OT Whining Lightning

Discussion in 'UK and Europe' started by Mark Thompson, Aug 19, 2004.

  1. As this group likes discussing lightning every few weeks you might be
    interested in this:

    http://www.newscientist.com/lastword/article.jsp?id=lw701

    Whence the whine?

    Question
    Last year our house was struck by lightning. Just before the strike,
    which manifested itself as a simultaneous bang and flash, my wife and I
    heard a strange noise. It lasted about 2 seconds and was similar to the
    whine a jet aircraft makes when landing --at first, we thought the house
    had been hit by a plane. Does anyone know what caused it?

    Carl Hampson , Sefton, Merseyside

    Answers
    As a cloud-to-ground lightning strike develops, an almost invisible
    "stepped leader stroke" descends from the cloud towards the ground. When
    this reaches about 100 metres from the ground, or your house in your
    case, the intense electrical field causes a coronal discharge to flow
    upwards.

    When the upward discharge meets the descending leader stroke, the circuit
    is completed and the main current flows. The result is lightning.

    The whine you heard was from this upwards electrical discharge. This also
    causes other effects people have observed before a lightning strike, such
    as hair standing on end or the peculiar odour of ozone.

    Sam Mulholland , Bristol



    Just before a lightning strike, energy discharges upwards from projecting
    points such as housetops, ionising the air. Because the charge of these
    ions opposes the electric field near the points, the discharge quickly
    stops, and then rapidly dissipates. The cycle can then begin again. This
    happens hundreds of times per second in a large volume of air, and is
    what makes the buzz or whine.

    The sound is a warning of an imminent lightning strike. If you hear it,
    take it seriously. Move away from walls or tree trunks, and crouch with
    your feet together. In darkness the discharge is visible and is known as
    St Elmo's Fire.

    For a small-scale demonstration, choose a very dry day and electrify one
    side of a large balloon by rubbing it on clean, dry hair until it
    crackles with static. Then bring your finger towards the charged surface,
    and listen carefully. The more rapid the approach, the higher-pitched the
    sound. A very fast approach yields a miniature lightning strike, which
    should be visible in a dark room.

    As a child, I set up a 10-metre wooden mast on an exposed hilltop. Using
    insulated wires, I connected a fluorescent-tube lamp to the ground and to
    a point near the top of the mast. As a storm passed overhead, the tube
    would flash with each discharge from the mast top, increasing in tempo
    from a few times a second to nearly continuous light. A lightning strike
    would either intensify or stop the display. The experiment came to an
    abrupt end one day when lightning struck the mast.

    Charles Sawyer , Camptonville, California

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