3 - Mirages

Summary

Quite early in spring, on any warm cloudless day, this watermirage was visible … an appearance of lakelets or sheets of water looking as if ruffled by wind and shining like molten silver in the sun. The resemblance to water is increased when there are groves and buildings on the horizon, which look like dark blue islands or banks in the distance, while the cattle and horse feeding not far away from the spectator appear to be wading knee-or belly-deep in the brilliant water.

W.H. Hudson: Far away and long ago, J.M. Dent & Sons, 1939, p. 57

Atmospheric refraction

When looking at something, we not unnaturally assume that what we see is exactly as we see it and precisely where we see it. In almost all situations in which we find ourselves this assumption is perfectly justified because we are close enough to the things that we see for their light to reach us by travelling in a straight line to our eyes through a homogeneous atmosphere.

Over larger distances, however, the atmosphere is not homogeneous because its density is not uniform. As you might expect, the density of air decreases with height because the lower layers are compressed by the weight of those above.