Explanation of a ‘Threatening’ Cloud Formation Observed on the Horizon

Photographs captured by BBC Weather Watchers depicting a ‘threatening’-looking cloud appearing on the horizon have generated interest regarding its cause. Tuesday offered a dramatic view of this phenomenon, which is familiar to local residents, particularly during calmer, colder days of autumn and winter, and consistently originates above the same location. It manifests as a towering cloud mass, ascending skyward as if linked to a thunderstorm, a massive fire, or something more sinister. However, its actual origin is considerably more benign. This question is frequently posed to me at this time of year, and the answer is that this cloud formation is 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, situated on Bristol’s shoreline at the mouth of the River Severn. But why does this occur? Rising warm, moist air from outflow stacks and chimneys at Avonmouth ascends into the colder air above. In doing so, it readily condenses and creates a plume of cloud, which can sometimes reach a considerable size and height. This generally happens under conditions of calmer, colder weather, when a ridge of high pressure temporarily establishes itself over the region. Typically, the plume of warm air would continue to rise higher into the progressively colder atmosphere above. However, this was not the case on Tuesday. Instead, the cloud reaches a point (at about 6,000ft in this instance) where the air suddenly begins to both warm and dry out—a condition meteorologists term a temperature inversion. Photos of Tuesday’s ‘Avonmouth Plume’ clearly demonstrate how the cloud attains a certain height but then flattens out, a clear indication of an inversion ‘capping’ its upward growth. Under high pressure during winter, these inversions are a common characteristic and explain, for example, why fog sometimes becomes trapped and can persist without ever rising and clearing. In particularly cold weather and under very specific conditions, these types of man-made clouds can even produce very light snow, as was previously observed originating from the Didcot Power Station in Oxfordshire. It will be interesting to see if further spectacular examples of the ‘Avonmouth Plume’ emerge over the coming weeks of winter. Follow BBC Bristol on Facebook, X and Instagram. Send your story ideas to us on email or via WhatsApp on 0800 313 4630. Copyright 2024 BBC. All rights reserved. The BBC is not responsible for the content of external sites. Read about our approach to external linking.