“Threatening” cloud seen on cold, calm days explained

It is a question regularly asked of me at this time of the year and the answer is that this cloud formation in entirely anthropogenic, in other words it is effectively man-made.

Often visible from dozens of miles away under clear, calm conditions, it forms directly above the industrial complex of Avonmouth, on Bristol’s shoreline of the mouth of the River Severn.

But why?

Rising warm, moist air from outflow stacks and chimneys at Avonmouth soars up into the cold air above.

In doing so, it readily condenses and forms a plume of cloud – this sometimes towering up to a considerable size and height.

This happens generally under conditions of calmer, colder weather, as a ridge of high pressure, external establishes temporarily across us.

Usually, the plume of warm air would continue rising higher into the ever-colder atmosphere above. But that’s not the case on Tuesday.