The fascinating science of Oases and Mirages

Q: In a desert, from how much distance can Mirage or Oasis be differentiated?

Krishna:  There is a difference between an oasis and a mirage. 

A mirage is a trick of the light while an oasis is real. An oasis is an actual water presence in the desert. A mirage is merely the appearance of water in the desert. 

In geography, an oasis is a fertile land in a desert or semi-desert environment. Oases also provide habitats for animals and plants. Surface water may be present, or water may only be accessible from wells or underground channels created by humans.Oases develop in “hydrologically favoured” locations that have attributes such as a high water table, seasonal lakes, or blockaded valleys. Stereotypically, an oasis has a “central pool of open water surrounded by a ring of water-dependent shrubs and trees…which are in turn encircled by an outlying transition zone to desert plants.

Oasis-image source: dreamstime.com

Oases are made when sources of freshwater, such as underground rivers or aquifers, irrigate the surface naturally or via man-made wells.Underground water sources called aquifers supply most oases. In some cases, a natural spring brings the underground water to the surface. At other oases, manmade wells tap the aquifer. In some oasis settlements, these wells might be centuries old and might have been diligently maintained for generations to preserve access to their life-giving water.

The location of oases has been of critical importance for trade and transportation routes in desert areas; caravans must travel via oases so that supplies of water and food can be replenished.Rivers that flow through some deserts provide permanent sources of water for large, elongated oases. They vary in size from a cluster of date palms around a well or a spring to a city and its irrigated cropland. Dates, cotton, olives, figs, citrus fruits, wheat and corn (maize) are common oasis crops.

So if an oasis is very big, you can locate it from a great distance. If it is small, you have to go near it. That depends on the size of the oasis!

The mirage is an optical phenomenon in which light is refracted through a layer of hot air close to the ground, giving the appearance of there being refuge in the distance .The phenomenon which causes the formation of mirage is known as total internal reflection.
Mirages are most common in deserts. They happen when light passes through two layers of air with different temperatures. The desert sun heats the sand, which in turn heats the air just above it. The hot air bends light rays and reflects the sky.When you see it from a distance, the different air masses colliding with each other act like a mirror. The desert ahead seems to have become a lake but it is actually a reflection of the sky above.Mirages can be seen almost anywhere – those shimmering heat hazes that appear on the road ahead of you on sunny days, but disappear as you approach are mirages.In the countryside you may seem to see a small lake or pond near trees or in a field, this is also created by differences in temperature between the ground temperature and the air just above ground level.

In a normal mirage, a ray originating in the sky is refracted sufficiently to arrive at your eye from "below" so that it appear to come from the ground.

Because air has a refractive index very close to unity, it cannot support a substantial index gradient, and therefore forms only a very weak lens. So the light that forms the mirage is always nearly horizontal (near grazing to horizontal.) For that reason a mirage appears only at substantial distance from us. The height of our eyes must be small compared to the distance to the mirage.

Now, if we put your eye at a height h and look to see if a mirage appears at a distance d in front of you that light ray has to refract through an angle slightly more than θ=arctan⁡hd. We you look at a close spot (or move toward the "location" of the mirage, d gets smaller and theta gets larger. For any given temperature gradient there is a maximum angle at which the ray still originates from the sky and therefore a minimum distance at which you still see the mirage.

The angle subtended by the light rays emanating from the mirage determines the distance at which an observer begins to see it and at which he no longer sees the mirage. The observer's height is another factor. A taller person is more likely to see it before a shorter person as they approach it from the same distance, and a shorter person is more likely to continue seeing a mirage as he gets closer to it than a taller person. Thus the angle of depression and perspective is key.

Mirage, in optics, the deceptive appearance of a distant object or objects caused by the bending of light rays (refraction ) in layers of air of varying density.

Under certain conditions, such as over a stretch of pavement or desert air heated by intense sunshine, the air rapidly cools with elevation and therefore increases in density and refractive power. Sunlight reflected downward from the upper portion of an object—for example, the top of a camel in the desert—will be directed through the cool air in the normal way. Although the light would not be seen ordinarily because of the angle, it curves upward after it enters the rarefied hot air

near the ground, thus being refracted to the observer’s eye as though it originated below the heated surface. A direct image of the camel is seen also because some of the reflected rays enter the eye in a straight line without being refracted. The double image seems to be that of the camel and its upside-down reflection in water. When the sky is the object of the mirage, the land is mistaken for a lake or sheet of water.

Sometimes, as over a body of water, a cool, dense layer of air underlies a heated layer. An opposite phenomenon will then prevail, in which light rays will reach the eye that were originally directed above the line of sight. Thus, an object ordinarily out of view, like a boat below the horizon, will be apparently lifted into the sky. This phenomenon is called looming.

Mirages can be categorized as "inferior" (meaning lower), "superior" (meaning higher) and "Fata Morgana", one kind of superior mirage consisting of a series of unusually elaborate, vertically stacked images, which form one rapidly-changing mirage.In contrast to a hallucination, a mirage is a real optical phenomenon that can be captured on camera, since light rays are actually refracted to form the false image at the observer's location. What the image appears to represent, however, is determined by the interpretive faculties of the human mind. For example, inferior images on land are very easily mistaken for the reflections from a small body of water.

In an inferior mirage, the mirage image appears below the real object. The real object in an inferior mirage is the (blue) sky or any distant (therefore bluish) object in that same direction. The mirage causes the observer to see a bright and bluish patch on the ground.

Heat haze, also called heat shimmer, refers to the inferior mirage observed when viewing objects through a mass of heated air. Common instances when heat haze occurs include images of objects viewed across asphalt concrete (also known as tarmac) roads and over masonry rooftops on hot days, above and behind fire (as in burning candles, heaters, and campfires), and through exhaust gases from jet engines.

A superior mirage is one in which the mirage image appears to be located above the real object. A superior mirage occurs when the air below the line of sight is colder than the air above it. This unusual arrangement is called a temperature inversion, since warm air above cold air is the opposite of the normal temperature gradient of the atmosphere during the daytime. Passing through the temperature inversion, the light rays are bent down, and so the image appears above the true object, hence the name superior.

A Fata Morgana  is a very complex superior mirage. It appears with alternations of compressed and stretched areas, erect images, and inverted images.  A Fata Morgana is also a fast-changing mirage.

The conditions for producing a mirage can occur at night as well as during the day. Under some circumstances mirages of astronomical objects and mirages of lights from moving vehicles, aircraft, ships, buildings, etc. can be observed at night.

A mirage of an astronomical object is a naturally occurring optical phenomenon in which light rays are bent to produce distorted or multiple images of an astronomical object. Mirages can be observed for such astronomical objects as the Sun, the Moon, the planets, bright stars, and very bright comets. The most commonly observed are sunset and sunrise mirages.

Sources: 

Science Art Lab, physics.stackexchange.com, wikipedia, britannica.com, and Quora

Qs based on this:

Q: Thanks for your good and elaborate answer madam. Suppose you are caught in the desert in the afternoon of the summer and you are observing that nearby something like water body is there. Then how can you find out what you are seeing is Mirage or Oasis? You are thirsty and your energy is draining out. Suppose to your right side it is oasis and a Mirage to your left side. You will live if it is an oasis and die if it is a Mirage. This is the situation. How to deal with the situation?

Krishna: My  answer has a very clear message: Oases have plants and trees growing around them! Life is a sure sign of having water.

Oases can be fairly easy to spot—at least in deserts that do not have towering sand dunes. In many cases, the oasis will be the only place where trees such as date palms grow for miles around.

The desert oasis is home to many different types of insects, aquatic invertebrates, and their larvae, including giant water bugs, back swimmers and dragonflies. Birds such as Cooper's hawks, sparrows, mourning doves, finches and quail use this habitat to nest and forage.

If you see birds and animals around a water body, it is a sure sign of oasis.

Moreover oases don’t disappear when you approach them.

Q: What is a cold water mirage?

Krishna: The 'cold weather mirage' occurs when a cold weather front collides with warmer air and causes light passing between the boundary of the two to be bent dramatically, distorting how an object appears. 

People say this is one of the reasons why The Titanic hit the ice berg and sank: the superior mirage at Titanic's crash site had the effect of appearing to raise up the horizon behind the iceberg, partially masking it from view, until it was too late. The low contrast between the haze on the horizon and the nearby iceberg caused it to appear later than it otherwise would.