Electric Fire
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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 to be a supernatural phenomena. But it has a basic physical cause, atmospheric electricity. Hi, I'm Dave Thurlow and this is The Weather Notebook.

The phenomenon of St Elmo's Fire, scientifically known as a corona or point discharge, is actually a glow caused by electrical forces between the atmosphere and an object. It occurs most frequently around spiked, pointed objects like ship masts or flag poles, trees. When the electric charge between the pointy object and atmosphere is way out of balance, measured in voltage, it's time for St. Elmo's display.

During typical fair weather, the electrical field strength of the lower atmosphere is only about one hundred volts per metre. However, in the initial stages of thunderstorm formation, it increases to ten thousand volts per metre or more, just below the thundercloud. And just before a lightning flash, the potential may exceed one million volts per metre in the vicinity of the coming strike.

Tall objects, which are good conductors, have the same electrical field strength at their top as at their base because they are grounded. Thus, the high end of the object has a higher voltage than the air around it. This causes 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: They are -- with the Fire of St Elmo.

Keith Heidorn wrote today's show. Subaru, provides funding for the Weather Notebook, a production of the Mount Washington Observatory.