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Following January's issue on severe winter weather which specifically mentions lightning damage in Guernsey (Prichard, 2011), you may be interested in my recollections of a frightening event that took place in Guernsey at the end of November 2010.

On 26 November, the weather charts showed slack areas of low pressure covering most of the UK with fronts over western England running north–south in a cold northerly airstream. The weather in Guernsey during the day, though cool, was lovely and sunny for early winter, but further north over the UK it had already turned much colder with areas of snow moving south. There was one such area of snow showers moving south over Wales, which by late afternoon had crossed the Bristol Channel and was then over southwest England. By sunset I observed that the sky was becoming cloudier from the north, and the radar showed precipitation continuing to move south into the English Channel. Winds during the early evening were very light from the west and temperatures were still well above freezing.

By 8pm, the air was very still and rather menacing with quite frequent lightning to the west and very deep rumblings of thunder. Within twenty minutes it was snowing very hard and suddenly there was a tremendous lightning bolt accompanied by an explosive crack of thunder. A large upstairs windowpane was blown into the house and the house was plunged into darkness with snow and glass all over our hallway. The lightning bolt had struck a property some 100 metres across a field to the west, causing it to catch alight with many thousands of pounds worth of damage.

This mix of lightning bolts and snow continued to occur over the following few days with further heavy snowfalls. During this period the island was hit by quite a number of cloud-to-ground lightning strikes with several prominent buildings being struck. In over 60 years I cannot remember thunder with snow occurring like this in Guernsey, or seeing up to five inches of snow as early as in November.

I have heard of the term thundersnow when one encounters lightning storms whilst it is snowing, particularly on Fox News describing the weather in the USA. I am therefore wondering if anyone could explain why it seems to me that lightning storms in conjunction with heavy snow appear to enhance the lightning bolts and the intensity of the claps of thunder.

Response from Jonathan Webb

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Observations and impacts of this exceptional spell of weather at Guernsey airport are described by Cunningham (2010). The underlying cause was cold arctic air becoming extremely unstable over the warm sea.

Though accounting for only about 5% of all lightning strikes, positive discharges can be the predominant cloud-to-ground lightning during the winter months (NOAA-NWS Jetstream, 2010), conspicuously so in thundersnow events (Schultz and Vael, 2009). Such discharges are estimated to be six to ten times more powerful than the much more common negative discharges, as well as lasting ten times longer. Positive discharges originate in the frozen, anvil area of the cloud. During intense winter convection (when freezing levels are low) this charged area is nearer to the ground and constitutes a large proportion of the cumulonimbus cloud. It all adds up to a powerful bolt, especially dramatic when punctuating the normal quietness of a snowfall.

References

  1. Top of page
  2. Response from Jonathan Webb
  3. References