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| Yacht on Waterfall Sea A small yacht sails innocent of nearing doom. It is grabbed. Its hull is bent down to match the plummeting waters. Rigging and sail rip, stretch sideways and upwards. A scene captured by Sandy Robertson at Alderney (Channel Islands) Breakwater, an immense 3000ft long structure jutting north east from the island. Sandy has witnessed the breakwater�s mirages several times and an earlier OPOD features it. In this sighting the yacht adds extra spice and optical complexity. All images ©Sandy Robertson, shown with permission |
| The breakwater is no stranger to mirages. This one was recorded in 2010 by Sandy Robertson. The offshore breeze was again coming from the left, seaward, side of the structure. The sea roars over the waterfall lip. At right the trailing and turbulent warm/cool air plume blurs the horizon and waves. |
| Scroll right for full mirage sequence |
| A combination of two mirages. An offshore breeze from the left sweeps cool air up and over the breakwater. The breakwater itself is strongly heated on its top and left side by the sun. A hot air layer formed next to the stone resists the breeze and creates strong temperature gradients � The stuff of mirages. Two mirages? The breakwater top and its hot air layer create a variant of the well-known hot road inferior mirage. The hot breakwater side creates a hot wall or lateral mirage. We see their combined effects. |
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| Modified Hot Road Mirage The camera views the distant sea through a long hot air path along the breakwater top. Cooler air is above it. Rays from the sea are refracted upwards by the strong temperature gradients between the two air layers. Rays always refract towards cooler, denser, air. The sea horizon closest to the breakwater appears to come from a point lower down. We see a 'waterfall' effect. The yacht - sailing past the breakwater end - has its hull similarly bent downwards. Hot road mirages do not usually show this effect because the road does not end and we do not view things at below road level. |
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| The mirage stretches the crewman towards the wall. |
| Lateral Mirage The hot sunward breakwater wall adds a sideways component to the refraction. We see it in the distortion of the yacht's rigging. Refraction of the yacht's rays across the cool/hot air boundary deflects them - towards the cooler air - and the rigging appears shifted to the right from its true position. The forestay is stretched outwards from the mast and even curls upwards. The latter is a more extreme mirage where mirroring occurs. Is the sea similarly miraged? Yes, but with its predominantly horizontal features it is hard to see. |
