How is a typhoon formed?

There are often many weak tropical eddies on the surface of tropical oceans, which we call "embryos" of typhoons, because typhoons always develop from such weak tropical eddies. Through meteorological satellites, it is found that only about 10% of a large number of tropical eddies on the ocean surface can develop into typhoons. How is a typhoon formed?

Generally speaking, the occurrence of typhoons requires the following basic conditions:

A. First of all, there must be a wide enough tropical ocean surface, which requires not only that the surface temperature of seawater is higher than 26.5℃, but also that the water temperature in a layer of seawater 60 meters deep should exceed this value. Among them, the vast ocean surface is the necessary natural environment for the formation of typhoons. Because of the friction between air molecules in typhoon, the daily average energy consumption is 3 100-4000 calories/cm2. This huge energy can only be supplied by the latent heat released by the vast tropical ocean. In addition, the strong wind around the tropical cyclone will make the sea water churn around the center, even in the typhoon center where the air pressure drops very low, and then spread around, so the sea water will churn around from the typhoon center. In a typhoon, this sea churning phenomenon can affect the depth of 60 meters. On the ocean surface where the seawater temperature is lower than 26.5℃, it is difficult to maintain a typhoon due to insufficient heat energy. In order to ensure that the sea surface temperature is always above 26.5℃ during this tumbling process, this warm water layer must have a thickness of about 60 meters.

B. Before the typhoon forms, there must be a weak tropical vortex in advance. We know that the operation of any machine consumes energy, which requires energy. Typhoon is also a "heat engine". It rotates there at such a huge scale and speed and consumes a lot of energy, so it needs an energy source. The typhoon's energy comes from the water vapor in the tropical ocean. In a pre-existing tropical vortex, the air pressure in the vortex is lower than that around it, and the air around it carries a lot of water vapor to the center of the vortex, causing upward movement in the vortex area; Wet air rises and water vapor condenses, releasing huge latent heat of condensation, which can make typhoons run. Therefore, even if there is a tropical ocean surface with high temperature and high humidity to supply water vapor, it is impossible to form a typhoon without the strong rise of air and the condensation and release of latent heat. Therefore, the upward movement of air is an important factor to produce and maintain typhoons. But the necessary condition is that a weak tropical vortex exists first.

C. there should be enough earth rotation deflection force. Because the geostrophic deflection at the equator is zero and gradually increases to the poles, the typhoon is located about 5 latitudes away from the equator. Due to the rotation of the earth, a force that changes the direction of air flow is generated, which is called "Earth rotation bias force". On the rotating earth, the rotation of the earth makes it difficult for the surrounding air to flow directly into the low pressure, but rotates counterclockwise along the center of the low pressure (in the northern hemisphere).

D above weak low pressure, the difference between high and low air direction and wind speed is small. In this case, the upper and lower air columns work together, and the high-altitude heat is easy to accumulate, thus heating up. Once the cyclone is generated, the ambient airflow above the friction layer will flow along the isobar, and the warming effect of the upper layer can be further completed. In the area north of 20 N, the climatic conditions have changed, mainly because the high-rise wind is very strong, which is not conducive to warming, and typhoons are not easy to appear.

These are just the necessary conditions for a typhoon. Having these conditions does not mean that there will be a typhoon. The occurrence of typhoon is a complicated process, which has not been fully understood so far.

Typhoon is actually a strong tropical cyclone. Tropical cyclone is a strong weather system that occurs in tropical ocean. Like a vortex moving forward in a flowing river, it rotates rapidly around the center and moves forward with the surrounding atmosphere. Like temperate cyclones, the airflow in tropical cyclones in the northern hemisphere rotates counterclockwise around the center, while in the southern hemisphere it is the opposite. The closer to the tropical cyclone center, the lower the air pressure and the stronger the wind. However, tropical cyclones with strong development, such as typhoons, have a calm and clear sky area at their center, that is, the typhoon eye.

The intensity of tropical cyclones occurring in tropical oceans varies greatly. Before 1989, the tropical cyclone with the maximum wind force of 8 or above near the center was called typhoon in China, and the tropical cyclone with the maximum wind force of 12 near the center was called strong typhoon. Since 1989, China has also adopted international classification standards, that is, when the maximum wind force near the center of a tropical cyclone is less than 8, it is called a tropical depression, and the winds of 8 and 9 are called tropical storms, and the winds of 10 and1are strong tropical storms. Only the tropical cyclone with the strongest wind near the center is called a typhoon.

From the above definition, it is not difficult to see that tropical cyclone is a general term for tropical depression, tropical storm, strong tropical storm and typhoon. However, due to the weak destructive power of tropical depression, traditional tropical cyclones generally do not include tropical depression.

The generation and development of tropical cyclones need huge energy, so they are formed on the tropical ocean surface with suitable meteorological conditions such as high temperature and high humidity. According to statistics, tropical cyclones are generated in tropical oceans all over the world except the South Atlantic.

Most tropical depressions cannot develop into tropical storms, and only a certain number of tropical storms can develop into typhoon intensity. The intensity of typhoons also varies greatly. Some typhoons have a maximum wind speed of 35 m/s near the strong wind center, but typhoons with a maximum wind speed of more than 50 m/s near the center are not uncommon. For example, Typhoon 94 17 landed in Ruian, Zhejiang Province, and the maximum wind speed near its center reached 45 m/s when it landed.

The life history of tropical cyclones can be divided into three stages: generation, maturity and extinction. Its life span can generally reach more than one week, and some tropical cyclones can survive for more than two weeks under favorable external environment. When tropical cyclones land or move northward to higher latitudes, they will soon become extinct because they lose the high temperature and high humidity conditions on which they depend.

Tropical cyclone disaster is the most serious natural disaster, because its frequency is much higher than that of earthquake disaster, so its cumulative loss is also higher than that of earthquake disaster. 1991The tropical cyclone that landed in Bangladesh at the end of April claimed139,000 lives. China is one of the countries that suffer the most tropical cyclones in the world. In recent years, the average annual loss is more than 100 billion yuan, and the loss caused by a strong tropical cyclone like typhoon 94 17 is more than 100 billion yuan.

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