Walking along the breakwater at in north-east England recently, my granddaughter and her mother noticed their hair was standing on end. It started to rain soon afterwards, but there was no thunder or lightning that day. What was happening?
鈥� From one of my physics textbooks I recall a hair-raising picture of a woman standing on an exposed viewing platform at Sequoia National Park in California. She was in grave danger; lightning struck only minutes after she left, killing one person and injuring seven others (Fundamentals of Physics, 6th Edition, by David Halliday, Robert Resnick and Jearl Walker). It鈥檚 likely that similar conditions were abroad on the day your photo was taken.
Most lightning clouds carry a negative charge at their base. Anything close to the cloud would feel the effect of electrostatic forces: electrons in a person鈥檚 hair would be repelled downwards, leaving the ends of the hair positively charged. The positive hair tips then get attracted to the cloud 鈥� and repelled by each other 鈥� and stand on end. It鈥檚 rather like rubbing a balloon on someone鈥檚 hair to make the hair stand on end: the balloon becomes negatively charged and the hair is attracted to it.
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Lightning victims often describe how they felt tingly and their hair stood on end before they got struck. Fortunately air is a good electrical insulator and, in this instance, the charge in the clouds wasn鈥檛 high enough to jump down to earth, so there was no lightning. However, this was probably a lucky escape for your family. If your hair stands on end outdoors or your skin is tingling, lighting may be imminent and it鈥檚 best to run for suitable shelter.
Iain Longstaff, Linlithgow, West Lothian, UK
鈥� The phenomenon described above is known as luck 鈥� the two people were fortunate not to have been struck by lightning. Experienced hikers and climbers know that this hair-raising phenomenon can be a precursor to a lightning strike and are taught to flatten themselves or, if climbing, dive for lower ground.
鈥淭he two people described in the question are very lucky not to have been struck by lightning鈥�
There is a vertical voltage gradient in the atmosphere, typically in the order of 100 volts per metre on a clear, dry day. For an average adult male, then, there will be a 180 to 200-volt difference between the toes and the top of their head.
When electrically charged would-be storm clouds scud overhead, an induced ground charge follows the clouds, markedly increasing the voltage gradient. If the potential difference is sufficient to overcome the resistance of the air 鈥� around 3 million volts per metre 鈥� then lightning leaps across the gap. In practice, lightning strikes are possible at substantially lower voltage differences. The fact that the reader saw no lightning and heard no thunder merely suggests that, luckily, the voltage never rose high enough for a lightning strike.
Larry Constantine, Department of Mathematics and Engineering, University of Madeira, Funchal, Portugal