Light does not travel in straight lines in the earth’s atmosphere. It bends according to the density of the air it is travelling through. Normally, the air near the earth is warmer than the air higher up, because air pressure is higher near the surface of the earth and air cools as it expands in the lower pressure of the atmosphere, higher up. This common situation results in standard refraction, where light is bent a normal amount. But when the air near the earth’s surface is unusually hot, such as in the desert or on a hot road, light bends more than normal towards the cooler air higher up. This has the effect of making the sky just above the horizon look like it is on the ground and our brains interpret this as water. Because distant objects appear lower than normal in these conditions, this is called an inferior mirage.
Diagram of light rays in standard refraction: Diagram of light rays in an inferior mirage:
These ray-bending diagrams are drawn by Dr. Andrew T. Young of San Diego State University. “Hobs” means “Height of Observer”.
The most common example of this is the hot road mirage, where there appears to be water on the road ahead, on a hot, dry day:
Hot road mirage © Physics Department, Warren Wilson College
Another classic example of this is the desert mirage, which appears like a lake of water in a dry desert, as can be seen in the following photograph by mirage photographer Ed Darack:
Most people are aware of this type of mirage, but what most people do not know is that the opposite occurs when the surface of the earth is unusually cold, such as over a very cold sea.
In this situation the air near the surface is colder than the air higher up. Because this is the opposite of the normal thermal situation in the earth’s atmosphere, this is called a thermal inversion. In this situation temperature rises with height and light bends towards the cooler, denser air near the surface, bending it down, around the curvature of the earth.
This is known as “looming”, which has the effect of making a distant object, such as the coast, appear higher – and therefore seem to be nearer – than it really is:
Exaggerated diagram of a looming coastline, seen from a ship.
But where the thermal inversion is so steep that the downward bending of light rays exceeds the curvature of the earth, light becomes trapped in what is known as a “duct”, where escaping light beams are continually bent back down, towards the earth, crossing and re-crossing each other as they go around the earth, trapped in the duct. This is known as a superior mirage, because here objects appear both higher than normal, but also distorted, with objects within the duct becoming inverted – and inverted again and again – each time the rays cross and re-cross each other:
Dr. Andrew T. Young Superior Mirage Diagram. Note the rays crossing within the grey “duct”. ‘Hobs’ means Height of observer.
The photograph below is a superior mirage of the coast around the small Swedish village of Höganäs, taken from a boat on the Danish coast, near Osterrenden Bridge. The land seen is therefore 100km away, but it has been lifted up above the horizon and distorted by the superior mirage:
Swedish coast seen at 100km range due to superior mirage © Captain Peter
Superior mirages are most often experienced in high latitudes and wherever the sea surface temperature is exceptionally low.
Titanic’s officers were well aware of abnormal refraction and the phenomenon of looming, as her Second Officer Charles Lightoller explained at the British Inquiry into the sinking:
“The man may, on a clear night, see the reflection of the light before it comes above the horizon. It may be the loom of the light and you see it sometimes sixty miles away.”
The opposite of Looming is known as ‘sinking’, where objects which would normally be seen above the horizon are hidden below it. And because light rays bend downwards at different rates, the apparent height of objects seen will differ according to the degree of bending of the light. The vertical stretching of an objects is known as ‘Towering’ and the vertical compression of objects is known as ‘stooping’, as demonstrated in the following series of photographs of the same lighthouse under different thermal conditions, by Pekka Parviainen:
Looming, Stooping and Miraging lighthouse © Pekka Parviainen
And because these light rays are bending down and reflected at different rates and points in the duct, they cross over each other, causing very distortive effects, such as can also be seen in this trio of photographs of the same ship, taken by mirage photographer Pekka Parviainen:
A distorting ship in a superior mirage. © Pekka Parviainen
See also this summary entry on my site.
I hope by blogging chapters from my book, A Very Deceiving Night, it will contribute to the ongoing discussions regarding the atmospheric conditions on the night of the tragedy and the true causes of the disaster. At the moment, the book is only available as an e-book. If you wish to purchase it then you can do so in Amazon Kindle format here and other formats, including Apple, Kobo and Nook, here. Thank you.