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Lightning + static electricity + moisture = confusion

  1. Mar 7, 2010 #1


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    I've never understood this, but none of the people that I thought would know could think of a reason, either. It's probably just some simple detail that I've overlooked, but I'm stumped.

    So... I've always been taught that "lightning is a gigantic spark of static electricity", but I've also always been taught that "moisture and/or humidity inhibit the build-up of static electricity".

    Which one is correct, if either? If both of them are correct, then why are there lightning storms when it's pouring rain, but not when there is a drought?
  2. jcsd
  3. Mar 7, 2010 #2


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    Moisture/humidity don't necissarily inhibit the build up of static electricity.

    They are excellent charge carriers, but it depends on the system how they carry charge. In the cumulonimbus, it's hypothesized they carry charges (through collisions) in a way that separates charge.

    Closer to the surface of Earth, humidity tends to balance charge. I would speculate this is because moisture is more diffusive near Earth and more dynamic in the clouds where you have collisions with larger water particles.

    Cumulonimbus clouds are basically giant solar powered heat engines.
  4. Mar 7, 2010 #3


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    I think that might have been just a tad bit "over my head" (although I now have more terms to add into my searches), but let me see if I can come up with an analogy to understand this.

    So, it's one of those things where "it depends on the situation"? [begin analogy] Sort of like how "hot air rises", and thus why basements are generally colder than the top floor of a building, yet it's colder at high altitudes than low altitudes (given the lower altitude is not very cold; maybe 80°F).

    If I remember correctly, this is due to the expansion of air; hot air is less dense, thus rises above colder air, then the particles spread out (due to lower surrounding pressure), causing the temperature to drop. (Demonstrated by breathing on your hand with your mouth wide open from a couple inches away, then by blowing through a tiny hole between your lips from the same distance.) [end analogy]

    Another thought... would it have anything to do with the fact that, when in our everyday lives, static occurs when dry materials experience friction with other dry materials, and water particles "interfere" with it? If so, would adding "dry particles" to the "wet material" (clouds) make then less likely to produce static build-up?

    Do I seem to be understanding this (even if I'm not correctly describing it), or am I way more lost than I thought I was?
  5. Mar 9, 2010 #4


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    Here's the Electrostatic Induction Hypothesis (it shows the alternate hypothesis too).

    On the other hand, if you have a conducting sphere with a charge on it in a humid environment, the moisture will act to carry the extra charges away from the sphere in a convective manner.

    Actually, it's the same kind of friction that is theorized to cause separation of charge in clouds in the case of Electrostatic Induction.

    Of course, if the alternate theory is true:
    "Falling droplets of ice and rain become electrically polarized as they fall through the Earth's natural electric field"

    Then water isn't required for the separation of charge. Then all I can say is that Cumulonimbus clouds are as well known for rain as they are for lightning. It may be possible that the lightning triggers the rain by affecting the cloud somehow (I have noticed the thunderclap followed by the rain before).
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