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St. Elmo's Fire
What is St. Elmo's Fire?
BY JULIA LAYTON
If
you were to look outside your home during a thunderstorm and see a tall
streetlamp glowing with blue flames, you might be tempted to call the fire
department.
Then
you might notice that the streetlamp is on fire but isn't actually burning --
and the water from the fire hose isn't putting out the flames.
At
this point, you might be about ready to call a priest, but that, like the call
to the fire department, would be unnecessary.
The
phenomenon you're witnessing is actually St. Elmo's Fire. (Which
has nothing to do with a 1980s coming-of-age film starring a young Emilio
Estevez.)
St.
Elmo's Fire is a weather phenomenon involving a gap in electrical charge.
It's
like lightning, but not quite.
And
while it has been mistaken for ball lightning, it's not that, either -- and
it's definitely not fire.
Early
observers of the phenomenon, mostly sailors on high seas during thunderstorms,
seem to have understood they weren't looking at actual fire, because instead of
abandoning ship, they took comfort in the sudden glow atop the masts.
Such
famous figures as Magellan, Caesar and Columbus experienced St. Elmo's Fire on
their journeys.
And
Pliny the Elder, who seems to have documented absolutely every natural
phenomenon back in the 1st century A.D., beat everyone else to the punch when
he described blue flames appearing out of nowhere during thunderstorms.
Sailors
tended to attribute the glow to "St. Elmo," a mispronunciation of St.
Ermo or St. Erasmus, the patron saint of Mediterranean sailors.
They
believed the fire was a sign of salvation from the saint, since the phenomenon
occurs most often toward the end of a storm.
Benjamin
Franklin and Charles Darwin viewed the weather event through a decidedly more
scientific perspective. But regardless of interpretation, it's clear they were
all observing the same phenomenon. And contrary to popular belief, St. Elmo's
Fire doesn't only occur at sea.
As
with all electrical phenomena, St. Elmo's Fire is about electrons. So, what is
St. Elmo's Fire if it's not a form of lightning? Find out in the next section.
Causes of St. Elmo's Fire: The Fire
That's Not a Fire
Like
lightning, St. Elmo's Fire is plasma, or ionized air that emits a
glow.
But
while lightning is the movement of electricity from a charged cloud to
the ground, St. Elmo's Fire is simply sparking, something like a shot of
electrons into the air.
It's
a corona discharge, and it occurs when there is a significant
imbalance in electrical charge, causing molecules to tear apart, sometimes
resulting in a slight hissing sound.
The
first step in generating St. Elmo's Fire is a thunderstorm.
As
you can learn in How Lightning Works, a thunderstorm creates an electrically charged
atmosphere.
There
is a charge difference between the storm clouds and the ground, and this
difference creates voltage, or electrical pressure.
In
between the clouds and the ground, the atoms in the air undergo
changes; most important to our discussion, electrons move farther away from
protons, creating an environment that allows electrons to move around freely.
In other words, the air becomes a good conductor.
Once
the air is conducive to the movement of electrons, those electrons continue to
increase the distance between their positively charged counterpart, protons.
This
is ionization, and plasma is simply ionized air.
The
phenomenon that causes St. Elmo's Fire is a dramatic difference in charge
between the air and a charged object, like the mast of a ship, the tip of an
airplane wing or the 30-foot steeple of a church -- things we often think of as
potential lightning rods.
When
the voltage gets high enough, usually around 30,000 volts per centimeter of
space, the charged object will discharge its electrical energy [source: Scientific American].
The
reason why St. Elmo's Fire occurs most often on pointed objects is that a
tapered surface will discharge at a lower voltage level. The tip of a steeple,
mast or airplane wing presents something like a condensed surface charge.
When
the air molecules tear apart, they emit light.
In
the case of St. Elmo's Fire, the discharge is continuous -- sometimes lasting
several minutes -- and creates a constant glow.
The
glow is blue because different gasses glow different colors when they become
plasmas.
Earth’s atmosphere
has nitrogen and oxygen in it, and this particular combination happens to glow
blue.
St.
Elmo's Fire is exactly what's happening in neon tubes -- essentially a
continuous spark.
If
Earth's atmosphere were made up of neon, St. Elmo's Fire would glow orange
instead of blue.
A
neon tube is simply St. Elmo's Fire contained in glass.
St.
Elmo's Fire also behaves something like a plasma globe.
One
pilot described the phenomenon occurring on the windshield of her small plane
while flying through a storm cloud; when she touched the inside of the
windshield, blue streaks reached toward the tips of her fingers [source: USA Today].
BALL LIGHTNING
St. Elmo's Fire
and "ball lightning" are two different things.
The scientific
community can't agree on what ball lightning is, but it's definitely not St.
Elmo's Fire.
Ball lightning
can float around the air, while St. Elmo's Fire stays put.
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