What is a mirage in summer?

In calm seas, rivers, lakes, snowfields, deserts or Gobi, occasionally towering towers, battlements and trees appear in the air or "underground", which is called a mirage. This illusion often appears on the sea surface of Penglai, China. The ancients attributed it to the dragon, and exhaled to form towers and castles, hence the name.

In summer, the humidity of seawater is relatively low during the day, especially on the sea surface where cold water flows, and the water temperature is even lower. The low-level air is affected by the water temperature, which is colder than the high-level air, resulting in the abnormal phenomenon of cooling and heating (under normal circumstances, the average temperature drops by about 0.6℃ for every rise of 100m). Because the air pressure in the lower air layer is very high, and now the temperature is lower than that in the upper air layer, the difference between density and rareness under the air layer is extremely significant.

If there is a ship under our eastern horizon, it is usually invisible. If the difference between dense air and thin air is too big at this time, the light from the ship will gradually refract from the dense air layer to the thin air layer, and then return to the dense air layer below after total reflection in the upper layer; After such a curve, it finally enters our eyes and we can see its image. Because people's vision always feels that objects come from a straight line, the ship image we see is much higher than the real thing, so it is called a mirage.

There is a miaodao islands in Bohai Sea, China. In summer, when the sea water temperature is low during the day, the air density will be obviously different. In Penglai County (called Dengzhou in ancient times) on the south bank of Bohai Sea, you can often see the phantom of miaodao islands. Shen Kuo in the Song Dynasty, in his masterpiece Meng Qian Bi Tan, has such a record:

"When you are in Zhangzhou Sea, there are clouds, such as watching the palace, the people in the city, and the crown of horses and chariots."

This is a mirage he saw in Penglai. 1In the early morning of May 22, 933 1 1 point, a mirage was also found on Zhucha Island in Qianhai, Qingdao (the outer mouth of Jiaozhou Bay), which spread all over the city for a time and many people went to watch it. 1975, a 6-hour mirage appeared in the sea near Guangdong province.

Mirages can be seen not only on the sea in summer, but also on the river sometimes. For example, 1934 appeared on the river near Nantong on August 2. It was a cool day and it was particularly hot. In the afternoon, I suddenly found towers, battlements and tree houses on the Yangtze River, all more than 20 miles long. About half an hour later, it moved eastward and suddenly disappeared. Then three mountains appeared, towering into the sky, with a mountain in the middle, much like a censer; It took half an hour to disappear.

In the desert, the sand is scorched by the sun during the day, and the temperature near the sand layer rises very quickly. Because air is not good at heat transfer, when there is no wind, the heat exchange between the upper and lower air is very small, which makes the vertical difference between cold and hot temperatures very significant, leading to the abnormal phenomenon that the density of the lower air is less than that of the upper air. In this case, if there is a tree in front of it and it grows in a relatively humid place, the light obliquely projected downward from the top of the tree will be refracted because it enters the air layer with high density. When the refracted light reaches the hot and thin air layer near the ground, total reflection occurs, and the light is reflected from the low-density air layer near the ground back to the higher-density air layer above. In this way, after a downward concave arc light, the image of the tree is sent to people's eyes and the reflection of the tree appears.

Because the reflection is below the real thing, it is also called a mirage. This kind of reflection can easily give people the illusion that there are trees by the water, thinking that there must be a lake in the distance. Most people who have traveled in the desert have had similar experiences. A photographer who filmed the film "Climbing Mount Everest in Hishapang" saw such a mirage while walking on a vast arid grassland. He ran in the direction of a mirage, trying to fetch water for cooking. When he ran there and saw that there was no water, he found himself cheated by a mirage. This is because hay, like sand, can be heated by the scorching sun, which gradually increases the density of the air layer from bottom to top, thus creating a mirage.

No matter what kind of mirage, it can only appear in windless or extremely weak weather conditions. When strong winds combine, which causes the agitation and mixing of the upper and lower air, the air density difference between the upper and lower air decreases, and there is no abnormal refraction and total reflection of light, then all illusions disappear immediately.