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Where does the lightning come from :
from the sky or the ground ?
A lightning is an electrical discharge in the air.
Air is a good electrical insulator.
It means that it is not easy
to have an electric current running through it.
And when the conditions meet,
this electric current passes
in the form of sparks or electrical arcs.
To produce sparks between two electrodes,
one of these electrodes has to be charged
with a negative electrical charge,
and the other with a positive electrical charge.
If both electrodes were connected with a copper wire,
we would have an electric current within the wire.
But here,
as the air is an insulator,
the electrical charges keep on storing up in the electrodes.
But there is a limit.
When tension reaches a certain value,
charges goes suddenly from one electrode to the other,
through the air:
it is the electrical arc.
The air has to reach a 10 000 volts tension
if both electrodes are separated of 1 cm
and the air is really damp.
In a dry air,
30 000 volts are required
so that the electrical arc can be activated.
So, in order to activate an electrical arc,
we need positive electrical charges on one hand,
and negative electrical charges on the other hand.
In a cumulonimbus,
a thunderstorm cloud,
there is actually water negatively charged
in the lower part of the cloud,
and ice positively charged
in the upper part of the cloud.
Scientists do not really understand yet
why we find those electrical charges in those spots.
But we know for sure
that there are a lot of complex airstreams
that can move the drops of water
and the ice within the cloud.
When they scrape up to each other,
the ice is likely to be positively charged
and the water is likely to be negatively charged.
As pieces if ice are lighter than water,
they land up in the upper part of the cloud,
whereas water fells at the bottom of the cloud.
So, negatives charges are stored up
at the bottom of the cloud
and positive charges at the top.
If the electrical voltage becomes enough,
the lightning
– which is nothing but a huge spark,
is activated.
The vast majority of lightnings are actually produced
inside the cloud.
But under the cloud,
as a reaction to the negative charge
of the lower part of the cloud,
the land is being positively charged.
This charge is moving concurrently with the cloud:
positive charges which naturally occur in the earth
are attracted to the negative charges of the cloud.
And here as well,
if the electrical voltage is becoming enough,
the lighting is activated.
But it is not that easy.
Because the voltage between the cloud and the earth
never reaches 10 000 volts/cm
that would allow the electrical arc.
So, what is actually happening?
Most of the time,
everything starts in the cloud.
A very small electrical discharge happens downwards,
at the bottom of the cloud.
Very small because not very powerful,
almost invisible to the naked eye,
and not very long, between 50 and 100m.
This little discharge ionises the air as it goes past,
i.e. it snatches electrical particles – the electrons –
from air molecules.
As a result,
as the little discharge goes past,
the air has become conductor.
An electrical conductive channel,
called a leader,
is created in the air.
This first discharge is followed by a second one,
which will extend the leader for a few metres,
and so on.
The leader develops doing zigzags and successive leaps,
with several branches,
in random directions but rather directed to the ground.
Leaders can be developed from the ground too.
But in fact,
most of the time,
they do not go really high up,
just some tens of metres.
Of course, upward leaders,
which means coming from the ground,
occur to meet downward leaders,
coming from the clouds.
When they link-up,
there is an electrical conductive channel
that links the ground to the cloud.
Obviously,
all the electrical charges surge into this channel:
this is the lightning bolt,
the main lightning,
far more powerful than the first discharges.
While a leader links the cloud to the ground
within several tens of seconds,
the main discharge happens in a 10 000 times shorter period,
that is several tens of millionth of seconds.
Under the effect of the electrical arc,
the air heat up to 20 000 degrees.
So the air is slowly expanding
and produces a shock wave,
that transforms in an acoustic wave
a few metres away from the lightning:
it is the thunder.
So, in the end,
where does the lightning come from?
Well, in 90% of cases,
the leader comes from the cloud,
does the greatest part of the way
until reaching a leader that is coming from the ground.
But the main lightning
is actually going up from the ground
at a dazzling speed,
in the order of 100 000 km/sec.
The positive charges of the ground
come to neutralise the negative charges
at the bottom of the cloud.
And so, eventually, in most cases,
the lightning bolt comes from the ground.
Production: Unisciel/ University of Lille 1
Conception/Production: Maxime Beaugeois, Damien Deltombe and Daniel Hennequin
Editing/Special effects: Perrine Lefrileux
Music: Sébastien Ride, « Thunder Chacha » (SR Music)
Presentation: Maxime and Nina Beaugeois
Graphic design/Credits animation: Michaël Mensier