Conjure up before your mind a vision of ‘Dickensian’ London, and as likely as not you will see in your imagination a street filled with yellow fog, dimly illuminated by a gas-lit street lamp. The classic ‘pea-souper’ was caused by a natural winter fog in the Thames basin, turned yellow by the coal fires and industrial chimneys of the Victorian city and held in place for days by the phenomenon of ‘temperature inversion’, when a layer of warmer air traps the cold, damp and increasingly impenetrable atmospheric mix in the streets below.