Conditions for the appearance of cyclones and anticyclones

Cyclone and anticyclone are common weather systems, and their activities have great influence on the heat exchange between high and low latitudes and the weather changes in various places. The characteristic cyclone of (1) gas cyclone and anticyclone is a horizontal vortex with the central pressure lower than the surrounding pressure. In the northern hemisphere, the air in the cyclone area flows counterclockwise, but in the southern hemisphere, it is the opposite; An anticyclone is a horizontal vortex with a high central pressure and a low surrounding pressure. In the northern hemisphere, the air in the anticyclone area flows clockwise, but in the southern hemisphere, it is the opposite. Cyclones and anticyclones are also commonly called low pressure and high pressure. In the lower atmosphere, especially near the ground, the wind direction is oblique isobar, so the cyclone in the northern hemisphere is an airflow system that rotates counterclockwise and converges to the center; In the southern hemisphere, it is an airflow system that rotates clockwise and converges to the center. Because the airflow meets in the center of the cyclone from all directions, it will inevitably produce upward movement, and the airflow will rise to the upper air and flow out around, thus ensuring that the lower atmosphere will continue to flow in from the periphery to the center, and the cyclone can exist and develop. Therefore, the existence and development of cyclones must have a circulation system consisting of horizontal motion and vertical motion. Because there is a vertical updraft in the center of the cyclone, it is easy to produce rainy weather if the water vapor content in the atmosphere is high. So whenever the low pressure (or cyclone) moves to this area, the cloud cover will increase, and even rainy weather will occur. In the low-pressure atmosphere, especially near the ground, because the anticyclone airflow flows outward from the center. Therefore, there must be downward airflow in the center of the anticyclone to supplement the air flowing around. Otherwise, anticyclone cannot exist and develop. Therefore, the existence and development of anticyclone must have a complete circulation system which is closely combined with vertical movement and horizontal movement. Because the center of anticyclone is downdraft, it is not conducive to the formation of clouds and rain. Therefore, the weather under the control of anticyclone is generally clear and cloudless. If it is in summer, the weather is hot and dry. If the anticyclone is stable and inactive for a long time, drought will often occur. The summer drought in the Yangtze River basin in China is caused by the long-term control of subtropical high. In winter, anticyclone comes from high latitude continent, which often brings dry and cold air flow, and can turn into cold current in severe cases. The diameter of a cyclone is generally 1000 km, the larger one can reach 2000-3000 km, and the smaller one is only 200-300 km or less. The anticyclone can be compared with the largest continent and ocean (such as the anticyclone in Asia in winter, which often occupies 3/4 of the whole Asian continent), and the diameter can reach hundreds of kilometers. (2) The intensity of cyclone and anticyclone is different. Their strength can be measured by their maximum wind speed: the maximum wind speed is strong and the maximum wind speed is small. In a strong cyclone, the maximum wind speed on the ground can reach more than 30 meters per second. In a strong anticyclone, the maximum wind speed on the ground is 20-30m/s. The central pressure values of cyclones and anticyclones are often used to express their intensity. The central pressure of surface cyclones is generally 10 10-970 mbar, and some of them are below 930 mbar. The central pressure of surface anticyclone is generally 1020-l030 mbar, and the highest cold wave pressure in winter is above 1078.9 mbar. (3) Classification of Cyclones and Anticyclones There are many methods to classify cyclones and anticyclones. According to its geographical location, cyclones can be divided into temperate cyclones and tropical cyclones. The anticyclone can be divided into temperate anticyclone, subtropical anticyclone and polar anticyclone. According to the different structures, temperate cyclones can be divided into frontal cyclones and non-frontal cyclones; The anticyclone can be divided into cold anticyclone (or cold high pressure) and warm anticyclone (or warm high pressure). There is no insurmountable gap between cyclones. Different types of cyclones and anticyclones; Under certain conditions, they will transform into each other. For example, frontal cyclone can be transformed into frontal cyclone (cold vortex) under certain conditions, and frontal cyclone (hot low pressure) can be transformed into frontal cyclone under certain conditions; Cold anticyclone can also be transformed into warm anticyclone. Cyclone and anticyclone should be regarded as conditional, variable and mutually transformed.