Waterfall Sea

To the left of the jetty the sea pours downwards into an abyss, as though over a giant waterfall. To the right it is also strangely sloping and depressed.

Sandy Robertson has seen this peculiar effect several times at the Alderney (Channel Islands) Breakwater. The structure is immense. It is currently 3000ft long (once it was 4900 ft) with a stone superstructure some 33 feet high. Construction started in 1848 in response to the building of a French breakwater at nearby Cherbourg and the perception that the Royal Navy needed better harbours to protect its channel fleet.

©Sandy Robertson, shown with permission


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The image, one of several, was taken late morning on a warm July day. A gentle breeze came off the sea – from left to right in the photograph.    The sea was cooler than the air but there are no measurements near the breakwater itself.    The left, seaward, wall remained in shadow until afternoon.   The breakwater is a solid barrier to the air flow with no recirculation possible.

The apparently precipitously curved sea surface and the left – right asymmetry of the mirage is most unusual.   That it has been seen more than once reassures that an explanation can be found without having to evoke freak and untestable circumstances.

The jetty top had seen several hours of bright sunlight and would in still air produce a layer of warmed air immediately above it.     The gentle left to right breeze carries air cooled by contact with the colder sea up the left seaward side of the jetty and over the warm air layer, thickening the latter in the process.   
At the left side of the jetty the layers have a relatively sharp boundary with a steep temperature gradient at the interface.      In passing over the jetty the layers interact and the vertical temperature gradient decreases from left to right.

Mirages and visual distortions arise from refraction in the temperature (and thus density) gradients between bodies of air at different temperature.   The refraction severity and the strength of the mirage depend on steepness of the temperature gradient.

Looking along the jetty towards its left hand edge, rays from the sea pass between the air layers where the temperature gradient is most extreme.   Rays are always refracted in the direction towards the cooler and dense air – upwards in this case.   Rays from the horizon appear to come from a point lower down in the sea.   The horizon and the nearer sea surface appear to be depressed downwards.  The more oblique the viewing angle along the jetty the stronger is the effect which produces the ‘waterfall’.   

On the right hand side of the jetty, mixing of the two air slabs has occurred and refraction is weaker.   We see an horizon less severely depressed but wavy in the turbulent wake from the jetty.

The left-right asymmetry depends also on the camera position and the waterfall is most apparent when it looks obliquely along the left - hand side. The cold air above the sea further enhances the overall effect by appearing to raise the distant horizon and waves.

The height of the jetty plays a role too in that the miraged waves are mostly at some distance beyond it. This enhances the mirage. Relatively nearby objects like the fishermen (see below) are within or close to the refracting regions and are not affected.

My thanks to Andrew Young for the above interpretation. Errors and ambiguities are mine!


Mirages alter with camera height. Images from 2 and 5 feet above the lower right hand deck. Here the warm air layer is less disturbed by the left to right cooler breeze.

The mirage from 9 feet above the deck level

Alderney in the English Channel off the coast of France and its breakwater