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  • Etching of a sailing ship with blue/violet sky of St Elmo’s fire
    Rights: Public domain Published 29 April 2014 Size: 430 KB Referencing Hub media

    In the region between a thundercloud and the ground, a very strong electric field1 can be set up. There is a huge potential difference (voltage2) established between the negative base of the cloud and the positive ground. When this potential difference reaches a certain value, sharply pointed ground-based objects are seen to glow, often with a hissing sound.

    Because this weather-related occurrence sometimes appeared on ships at sea during thunderstorms, it was given the name ‘St Elmo’s fire’. Saint Elmo is the patron saint of sailors, and in the past, sailors regarded such an event as an omen of bad luck and stormy weather3.

    St Elmo’s fire is a bright blue or violet glow due to the formation of luminous plasma4. It appears like fire in some circumstances coming from sharply pointed objects such as ships’ masts, spires, lightning5 rods and even on aircraft wings.

    Etching by Dr G Hartwig/Treasures of the NOAA Library Collection/National Oceanic and Atmospheric Administration/Department of Commerce

    1. electric field: Any region where a charged object experiences an electric force.
    2. voltage: The potential difference (PD) between two points in a circuit. It measures the potential of the electrons to flow between two points. The higher the PD, the greater the electron flow. PD is sometimes thought of as electrical pressure.
    3. weather: Daily or short-term conditions like temperature, cloud cover, precipitation and wind affecting a certain area.
    4. plasma: 1. The fourth state of matter – a gas that is ionised and consists of positive and negative ions (or particles), with no overall charge. It is affected by magnetic fields and has high electrical conductivity. 2. The colourless or pale yellow liquid in blood and lymph.
    5. lightning: A large-scale natural spark discharge, visible as a flash of blue-white light, that occurs within the atmosphere or between the atmosphere and the Earth’s surface. The lightning channel consists of extremely high-temperature plasma.
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      electric field

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    2. Any region where a charged object experiences an electric force.

      plasma

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    4. 1. The fourth state of matter – a gas that is ionised and consists of positive and negative ions (or particles), with no overall charge. It is affected by magnetic fields and has high electrical conductivity.

      2. The colourless or pale yellow liquid in blood and lymph.

      voltage

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    6. The potential difference (PD) between two points in a circuit. It measures the potential of the electrons to flow between two points. The higher the PD, the greater the electron flow. PD is sometimes thought of as electrical pressure.

      lightning

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    8. A large-scale natural spark discharge, visible as a flash of blue-white light, that occurs within the atmosphere or between the atmosphere and the Earth’s surface. The lightning channel consists of extremely high-temperature plasma.

      weather

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    10. Daily or short-term conditions like temperature, cloud cover, precipitation and wind affecting a certain area.