Torrential rain comes into being in a rather complicated way, Generally based upon the macrophysics, the main physical condition for torrential rain is the a steady flow of adequate aqueous vapour, strong and continuous upward movement of the air and the instability of the atmospheric structure.
Good combination of weather systems in various size, underlying surfaces and terrains may cause torrential rain. Main weather systems that give rise to torrential rain in China are front, cyclone, shear line, vortex, trough, typhoon, easterly wave, Intertropical Convergence Zone and so on.
Besides, the thermal thunderstorms among arid and semiarid areas can contribute to transitory torrential rain with small coverage.
Torrential rain often drops from cumulonimbus clouds. The clouds need sufficient steam in the air and intense upward movement pushing the steam up. When the water drops in the clouds are more and more affected by this movement and can not be hold by the rising airflow anymore, they will rapidly drop onto the ground.
Other scientists suggest that the soil erosion along with the destruction of forests is one of the most important reasons for torrential rain. Because forest deterioration will increase the amount of the water that flows out the catchment areas in forest when raining.
There is also someone holding a view that the El Nino Phenomenon and La Nina Phenomenon may cause abnormal weather with the result of mass torrential rain.
El Nino is a warm current from north to south along the Coast of Peru in South America after Christmas. Generally speaking, it may bring torrential rain to the eastern tropical Pacific Ocean, where there is not much rainfall at ordinary times. On the contrary, decrease is seen on the rainfall in western tropical Pacific Ocean, where it is supposed to be rainy but when El Nino comes even drought can happen.
La Nina Phenomenon is what happens when the temperature of eastern and middle surface of tropical Pacific Ocean drops unusually and the trade wind intensifies. The eye-catching torrential rain is attributed to the effects of the higher temperature worldwide. Researches have pointed out that the increasing average amount of vapour of earth caused by global warming also contributes to the growth in the frequency of intensified rain and the amount of low-pressured tropical rainfall.
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