Looming and similar refraction phenomena

Looming

Looming is the most noticeable and most often observed of these refraction phenomena. It is an abnormally large refraction of the object that increases the apparent elevation of the distant objects and sometimes allows an observer to see objects that are located below the horizon under normal conditions. One of the most famous looming observations was made by William Latham in 1798, who wrote:

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.

Thomas Jefferson noted the phenomenon of looming in his book Notes on the State of Virginia:

There is a solitary mountain about 40 miles off, in the South, whose natural shape as presented to view there, is a regular cone; but, by the effect of looming it sometimes subsides almost totally into the horizon; sometimes it rises more acute and more elevated, its top flat, and as broad as its base. In short it assumes at times the most whimsical shapes, and all these perhaps successively in the same morning.

He was unable to explain this phenomenon and did not think refraction could account from the perceived changes of shape of the object in question.

Other famous observations that were called "mirages" may actually be referring to looming. One of those was described in Scientific American on August 25, 1894, as "a remarkable mirage seen by the citizens of Buffalo, New York". Such looming—sometimes with apparent magnification of opposite shores—have been reported over the Great Lakes. Canadian shorelines have been observed from Rochester, New York, across Lake Ontario, and from Cleveland across Lake Erie. The landforms over 50 miles (80 km) distant, normally beyond the horizon, were sometimes perceived as 6 miles (10 km) away.

Looming is most commonly seen in the polar regions. Looming was sometimes responsible for the errors made by polar explorers; for example, Charles Wilkes charted the coast of Antarctica, where later only water was found.

The larger the size of the sphere (the planet where an observer is located) the less curved the horizon is. William Jackson Humphreys' calculations showed that an observer may be able to see all the way around a planet of sufficient size and with sufficient atmospheric density gradient.