as thunder. The sound can be heard for varying distances, depending on the wind
direction and speed, the presence or absence of favorable sound propagation
conditions, and the amount of background noise. Usually, thunder is audible up
to about 8 km (5 mi) from the point of strike.
If the lightning flash can be seen, the thunder is heard somewhat later. Sound
travels through air at a speed of approximately 335 m/s, which is roughly 1 km
per 3 s or 1 mi per 5 s. You can determine the distance to a lightning flash by
counting the number of seconds it takes for the thunder to arrive. Divide the
number of seconds by 3 to get the distance in kilometers, or by 5 to get the dis-
tance in miles.
The rumbling, or booming, noise of thunder puzzles some people. A lightning
flash has short duration, but thunder seems to last for several seconds. This
occurs for two reasons: echoes and propagation delays. In hilly or mountainous
terrain, or in cities with many tall buildings, the acoustic noise from a lightning
stroke gets a chance to bounce around. We hear not only the original thunder, but
its echoes. This creates a prolonged rumble. In flat, open country, or on a lake or
at sea, there are no objects to cause the echoes, and the rumbling is less pro-
nounced. Propagation delays also contribute to the rumbling effect. Suppose you
stand 1 km from a stricken object, and the lightning flash occurs vertically from
a cloud base 1 km high. You are about 400 m closer to the bottom of the light-
ning bolt than you are to the top, and the sound is therefore spread out over a
time interval of slightly more than 1 second.
THE ELECTROMAGNETIC PULSE
In a lightning stroke, electrons are rapidly accelerated as they jump the gap
between poles of opposite charge. Whenever electrons are accelerated, EM fields
are produced. A radio broadcast transmitter works according to this principle; elec-
trons are made to accelerate back and forth in the antenna at a precise and constant
frequency. In a bolt of lightning, electrons are accelerated in a haphazard way, and
this produces radio noise over a wide range of frequencies. The radio “signals”
from thunderstorms are known as sferics. Sometimes the sound they produce in a
radio receiver is called “static,” although technically this is a misnomer.
Sferics travel for long distances at low radio frequencies, and for progres-
sively shorter distances as the frequency becomes higher. A fairly distant group
of thundershowers can cause interference to low-frequency radio communica-
tions. Low-frequency radio direction-finding (RDF) apparatus can actually be
used to locate regions of intense thunderstorm activity. As thundershowers
approach, the sferics become more frequent and intense, and they are observed
CHAPTER 4 Rain and Lightning
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