<scp>CTR</scp> Wilson Meeting 2024 on atmospheric electricity

Atmospheric electricity chiefly concerns lightning, thunderstorms, ionisation and the charging of droplets and particles. The 12th CTR Wilson Meeting on Atmospheric Electricity was held at the University of Bath on 21 November 2024. This year there were 57 registrants for this meeting, which was organised and chaired by Martin Fullekrug, Xue Bai, Karen Aplin, Alec Bennett, Keri Nicoll and Giles Harrison. Such is the enthusiasm for this meeting nationally and internationally that even the vice chancellor of the University of Bath attended. A photograph of meeting attendees is shown in Figure 1. This series of annual meetings is organised between the Atmospheric Electricity Special Interest Group of the Royal Meteorological Society and the CTR Wilson Institute for Atmospheric Electricity. The meetings are named after Charles Thomas Rees ‘CTR’ Wilson, whose Nobel prize-winning method for visualising energetic particles – the cloud chamber – can be traced back to his atmospheric science interests (see e.g. Harrison, 2011). CTR Wilson also pioneered the global atmospheric electric circuit concept, which explains how electric charge generation in disturbed weather is coupled to distant regions, via conduction through the Earth's surface and upper atmosphere. The global circuit sustains the fair weather electric field, also known as the potential gradient (PG) – which is the voltage difference (~150V) consistently observed in fair weather between the surface and one metre above (Nicoll, 2016). The first talks in the morning sessions mostly focused on studies of charge and electric fields in non-thunderstorm conditions. Caleb Miller (Meteorology, University of Reading) opened the meeting discussing the effects of fog on the atmospheric PG, and how further insights into using these changes for fog forecasting can be obtained by modelling and measuring fog's charge structure. Sakina Alblooshi (Meteorology, University of Reading) then described her work on the effects of the sea breeze on dust electrification at a site in the United Arab Emirates. The consistency and regularity of the sea breeze from the Gulf allow the substantial charge generation in dust to be explicitly identified and analysed. The remaining talks concerned observing energetic particles in the atmosphere (e.g. from cosmic rays) and establishing whether electric field effects could be remotely sensed. Karen Aplin (Engineering, University of Bristol) discussed recent results from a balloon flight carrying a miniature detector able to resolve the energies of atmospheric particles. A highlight was identifying a part of the gamma energy spectrum generated by cosmic ray neutrons, which may ultimately provide an alternative to the neutron monitors currently widely used for space weather and cosmic ray monitoring. Caroline Cox (from RAL Space) described the latest work on remote detection of atmospheric electric fields through the modification of spectral lines in the electromagnetic spectrum. Using the ARTS (Atmospheric Radiative Transfer Simulator) code and realistic atmospheric conditions, electric field-induced effects were found in the Terahertz region. Moving beyond Earth, Blair McGinness (Meteorology, University of Reading) used electrical data recovered from the 1970s Venera missions to Venus. Using new insights derived from experiments investigating similar sensors to those originally carried by Venera space probes, Blair discussed whether there was likely to be a Wilson-like global circuit on Venus. To facilitate new fieldwork charge measurements, Giles Harrison (Meteorology, University of Reading) described a miniature design of Faraday cup charge collector for small charged objects such as snow and insects. Liam O'Reilly (Biosciences, University of Bristol) gave an overview of electrosensory responses in bees and how positive and negative electric fields influence their foraging behaviour. The afternoon sessions were concerned with lightning and its effects. Xue Bai (Engineering, University of Bath) spoke about lightning threats to low carbon infrastructure and the lightning damage to associated low elevation towers. Lightning-generated faults were found to be caused in central regions, such as Birmingham and Manchester. Laiz Souto (Engineering, University of Bath) continued this theme, discussing weather patterns leading to lightning-related transmission line failures. An example was the Little Barford fault on 9 August 2019, which cascaded to affect 4% of power generation, including Newcastle airport and Ipswich hospital. Martin Fullekrug (Engineering, University of Bath) summarised the developing knowledge on lightning and climate, now that lightning is a World Meteorological Organization Essential Climate Variable. It is known, for example, that El Niño increases Pacific lightning but decreases marine continent lightning. Mingyi Xu (Engineering, University of Bath) presented an 11-year lightning climatology for China. This provided a range of statistical information, such as a mean flash rate of 0.9 flashes km−2 year−1 across China's land area, but exceeding 10 flashes km−2 year−1 in the south and east provinces (Xu et al., 2022). Masashi Kamogawa (University of Shizuoka, Japan) provided a stimulating discussion on an exceptional lightning hazard, that of the tether for the proposed space elevator (McGowan et al., 2007). Amongst many other factors, the risk from superbolts argues for a location in the Pacific. Superbolts received further attention from Yoav Yair (Reichmann University, Israel), who presented analyses of lightning during winter thunderstorms in the Eastern Mediterranean. Superbolts (energy >1MJ) are only a small fraction of natural lightning, but they are highly damaging. They are thought to be particularly associated with squat clouds close to the surface. Tamás Bozóki (Institute of Earth Sciences, Hungary) described the identification of lightning properties using extremely low frequency (ELF) measurements, which have good detection efficiency. Powerful lightning discharges with long-duration continuing current are especially well detected. Daniel Mitchard (Engineering, University of Cardiff) described the effect of reducing air pressure on reducing breakdown electric fields. The Paschen curve (a theoretical description of electrical breakdown fields in gases) provides a good approximation of the experimental findings. Meirion Hills (Engineering, University of Cardiff) presented an overview of a range of high-speed image technology for lightning studies, including Schlieren imaging (i.e. visualising density changes) for streamer formation and determining elemental composition using atomic emission spectroscopy or streak spectroscopy. Posters were presented by Masashi Kamogawa on the Fukushima accident and PG measurements at Kakioka observatory, Nathan Croll Dawes on infrared absorption by negative ion clusters, Ola Al-Khuraybi on ionisation in Uranus' atmosphere, Tabitha Hirst on a loop antenna for lightning detection, Tamina Boss on a very high frequency (VHF) lightning detection site protection system, Tom O'Hara on ash charging in volcanic lightning and Tom Potter on machine learning methods for lightning location. Many of the participants continued their discussions over pizza in the centre of Bath. R. Giles Harrison: Writing – original draft. Sakina Alblooshi: Writing – review and editing.