Remember, remember the fifth of November: Was that thunder I heard or not?

Abstract

‘Thunder days’ are simple records of thunderstorm activity, logging whether a human observer heard thunder on a particular day or not. Despite their low dynamic range and inherent subjectivity, thunder days are invaluable as the only long‐term observations of thunderstorm occurrence, with some records stretching back into the nineteenth century. Thunder days, however, are potentially susceptible to false positives, particularly from explosions. Thus one might expect UK thunder days to show anomalously high counts on New Year's Eve and the days around 5 November, Bonfire Night, both of which are celebrated with large firework displays across the country. It is demonstrated that UK Met Office records of thunder days between 1980 and 2010 do not show any significant increase in thunder reporting around 5 November or 31 December. In fact, the days around 5 November exhibit the largest reduction in the amount of reported thunder relative to annual climatology. While meteorological variability cannot be completely ruled out, this result is suggestive of observer bias; it is speculated that human observers, armed with a priori knowledge of the likelihood of false positives, ‘second guess’ themselves to a greater degree around 5 November than the rest of the year. In fact, the data suggest they should trust in their ability to correctly discriminate between thunder and fireworks.

Thunderstorm occurrence at ten sites across Great Britain over 1884–1993

“…However, with the shift towards automated weather recording, it is perhaps not surprising that multiple stations would under‐report thunder during this period. A better proxy for accurate thunderdays may be the highest reporting station in the neighbourhood, as false positives are rare (Owens, ). Using this measure, it is clear that the declines at Cambridge, Oxford, Spurn Head and York are not genuine.…”

“…However, only a small fraction of the thunderday observations have been digitized, and exclusively after 1950 (e.g. Owens, ).…”

“…Given false positives in thunderdays are rare (e.g. Owens, ), a reasonable approximation is that the highest reporting station in a given region produces the most compete record. This is shown in Figure , where the red line shows the annual regional maximum thunderday time series, constructed from the maximum monthly thunderday count from any neighbouring MIDAS station.…”

“…Anecdotally, thunderdays are sometimes criticized as being subject to false positives from explosives or industrialization, but there is little evidence for this. Indeed, periods of expected false positives, such as from widespread firework displays on 5 November, so no such bias and may even lead to an under‐reporting (Owens, ). Thus, the dominant dataset bias is likely to be false negatives from a lack of reporting of actual storms.…”

“…In the MIDAS dataset, the earliest thunderday observations are 1950, where data are available from 9 stations, increasing to around 100 until 1970, before peaking around 400 stations through 2000 and gradually declining to around 100 by 2010 (Owens, ). The decline in number of stations reporting thunderdays is largely due to the adoption of automation at stations which were previously staffed by a human observer.…”