Looming is defined as “the appearance above the horizon of a distant object that would normally be hidden below it.” This effect is caused by unusually large terrestrial refraction—usually caused by a thermal inversion.
Old looming or light refraction accounts are just one of the many interesting subjects—ancient and more modern—discussed in the lengthy endnotes of The Electric Mirror on the Pharos Lighthouse and Other Ancient Lighting. Here is just one example: Writing in 1857, Dr. Thomas Dick, in his voluminous work on astronomy and other curious subjects, under his topic titled “Extraordinary Cases of Refraction in Relation to Terrestrial Objects,” points out that "In consequence of the accidental condensation of certain strata of the atmosphere, some very singular effects have been produced in the apparent elevation of terrestrial objects to a position much beyond that in which they usually appear. The following instances are worthy of notice. It is taken from the Philosophical Transactions of London for 1798, and was communicated by W. Latham, from Hastings, on the south coast of England: 'On July 26, 1797, about five o’clock in the afternoon, as I was sitting in my dining-room in this place, which is situated upon the parade, close to the seashore, nearly fronting the south, my attention was excited by a number of people running down to the seaside. Upon inquiring the reason, I was informed that the coast of France was plainly to be distinguished by the naked eye. I immediately went down to the shore, and was surprised to find that, even without the assistance of a telescope, I could very plainly see the cliffs on the opposite coast, which, at the nearest part, are between forty and fifty miles distant, and are not to be discerned from that low situation by the aid of the best glasses. They appeared to be only a few miles off, and seemed to extend for some leagues along the coast. I pursued my walk along the shore eastward, close to the water’s edge, conversing with the sailors and fishermen upon the subject. They at first would not be persuaded of the reality of the appearance; but they soon became so thoroughly convinced by the cliffs gradually appearing more elevated, and approaching nearer, as it were, that they pointed out and named to me the different places they had been accustomed to visit, such as the Bay, the Old Head, or Man, the Windmill, &c., at Boulogne, St. Vallery, and other places on the coast of Picardy, which they afterward confirmed, when they viewed them through their telescopes. Their observations were, that the places appeared as near as if they were sailing, at a small distance, into the harbors. The day on which this phenomenon was seen was extremely hot; it was high water at Hastings about two o’clock P.M., and not a breath of wind was stirring the whole day.'
"From the summit of an adjacent hill, a most beautiful scene is said to have presented itself. At one glance the spectators could see Dungeness, Dover Cliffs, and the French coast, all along from Calais to St. Vallery, and, as some affirmed, as far to the westward as Dieppe, which could not be less than eighty or ninety miles. By the telescope the French fishing boats were plainly seen at anchor, and the different colors of the land on the heights, with the buildings, were perfectly discernible.
"This singular phenomenon was doubtless occasioned by an extraordinary refraction, produced by an unusual expansion or condensation of the lower strata of the atmosphere, arising from circumstances connected with the extreme heat of the season. The objects seem to have been apparently raised far above their natural positions; for, from the beach at Hastings, a straight line, drawn across towards the French coast, would have been intercepted by the curve of the waters. They seem, also, to have been magnified by the refraction, and brought, apparently, four or five times nearer the eye, than in the ordinary state of the atmosphere.
"The following is likewise an instance of unusual refraction: When Captain Colby was ranging over the coast of Caithness, with the telescope of his great Theodolite, on the 21st of June, 1819, at eight o’clock, P.M., from Corryhabbie Hill, near Mortlich, in Banffshire, he observed a brig over the land of Caithness, sailing to the westward in the Pentland Firth, between the Dunnet and Duncansby heads. Having satisfied himself as to the fact, he requested his assistants, Lieutenants Robe and Dawson, to look through the telescope, which they immediately did, and observed the brig likewise. It was very distinctly visible for several minutes, while the party continued to look at it, and to satisfy themselves as to its position. The brig could not have been less than from ninety to one hundred miles distant; and, as the station on Corryhabbie is not above 850 yards above the sea, the phenomenon is interesting. The thermometer was at 44 degrees. The night and day preceding the sight of the brig had been continually rainy and misty, and it was not till seven o’clock of the evening of the 21st that the clouds cleared off the hill. [Edinburgh Philosophical Journal for October, 1819, p. 411]."
“At Mauritius a pilot once saw a vessel when it was actually 200 miles from him,” wrote Dr. John A. Timm, a Yale Chemistry professor, “and on another occasion the Bombay packet was sighted from Aden when 200 miles off.”
In his Gleanings from the Harvest-Fields of Literature, Science, and Art, with no mention of “looming” as an aid, the fertile researcher C. C. Bombaugh reported that
“It is certain that, under some circumstances, objects may be seen at a much greater distance than is generally supposed. For example, it is said that the Isle of Man is clearly visible form the summit of Ben Lomond, in Scotland, which cannot be less that a direct distance of one hundred and twenty miles. The peaks of the White Mountains in New Hampshire, which are about the same distance from Boston, may be seen when the atmosphere is clear, from the top of Bunker Hill Monument. Glas, in his History of the Canary Islands, affirms that the Peak of Teneriffe is visible at a distance of one hundred and twenty miles in approaching it, and of one hundred and fifty in leaving it; and Brydone, if we recollect rightly, says that from the summit of Mount Aetna mountains two hundred miles off may be distinguished. But the most extraordinary fact of the kind we have met with is to be found in the Encyclopaedia Britannica, article London, where we are told that the illumination of the atmosphere by the great fire of London was visible at Jedburgh, in Scotland, three hundred and seventy-three miles distant!”
Normally, on a clear day, a person standing on level ground can only see an object at the same height on the same type of (extended) terrain a few miles away. The chart above, copied from Radka's Electric Mirror, from its collection of hundreds of illustrations and photographs, indicates the distance an object may be seen at a certain height of both observer and object like a lighthouse beacon—at sea. Obviously, no consideration is given here of any effects of weather or "looming."—an outstanding type of light refraction.