How is lightning formed？

The formation of lightning begins with the formation of thunderclouds. In fact, there are several clouds related to lightning, such as stratocumulus cloud, laminatus cloud, cumulus cloud, cumulonimbus cloud, and most importantly, cumulonimbus cloud, which is a thunderstorm cloud. Thundercloud is a huge, opaque and charged black cloud composed of water droplets, ice crystals and gas dust in the atmosphere. The fundamental cause of its formation is the air flow containing water vapor. With the continuous development of thunderclouds accumulation, will cause lightning, thunder phenomenon, this is thunderstorms.

1)Classification of thunderstorms

There are two main types of thunderstorms: frontal and ther。

Frontal thunderstorms are caused when two air masses flowing on the surface meet, the cold air mass flows under the hot air mass due to its high density, forming relative motion at the interface of the two and lifting the hot air mass up sharply, and the hot air forms a strong ascending column and vortex, thus forming a cumulus cloud. If the hot gas mass is hot enough and moist enough, it can form a large thunderstorm cloud.

Thermal thunderstorms occur in mountainous areas. Due to the sunlight, the temperature of the hill and its ground rises, the hot air flows to the sky because of the small density, the temperature of the nearby trees, lakes and rivers is low, and the surrounding relatively cold air flows to the higher temperature of the hill and the lower density of the area, at the same time, the air is heated by the high temperature of the hill surface and flows to the sky, so as to form a thermal thunderstorm.

2)Electrification mechanism of cumulonimbus cloud

There are three main theories of the electrification mechanism of cumulonimbus cloud:

①Water absorption charge effect. There is a downward electric field in the atmosphere, which causes the positive and negative ions of the air to move down and up respectively. Neutral water droplets are also polarized in the electric field, with negative charges at the upper end and positive charges at the lower end. When the big water drop falls, its lower end absorbs negative ions and repels positive ions. Because the big water drop falls fast, the negative charge at its upper end is too late to absorb the positive ions above it, so the whole water drop is negatively charged. The water drop is carried upward by the air flow, and the polarized negative charge on its upper end will absorb the positive ions, so the water drop is positively charged.

②The drip freezing effect. The experiment found that when water freezes, the ice has a positive charge, and the unfrozen water has a negative charge, so when the updraft in the ice crystal region of the cloud carries away the water above the ice particles, it causes the separation of charges and the different cloud regions are charged.

③Water drop breaking effect. By blowing water droplets in the air with a strong airflow, the larger residual droplets have a positive charge, and the smaller droplets have a negative charge, which is because there are many electrons on the surface of the droplets.

3)Thundercloud discharge mechanism

Because of the uneven distribution of charge in the cloud, many charge centers are formed, so the electric field strength is different between the clouds, within the clouds, and between the clouds to the earth. Only when the cloud-to-earth field intensity is the highest and reaches a certain value can the ground discharge occur. Similarly, intercloud discharge occurs when the electric field intensity between clouds reaches a certain critical value. In fact, the vast majority of discharges occur between or within clouds.

The mechanism of lightning cloud discharge to the ground: the cloud with a large amount of charge generates electrostatic induction to the earth, and the earth induces a large number of heterosexual charges, so that a strong field strength is formed between the thunder cloud and the earth, when the electric field strength at a certain place reaches 25 ~ 30kV/cm, the lead discharge will be generated from the thunder cloud to the earth (in a few cases, the lightning leader is emitted from the surface upward). When the leader reaches the ground or meets the ground leader, a lightning strike is generated by forming a strong discharge through charge neutralization. The discharge usually occurs more than once, with a large current in the first strike and a much smaller current in subsequent lightning strikes.