What is artificial precipitation enhancement? Under what circumstances will the meteorological department start artificial precipitation enhancement?

Weather modification refers to the activities to avoid or reduce meteorological disasters, make rational use of climate resources, and artificially influence local atmospheric physical processes through scientific and technological means under appropriate conditions to achieve the purposes of increasing rainfall (snow), hail suppression, rain elimination, fog elimination and frost prevention. We know that clouds in the sky are composed of small water droplets and small ice crystals, so clouds can be regarded as movable reservoirs. Cloud to rain is not only related to the amount of water vapor in the cloud, but also related to the content of condensation nuclei in the cloud. So people spray refrigerant, crystallizer, hygroscopic agent, water mist and so on. According to the specific situation of the cloud. Improve the concentration of ice crystals in the cloud, make up for the deficiency of condensation nuclei in the cloud, strengthen contact in the cloud, increase cloud droplets, change the size, distribution and properties of cloud droplets, accelerate the growth process of raindrops, and achieve the purpose of increasing precipitation.

One is to fire guns or rockets at the clouds, and the other is to spray cooling water or catalysts on the aircraft. In the weather process, when there is a suitable rainfall system, the existing precipitation can be increased through manual intervention and catalysts, but the core is to use natural precipitation. Without the corresponding conditions, there is no way to increase labor costs. Artificial precipitation enhancement is to choose the right time according to the physical characteristics of different precipitation clouds, and spray dry ice, silver iodide, salt powder and other catalysts into the clouds with airplanes and rockets. The catalyst becomes the condensation nucleus of ice crystals (water droplets) in the cloud. After reaching a certain volume, the ice crystals (water droplets) quickly gather around and fall, and the target cloud increases precipitation according to natural precipitation.

China mainly uses dry ice, silver iodide, salt powder and other catalysts. The role of dry ice (solid carbon dioxide) in artificial rainfall enhancement is that after dry ice is put into the cooling cloud, a cold zone is formed in the surrounding thin layer. In this cold area, the supersaturation is very high, so the water vapor molecular combination exists and grows, and a large number of ice crystal embryos can be formed. Among them, the larger ice crystals spread to the surrounding space through turbulence, and will continue to grow to a larger precipitation quality point and decline in the future. The number of ice crystals in 1g dry ice increases with the decrease of air temperature. When the temperature dropped from-1 to -20/,the number of ice crystals produced increased from 5.55101to1.2210/4.

The role of silver iodide in artificial precipitation enhancement is called Leng Yun catalyst in meteorology. After silver iodide is heated, multipolar silver iodide (1 ~ 1000 of hair diameter) in the air forms particles. 1g silver iodide can form billions of particles. These particles enter the clouds with the airflow, and billions of ice crystals are produced in the cold clouds and water tanks. The role of salt powder in artificial precipitation enhancement is mainly aimed at the process of warm cloud precipitation. Fine salt powder is sprayed on warm clouds (that is, hygroscopic nuclei are introduced) and condenses and grows at low saturation, so it can form large water droplets of tens of microns or even larger in a short time, expand the water droplet spectrum, accelerate the sudden growth process and achieve the purpose of precipitation.