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St. Elmo
Tue Jul 20, 2004
Listen in RealAudio 
Journal entries of the great sea explorers Columbus and Magellan and other sailors
as well as Shakespeare and Herman Melville describe St. Elmo's Fire as a
supernatural phenomena. But it has a basic physical cause, atmospheric electricity.
Hi, I'm Bryan Yeaton and this is The Weather Notebook.
The glow of St Elmo's Fire is not really fire at all—it is scientifically known as a corona
or point discharge, and is caused by an electrical imbalance between the atmosphere
and an object. It occurs most frequently around spiked, pointed objects like ship
masts, or flagpoles, or trees. When the electric charge between the pointy object and
the atmosphere is way out of balance, as is measured in voltage, it's time for a St.
Elmo's display.
During typical fair weather, the electrical field strength of the lower atmosphere is only
about one hundred volts per meter. However, in the initial stages of a thunderstorm,
that increases to ten thousand volts per meter or more, just below the thundercloud.
And just before a lightning flash, the potential may exceed one million volts per meter
in the vicinity of the coming strike.
Tall objects, especially pointed ones, can initiate a weak flow of electrons— an
electrical current— to flow from the object to the air. Normally the flow is an invisible
process, but when the electrical potential difference becomes strong, as it does under
a thundercloud, the flow of electrons increases like crazy. This excites air molecules
around the object causing them to glow with a blue or bluish white light, as if the point
tips are on fire: with the Fire of St Elmo.
Thanks today to meteorologist Keith Heidorn. The Weather Notebook is funded by
Subaru of America and the National Science Foundation. We are a program of the
Mount Washington Observatory.
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