Friday, 5 May 2006

Thunder & lightning

<![CDATA[ A very unusual occurrence last night: a thunderstorm. The last one I can remember to any degree of certainty happened in January 2005. It was in the middle of a hail, snow, sleet and kitchensink shower on the road near Leurbost, 7 miles south of Stornoway. It left a layer of ice on the road about 2 inches thick, and traffic was slowed to a 10 mph crawl.

Thunderstorms occur if the difference between the top of a cloud and the bottom is more than 40 degrees C. This usually happens when a layer of cold air moves over a layer of warm air. The rising air from the surface cools as it goes up, and condenses into a cloud. It develops an electrical charge, and in the right circumstances the difference between the negatively charged cloud and the positively charged earth results in a spark, which we know as lightning. As I say, those in more southern latitudes associate thunder with the aftermath of a hot day. This was actually the case last night. Although the temperature in Stornoway did not exceed 15 degrees C (59 F), further south, the mercury had soared to the upper 20s C, in excess of 80 F. Cold air is flowing down from the north, and its collision with the warm air moving up from France resulted in a spectacular electrical storm over Glasgow. Reports from there speak of flash flooding and one bus passenger left stranded at a busstop up to his waist in water.

Here in Stornoway there were three lightning discharges, two peals of thunder and one powercut. This lasted less than a minute, and had been preceded by a dip and a surge in power. That is about par for the course, and it results in a shorter than average lifespan for electrical equipment, such as lightbulbs. People in the islands are recommended to install surge breakers to protect sensitive electronics such as computers and hifi equipment. ]]>

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