The secret of the 'superior mirage'

6 August 2014

Since the 17th century, sailors have been haunted by the story of the Flying Dutchman, an airborne ghost-ship that can never put in to port and is forced to endlessly sail the world’s oceans.

While the sight of a ship floating above the horizon could unsettle any seafarer, meteorologists can explain it as the result of a Fata Morgana – a dramatic ‘superior mirage’ caused by the air below the line of sight being significantly colder than that above it.

Named after King Arthur’s sorceress half-sister, Morgan Le Fay, Fata Morgana tend to be more common in the polar regions, where the surface is often topped with shallow layers of cold air.

Where this air meets a higher layer of warmer air, particularly in calm conditions, it can create an atmospheric duct that acts like a refracting lens, causing light travelling between the two masses of air to ‘bend’ – and creating some very elaborate distortions. Ships on the horizon can appear elongated vertically or mirrored upside-down.

The Bureau of Meteorology’s Tasmanian media manager, Mal Riley, witnessed a striking Fata Morgana recently off the western coast of Victoria, while he was travelling – ironically – aboard a Dutch tall ship bound for Sydney.

There was a huge black square extending 1 or 2 degrees above the horizon Riley recalled. “Nobody could make out what it was, until the Automated Identification System confirmed it was a car carrier, one of those box-like cargo ships, and our vision of it was stretched to probably three times its actual height. There was an inverted reflection of the ship on top of the real ship: two boxes, one on top of the other.”

During the afternoon, Riley and the crew of the Oosterschelde watched a procession of strange-looking ships, some stretched vertically and with mirror images in the sky, others short and squat.

On the day of Riley’s voyage, winds from the northwest had carried a layer of warm air over the area. The temperature over the land was in the mid-twenties and this air moved over the water of the Bass Strait, which was 13-14°C at the time. The bottom layers of the atmosphere were thus being cooled by the cold sea temperatures.

Fata Morgana not only produces mirror images, but can magnify objects that lie beyond the horizon. Ships can therefore be below the horizon but their reflected light is distorted to such an extent that they appear to be ‘sailing’ in the sky. This is the likely explanation of the Flying Dutchman – the ship literally appeared to be flying.

When you next find yourself enjoying a day on the high seas, do not panic at seeing sky-scraping ships in hazy air – it’s just Fata Morgana, and not your mind playing tricks. Although the sailors who spotted the Flying Dutchman were absolutely convinced ships could fly.

Original article on the Bureau of Meteorology website

Image: reflected ship