Thunder and lightning - What determines where and when thunderstorms occur?

Where and when thunderstorms occur is a topic of considerable practical importance for human society on which some meteorologists and atmospheric and space scientists carry out research. Owens et al (2104 Environ. Res. Lett. 9 115009) have found that the occurrence of lightning over the UK is up to ∼50% greater than usual when the magnetic field outside the Earth's magnetosphere, in interplanetary space, points towards the Sun rather than away from it. But why this happens is not yet totally clear.