Air is a gas, and a very important
property
of any gas is the
speed of sound
through the gas.
The speed of "sound"
is actually the speed of transmission of a small disturbance through
the gas. Sound is a sensation created in the human
brain in response to sensory inputs from the inner ear.
The speed of sound through the
atmosphere
is a constant that depends on the daily
temperature.
On a
standard day
at sea level static conditions, the speed of sound is about
760 mph, or 1100 feet/second.
We can use this knowledge to approximately determine
how far away a lightning strike has occurred.
When lightning strikes, a bright flash of light is generated.
Light travels at a constant
186,000 miles/second,
which means that we see the flash immediately as it happens.
The intense
heat
of the lightning generates a sound called thunder which
is transmitted through the air at the speed of sound.
Because the speed of sound is so much slower than the speed
of light, we always see the flash before we hear the thunder.
On the figure, we show the
sound waves
that are generated by a lightning strike. The waves move at
about 1100 feet/second. How far has the wave moved from the
lightning strike after 2 seconds? We can use a rate equation
to solve this problem:
d = s * t
distance d equals speed s times time t.
In 2 seconds, at 1100 feet/second, the wave has moved 2200 feet.
How far does it travel in 5 seconds? 5500 feet which is just a
little over a mile (1 mile = 5280 feet). So, since the flash
reaches our eyes instantly, if we count the number of seconds
between the flash and when we hear the thunder, we can approximate
the distance to the lightning strike:
d = 1100 * t (distance in feet)
Or, if we divide by 5280 feet/mile:
d ~= t / 5 (distance in miles)
Try it out during the next thunderstorm! But take shelter if the
time is less than a couple of seconds!
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