RESEARCH//THUNDERSTORMS

Tips for Thunderstorm Observers

Thunderstorm observation is an aspect of TORRO's work where every member should experience events which they can report. Such contributions are of great value to TORRO's ongoing research. The main details required are the place, date and time of the occurrence of thunder in any part of the British Isles. The following information will be invaluable:

• Time of first observation of thunder (with or without lightning) with direction and estimated distance.
• Times of heavy rain and/or hail if observed.
• Approximate nearest approach of the storm with direction and estimated distance.
• Approximate time of final observation of thunder (with or without lightning) with direction.
• A note on the intensity of the storm.
• Information on any damage caused by the storm, especially that attributable to lightning (both in the immediate vicinity of the observer and that reported by the local news sites/regional press, tv, radio etc), hail (property, vegetation) or severe local winds.

The briefest note for any date will still be very helpful (it is not expected that a 24 hour watch will always be possible!)
Where possible, incidents of damage should be reported as soon as possible so that on-site investigations can be arranged
Copies of local press reports (or links to local news internet sites) are a very important additional source of information; please submit any items relating to storm damage or post the web site links to the TORRO members’ forum (NB it is important that the date of each incident is indicated as news reports can appear a while after an event).

Photographs (or video clips) of damage caused by lightning, hail, severe local winds or (thunderstorm related) flood are always welcome for our growing photographic collection. Photographs of large hailstones are also invaluable; these should include a scale reference to indicate the hail diameter - such as a ruler (ideal), coins (e.g. pound coins) or a hand with fingers together.
Observers may use either Greenwich Mean time (GMT/UTC) or clock time (BST in summer) provided the standard of time is indicated.

The distance of a lightning discharge may be estimated by timing the interval between seeing the lightning and hearing the thunder. One mile is indicated by five seconds or one kilometre by three seconds. Thunder occurring at a distance of approximately three miles (5km) or less can be considered as "close".
Details of isolated storms and/or distant storms over mountainous and sparsely populated areas (and over the sea) are of special importance. However, as with all reports, observers are urged to carefully note the dates and times.

Many members already contribute Thunderstorm reports, for which we are very grateful. If you are not currently doing so, we would very much encourage you to join our network of Thunderstorm Observers. Reports can be submitted in three alternative ways -

• On individual thunderstorm report forms, which can be downloaded from the TORRO web site (see below) -these can be submitted by post or scanned and sent by e mail- either immediately after each event, or, if more convenient, in monthly batches.
• By E-mail, either using the template available on the TORRO web site. (TORRO thunderstorm report form), or your own choice of template (e.g for a monthly report).
• Alternatively, details can just be included in an e mail (see contact details)
• On the TORRO Members’ Forum.
• As noted above, please also send in any press cuttings, or links to relevant online news reports/blogs/social media items, relating to damage caused by lightning, hail and (if thunderstorm related) flood or wind. This information can also be scanned and sent by e mail

Regular annual summaries, and case studies of significant events, are published in TORRO's International Journal of Meteorology. The annual Thunderstorm report is reproduced in the International Journal of Meteorology (together with the other annual reports - such as that on lightning impacts), and is presented, as part of TORRO's annual Weather review, at the TORRO Spring meetings, usually held in April.

Please also note that articles (or letters) on interesting/severe events are always welcomed as submissions to the International Journal of Meteorology. We have the capacity to provide full details of our storm records for specific days or periods of interest. Both detailed "case studies", and letters describing personal experiences of recent (or even not so recent!) thunderstorms, are very popular among readers, and contribute to the Journal providing an informative record for future generations.

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Widespread reports of thunder/lightning can sometimes be attributable to thundery showers occurring widely but with gaps in the ‘overhead activity’. If there are reports of widespread overhead thunder, this indicates highly organised convection such as Mesoscale Convective Systems (MCSs) or (a subdivision of the latter) thundersquall lines (Pike, 1994; 1999).Widespread overhead thunder can be identified by (a) observers’ reports of overhead thunder, the latter defined by TORRO as within 5 km (3 miles) or ‘close’; (b) the distribution of lightning strikes showing a high-density ‘whiteout’; and (c) strong evidence, albeit not direct proof, from an MCS anvil cloud shield on infrared satellite imagery (Figure 7.3).

Showery situations are more likely to be replicated on several successive days than are situations favouring major thunderstorm systems, though even with the former, subtle day by day variations in the distribution of convective showers must be expected. Prichard (1987) and Grant (1995) both refer to the unusual sequences of widespread thunderstorm days from 22 to 26 June 1980 and from 12 to 22 May 1983. June 1982 was the most thundery calendar month in the United Kingdom with an average of 11% of stations reporting thunder each day

Thunderstorm severity is also difficult to define precisely. It can be based on the peak intensity (lightning flash frequency) of a storm or on the total number of discharges. The main risks of personal injury and damage to buildings and infrastructure derive from cloud-to-ground strokes though aerial lightning strikes are significant hazards for aircraft. Moreover, ground strikes vary in power; considerable damage and disruption can be associated with a single powerful cloud-to-earth discharge – usually a positive strike. Positive lightning discharges (Rakov, 2003) are able to propagate continuously without ‘steps’, whereas the more common negative discharges cannot propagate continuously so the propagation is ‘stepped’. Public (and observers’) perception of storm severity will also be influenced by the intensity of precipitation. The international definition of a severe thunderstorm does not refer to electrical activity, but it is a definition which implies the need for organised convection. It is defined as a storm with one or more of the following: a tornado, wind gusts over 50 kt or hail 20 mm diameter or more.