Is it safe for our visitors to be on the lake during an electrical storm? What exactly are the hazards and how should the concession or Park Service respond to the appearance of lightning around the rim of Crater Lake? First, let's consider the hazards associated with lightning.
Each summer at Crater Lake, conditions are right for the formation of thunderstorms. Moisutre is lifted by strong updrafts which condenses into the cumulus clouds that become thunderstorms. As droplets, ice particles, and hail within the growing cloud come into contact, positive and negative charges are separated so the base of the cloud becomes negatively charged. As these clouds move over the Lake, large electrical potentials are separated by only a few thousand feet. Because air is a poor conductor, the electrical potential may grow to 300 volts per meter or 1000 volts per foot prior to an electrical discharge, or lightning strike, between the base of the cloud and the surface. This electrical potential is the result of a negative charge at the base of the cloud and an opposite positive charge that develops below the surface. If the surface is water, a better conductor than soil or rock, the electrical force driving the lightning is more easlily developed. When a discharge occurs between the base of a cloud and the surface, electrons stored in the cloud are transported. This neutralizes the charge separation momentarily. Surprisingly, the base of the cloud can be recharged in as little as 20 seconds, setting up the conditions again for another discharge. On average, each discharge represents 20 coulombs of charge with a measured 30,000 amperes or current. Death by electrocution can occur with as little as .1 ampere.
What hazard do the boat tours face on Crater Lake during a thunderstorm? The Lake covers some 25 square miles and except for Wizard Island and Phantom Ship, the tour boat is the tallest feature on the Lake as it moves form point to point. As mentioned above, the water under the base of thunderstorms will be positively charged. Because these charges are attracted to the negative underside of the cloud, the tour boat will be particularly attractive to electrical discharges from above as positive charges swell up and over the vessel. This would be true of any object adrift in the Lake if it rose above the surrounding surface. Since air is an insulator, the lightning will attempt to cover the least distance between surfaces to complete the discharge. This is why trees are often struck by lightining and why lightning rods are fixed to the tops of houses. In addition, the metal engine of the tour boat will also be attractive since it is made of a conducting material. 1% of all engine fires reported by boat operators resulted from lightning strikes.
How should the concession and the Park Service respond to a lightning storm on or around Crater Lake? Nearly all experts agree that the best place to be during a thunderstorm is on shore and under some shelter. This would suggest that tour boats remain at Cleetwood Cove or Wizard Island until the storm moves on. A ranger atop the Watchman could be very helpful in both warning tour operators of on coming danger as lightning approaches the rim and when the storm has moved away from the Lake. Tour boats caught on the Lake during a thunderstorm should move away from the center of the Lake and towards the nearest shore. This should reduce, somewhat, the exposure of the vessel to lightning. As mentioned, if a lightning discharge should occur with the boat near shore, trees or rocks higher on the rim are more likely a target. In any case, boat operators and park personnel should refrain from using portable radios since communications are based upon ionizing the atmosphere between stations. Lightning discharges could use this ionized channel in the air to reach the surface.
Tour boats have served park visitors during the summer for much of this century. Our summer weather is usually warm and dry which is ideal for sightseeing natural features around the rim. At some point every summer our weather becomes dominated by afternoon thunderstorms, making our Lake a place for visitors and others to avoid. With some understanding of the hazards associated with these storms and a plan of action, we can continue the spotless safety record the tour boats have earned.
by - Tom McDonough, seasonal Park Ranger at Crater Lake since 1969
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References:
F.K. Lutgens and E.J. Tarbuck, The Atmosphere, 4th edition, pg.267
P.E. Viemeister, The Lightning Book, pg. 143, 241-245
Fleagle and Businger, An Introduction to Atmospheric Physics, pg.121
Wilson, Technical College Physics, 3rd edition, pg. 524
