The condition of lightning is that there is accumulation and polarity in thunderstorm clouds. Scientists have made a lot of observations and experiments on the charging mechanism and the regular distribution of electric charges in thunderstorm clouds, accumulated a lot of data and put forward various explanations, some of which are still controversial. 1. Hypothesis of "ion flow" in the initial stage of convective clouds. There are a lot of positive ions and negative ions in the atmosphere. On the raindrops in the cloud, the charge distribution is uneven, the outermost molecules are negatively charged and the inner layer is positively charged. The potential difference between the inner layer and the outer layer is about 0.25V higher. In order to balance this potential difference, water droplets must preferentially absorb negative ions in the atmosphere, which gradually makes water droplets negatively charged. When convection begins to develop, lighter positive ions are gradually carried to the upper part of the cloud by the rising airflow; However, the negatively charged cloud droplets are relatively heavy and stay at the lower part, resulting in the separation of positive and negative charges. 2. Charge accumulation in cold clouds When the convection reaches a certain stage and the cloud reaches a height above 0℃, there are supercooled water droplets, graupel particles and ice crystals in the cloud. This cloud, which is composed of water vapor condensate with different phases and the temperature is lower than 0℃, is called Leng Yun. There are several processes of charge formation and accumulation in Leng Yun: ① supercooled water droplets collide with graupel particles to generate electricity. When there are many clouds, the temperature is below 0℃, and many water droplets will not freeze. This kind of water drop is called supercooled water drop. Supercooled water droplets are unstable. With a slight shake, they will immediately freeze and weigh the ice particles. When supercooled water droplets collide with graupel particles, they will freeze immediately, which is called collision freezing. When the collision occurs, the outer part of the supercooled water droplets immediately freezes into an ice shell, but the inner part remains liquid for a period of time. Because the latent heat released by the external freezing is transferred to the inner part, the temperature of the inner liquid supercooled water is higher than that of the outer ice shell. The temperature difference makes the frozen supercooled water droplets positively charged outside and negatively charged inside. When the interior is also frozen, the cloud droplets expand and split, and the outer skin breaks into many positively charged ice chips, which fly to the upper part of the cloud with the airflow. The negatively charged frozen cloud droplets attach to the heavier graupel particles, making the graupel particles negatively charged and stay in the middle and lower parts of the cloud. (2) The particles are charged by the friction collision between ice crystals and graupel particles. Polonium particles are composed of frozen water droplets, which are white or milky white and have a brittle structure. Because cold water droplets often collide with it and release latent heat, its temperature is generally higher than that of ice crystals. Ice crystals contain a certain amount of free ions (OH- and H+), and the number of ions increases with the increase of temperature. Because of the temperature difference between the contact part of graupel and the ice crystal, the free ions at the high temperature end must be more than those at the low temperature end, so the ions must migrate from the high temperature end to the low temperature end. In the process of ion migration, positively charged hydrogen ions are faster, while negatively charged heavier hydroxide ions are slower. Therefore, in a certain period of time, the phenomenon of excess hydrogen ions at the cold end appeared, which led to the negative polarization at the high temperature and the positive polarization at the low temperature. When ice crystals come into contact with graupel particles and separate, the graupel particles with higher temperature are negatively charged, while the ice crystals with lower temperature are positively charged. Under the action of gravity and updraft, the lighter positively charged ice crystals are concentrated in the upper part of the cloud, while the heavier negatively charged graupel particles stay in the lower part of the cloud, resulting in the Leng Yun being positively charged in the upper part and negatively charged in the lower part. (3) Water droplets are charged by dilute salt, which leads to the above two charging mechanisms of Leng Yun. Others think that water droplets in the atmosphere are charged by thin salt. When the cloud drops freeze, the crystal lattice of ice can accommodate negative chloride ions, but repel positive sodium ions. Therefore, the frozen part of water droplets is negatively charged, and the unfrozen part is positively charged (water droplets are frozen from the inside out). In the process of falling, the graupel particles frozen by water drops fall off from the surface water before freezing, forming many positively charged clouds, while the frozen core is negatively charged. Due to the separation of gravity and airflow, positively charged droplets are carried to the upper part of the cloud, while negatively charged particles stay in the middle and lower part of the cloud. 3. The charge of warm clouds accumulates in tropical areas, and some clouds are above 0℃. So it only contains water droplets and no solid water particles. This kind of cloud is called warm cloud or water cloud. There will be lightning in warm clouds. In the thunderstorm clouds in the mid-latitude region, the part of the cloud below the 0℃ isotherm is the warm region of the cloud. There is also an electrification process in the warm area of the cloud. During the development of thunderstorm clouds, the counting mechanism plays a role in different development stages. However, the main charging mechanism is still caused by the freezing of water droplets. A large number of observation facts show that only when the cloud top presents a fibrous and filamentous structure, the cloud develops into a thunderstorm cloud. Aircraft observation shows that there are a large number of cloud particles mainly composed of ice, snow crystals and graupel particles in thunderstorm clouds, and the accumulation of a large number of charges, that is, the rapid charging mechanism of thunderstorm clouds, must rely on the collision, freezing and friction during the growth of graupel particles.