each second represents about 300m
- Thunder is not only heard during thunderstorms. It
is uncommon, but
not rare, to hear thunder when it is snowing.
- Lightning does not always create
thunder. In April 1885, five lightning bolts struck the Washington
Monument during a thunderstorm, yet no thunder
Phillis. Thunderstorms. In The complete weather resource,
v. 2, Weather and phenomena. Detroit, UXL, c1997: p.
Martin L. Lightning and thunder. New York, Julian Messner,
c1969. 94 p. (Juvenile literature)
Wendy. Thunder and lightning. New York, Scholastic Reference,
c2002. 32 p. (Juvenile literature)
Vladimir A., and Martin A. Uman. Lightning: physics
and effects. Cambridge, UK, New York, Cambridge University
Press, 2003. 687 p.
Martin A. All about lightning. New York, Dover Publications,
1986. 167 p.
more print resources...
Search on "lightning," " sound," or "thunderstorms,"
in the Library of Congress Online
bolts of lightning strike the ground. From the NASA Website.
Near vertical view of thunderhead over
South America as seen from
Apollo 9. From
NASA JSC Digital Image Collection.
line of cumulonimbus thunderclouds. From
the NOAA Photo Library.
mammatus clouds, often associated with thunderstorms and
severe weather. From the NOAA Photo Library.
striking the Eiffel Tower. From NASA's SciJinks Web site.