How to make rain and drive clouds?

For many years, people have been longing for the day when the weather can change according to people's wishes: when the wind calls, it will come; As soon as the rain calls, it comes; If only the wind and rainfall could be controlled manually! In the era of underdeveloped technology, this is just a myth; However, with the development of science and technology, the work of artificially influencing local weather has begun, and it can call the wind and rain under certain conditions.

What is artificial influence on local weather? That is to use artificial methods to make local weather develop in the direction of people's wishes, produce favorable weather, avoid unfavorable weather, and then achieve the purpose of transforming the climate. The weather has a wide range of changes and great energy. At present, it is difficult for people to directly control the weather changes at will. But with the continuous development of meteorological science, some methods have been found. Under certain meteorological conditions, as long as a small amount of energy is consumed, some existing weather processes can be promoted to develop in the direction of people's wishes in a local area, which is the artificial influence on local weather.

In China, artificial hail suppression and artificial precipitation enhancement are widely carried out at present. It is of great economic significance to carry out this work in areas where hail and drought often occur.

Let's talk about artificial hail suppression first. Artificial hail suppression was tried more than 200 years ago. In fact, hail suppression only plays the role of "making the hail bigger and smaller". Specifically, the application of catalysts such as silver iodide or lead iodide in the cloud will increase the number of ice crystals in the cloud. When ice crystals form hail embryos, they consume a lot of Leng Yun drops, and as a result, all hail embryos will not grow too big. Some hail blocks will melt when falling, forming water droplets or shrinking into small hail, thus achieving the purpose of hail suppression. Like artificial rainfall, some people use hygroscopic substances to suppress hail, such as salt, which will absorb the water in the cloud, so that hail embryos will not expand too much and land on the ground in time.

Cumulonimbus clouds that can hail are generally divided into three layers, with the bottom temperature above 0℃ and mainly composed of water droplets; The highest temperature is below -20℃, which is almost entirely composed of ice crystals. In these areas, updraft is generally not strong. Even if there is updraft, water contacts water and ice contacts ice, it is not easy to form hail. Only the middle part of the cumulonimbus cloud (that is, the area where the temperature is between 0℃ and 20℃) has a particularly strong updraft, which is an ice water storage area. Under the action of strong updraft, ice is most likely to freeze when it meets water to form hail, so this part is generally called "hail stove". If special rockets or anti-aircraft guns are used to launch chemicals such as silver iodide into the "hail stove" in the cumulonimbus cloud, this will often cause the supercooled water droplets here to become ice crystals, which will gradually change the state of ice water, thus destroying the conditions for the formation of hail and turning the ice crystals and supercooled water droplets in the cumulonimbus cloud into rain instead of hail.

Hail suppression can use airplanes, anti-aircraft guns, rockets, etc. Under the surveillance of radar, it is common to use anti-aircraft guns and rockets to launch artificial icing agents in China and the former Soviet Union (now mainly Russian). Russia has special hail suppression rockets and special radars. This kind of radar can determine whether there is hail within 300 kilometers, determine hail area, moving speed, hail intensity, and even calculate the average diameter of hail.

It needs to be clear that artificial hail suppression can also adopt the method of air explosion operation. After the explosion, the heavy hail will be shattered by the shock wave, but it will freeze directly after passing through Leng Yun, thus achieving the purpose of hail suppression.

In addition, in areas vulnerable to hail, a variety of root crops (such as sweet potatoes, potatoes, etc. ) and small areas of seedlings or nurseries can also be covered with scaffolding; Irrigation can also be used for rice seedlings; For crops that are close to maturity, it is necessary to organize harvesting as soon as possible, so as to harvest one piece when it is mature. These are all good ways to reduce or avoid losses.

As we all know, if there is rain, there must be clouds first, and if there are clouds, there may not be rain. The natural rainfall (or snow) passing through Leng Yun is due to the fact that there are enough ice crystals in the cloud-saturated water vapor or supercooled water droplets, which are condensed or frozen by ice cores (water-insoluble dust particles) to form ice embryos. Water molecules of water droplets in supercooled clouds will evaporate and condense on ice crystals, which will grow and fall into the snow. If the temperature under the clouds is higher than 0℃, they will melt into rain. If there are not enough ice crystals in this cloud in nature, because the water droplets in the cloud are very small and can be suspended in the air for a long time without falling, then there is only cloud without rain. At this time, if silver iodide particles are sown in this cloud, many ice crystals will be produced, and the water on the water droplets in the cloud will be quickly transformed into these artificial ice crystals through evaporation and condensation, which will make the ice crystals grow rapidly and produce snowfall; If the ground temperature is high, the snow will melt in the process of falling, merge into water droplets and eventually become rain. This is artificial rainfall.

Artificial precipitation enhancement is to artificially supplement some necessary conditions for precipitation formation according to the principle of precipitation formation, and accelerate the condensation or merger of cloud droplets to form precipitation.

We can often encounter dense clouds without rain, which can't cause ice water accumulation, and are suitable for cloud droplet condensation (condensation) increase, or lack of large water droplets, which can't quickly increase into precipitation. So, how to increase the ice crystals and large water droplets in the cloud? At present, the commonly used method is to sprinkle dry ice and silver iodide in the cloud to promote the increase of ice crystals in the cloud. Dry ice is solid carbon dioxide, white as ice, and its own temperature is very low (-78℃), so it is a good refrigerant. When it sublimates, it should absorb a lot of heat to make the ambient air temperature very low, so that the supercooled water droplets in the cloud quickly freeze into many tiny ice crystals, or the water vapor in the cloud directly condenses into ice crystals, thus producing ice water. Silver iodide particles are condensation nuclei with good performance. As long as the temperature reaches -4℃, water vapor will condense into ice crystals with it as the core, and it will not fail if it stays in the air for a long time. If dispersed in the cloud, it can also freeze supercooled water droplets, or condense water vapor into ice crystals, resulting in the existence of ice water in the cloud.

At present, the main method to promote the merger of cloud droplets is to sprinkle salt powder in the cloud, because salt particles have strong hygroscopicity, and they can quickly grow into large water droplets after absorbing water, resulting in the coexistence of large and small water droplets in the cloud. In this way, because of the continuous convergence of other small water droplets, large water droplets are easy to form precipitation. In addition, the vibration of sound waves (including ultrasonic waves and secondary waves) can be used to promote the merger of cloud droplets. Because when sound waves propagate in the air, all kinds of particles in the air will vibrate, and large cloud droplets have large inertia and small displacement when vibrating; Small droplets have small inertia and large displacement when vibrating. In this way, cloud droplets of different sizes may collide due to different degrees of displacement, thus merging to form precipitation.

Generally speaking, artificial catalysis is best for low-thickness clouds. Usually dry ice, silver iodide, etc. They are scattered in the supercooled water drop area of the cloud, while salt powder should be scattered in the cloud with a temperature higher than 0℃. At present, dry ice and silver iodide or salt powder are mainly used for artificial catalysis of Leng Yun in China. In the method of broadcasting, the method of plane broadcasting is generally adopted. In addition, it can also be carried by small rockets or balloons; After rockets and balloons explode at a certain height, the catalyst is scattered to the clouds by the explosive force. Another method is to turn the solution of silver iodide or salt into steam in a high-temperature furnace and blow it into the cloud through updraft.

No matter which method is used for artificial precipitation enhancement, the existence of clouds is the fundamental condition, which is the internal cause; Spraying catalyst into the cloud is an external cause, which can only work through internal factors, so it has certain limitations.

Artificial rainfall facilities

The experimental and theoretical research on artificial precipitation enhancement began in the 1940s. Over the past 60 years, countries have made great progress in this field. Technically, it is still in a passive position, and "collision" can only be carried out under certain existing meteorological conditions. People can't create some favorable conditions in advance to carry out artificial precipitation enhancement by catalytic method. At best, they can only drop most of the water in the cloud to the ground, but they can't "make" rain from scratch. If people want to "get freedom from nature", they must limit the movement of the atmosphere as much as possible while affecting the microphysical processes of some weather processes, so as to truly realize the great ideal of calling for rain, driving away clouds and eliminating fog. Although it may take a long and arduous process to realize this ideal, we believe that this day will come.