In the Turpan region of Xinjiang, in December 2020, there were successive mirages that resembled lakes, sand dunes, or buildings and tall buildings, and the water and sky met, which was very spectacular. Netizens exclaimed, is this the parallel world told in science fiction movies?
In fact, this is a simple optical phenomenon, which is the result of a combination of sunlight reflection and refraction.
Schematic diagram of the "Mirage on the Present"
The occurrence of mirages is mainly related to geographical location, landform, and weather. It is not very much related to the seasons, but in general winter it still happens less often. This is because the air flow must be thick above and thin underneath, like hanging a mirror in the sky, reflecting the distant scenery. Every year, the mirage in Penglai, China, appears in May or June; some places such as Long Island often appear in July and August. When it comes to seasonality, summer is the season when mirages are most likely.
But the Turpan region appeared in December, it is not easy to occur in the winter of mirages, why does Turpan have the grace alone?
To understand how this rare situation occurs, we must first understand the conditions under which mirages occur, and the abnormal distribution of temperatures is a necessary meteorological condition for most mirages.
Mirage, referred to as mirage, mirage can be divided into "upper mirage", "lower mirage" and "side mirage". It is a natural phenomenon formed by the refraction and total reflection of light, and it is a virtual image formed by the refraction of light reflected by objects on the earth through the atmosphere.
Schematic diagram of the "Next Mirage"
The simplest and easy to remember is that "the upper mirage" generally appears on the sea, and the fairyland is upright; the "lower mirage" is more likely to appear in the desert, and the fairyland is upside down. In addition, there are occasional "side mirages" in the desert, where the scene is reversed at 90° and falls across the horizon.
Take the "Mirage" as an example, when the summer desert sun is hot and the sand is scorched. The specific heat of the sand is small, the temperature rises very quickly, and the temperature of the lower air near the sand also rises, while the temperature of the upper air is still very low, thus forming an abnormal distribution of temperature. Due to thermal expansion and contraction, the density of hot air in the lower layer close to the sand is small and the density of the upper cold air is large, so that the refractive index of the air is small in the lower layer and large in the upper layer.
When the light reflected by the higher object in the distance is continuously refracted from the denser air in the upper layer into the thinner air in the lower layer, its angle of incidence gradually increases, and it is fully reflected when it is increased to an equal critical angle, at this time, if people look against the reflected light, they will see the mirage.
Turpan's mirage in December is also due to the large temperature changes. When a desert area undergoes a cooling process, the air temperature drops sharply, but after the cold air passes, direct sunlight can quickly increase the temperature of the desert surface. As a result, the near-surface air near the desert warms up rapidly and the density decreases, while the upper temperature has not yet had time to rise, and the temperature difference is formed, and the air density difference is also formed, which produces the meteorological conditions for the formation of a mirage.
In general, winter is still a season when mirages are not easy to appear, and the condition that there is a large temperature difference between the upper and lower air is often only satisfied in desert areas.
Author: Chen Shan, Meteorological Analyst and Meteorological Engineer of China Meteorological Administration.