Anything they want, which is human has always been a dream, but this dream should become a reality, we must rely on science. Atmospheric science is the study of natural phenomena and disciplines cycle occurs in the atmosphere, it is also the realization of the scientific basis of human rage dream.
Atmospheric circulation is the ground and the water cycle in the atmosphere substance, which is the main source of clean water on the ground, the atmosphere is one of the main sources of energy required to drive all kinds of sports. Atmospheric circulation is not to say the whole complex, which includes the evaporation process, the process into the cloud and rain into the process.
More specifically, the surface water is liquid or solid, they become gaseous water vapor by evaporation process, which is the evaporation process, the first phase change of water. Water vapor with the air movement in the atmosphere, but once again before the phase transition it is impossible to return to the ground. Water vapor to return to the ground, you must go through a phase change, such as condensation or sublimation, from the gas turned into liquid or solid form cloud droplets, which is a cloud process. After the second vapor phase change back to the ground to have the condition, but it does not necessarily return to the ground, because the cloud droplets are too small too light, it floats in the air not fall, under certain conditions it would once again evaporated back to gaseous state. In order to make the cloud droplets back to the ground, it must grow to a certain size, this process many Xiaoyun raindrops merge into large drops of rain is a process. Natural cloud cloud droplets can touch or coagulation process and make cloud droplets grow up, when it grows to a certain scale can become precipitation, fell to the ground to overcome gravity, such as cloud droplet diameter greater than 200 microns is likely to fall into the ground drizzle became a diameter greater than 500 microns will form a raindrop or a variety of solid precipitation thereof.
Conditions for the formation of rain
From the above discussion of the water cycle, we must pay attention to two key questions: First, the phase the second is where the energy demand.
As already mentioned the atmosphere contains water cycle 2nd phase change process, which is due to surface water and atmospheric water vapor phase is different, water cycle through the first phase of the change of surface water into the gaseous state, and then after and the second phase transition into a liquid or solid, which can return to the ground. This is very important because atmospheric water vapor, if it does not have the opportunity to go through a second phase transition into a liquid or solid water, they can not return to the ground, that is to say, although a lot of water vapor, but they no chance of a phase change, then they can not form precipitation.
Therefore, only those who have the opportunity to vapor phase change will have on precipitation offerings. In this sense, the area of a certain period of time, the number of water vapor alone does not determine how much precipitation will determine how much precipitation water are those of the second phase change has occurred, this is the resource of precipitation .
The second phase after change after condensation (sublimation) water constitutes the region of the maximum possible period of precipitation, which is a very significant amount, which is the basis of artificial rainfall potential. Of course, this maximum possible precipitation and precipitation can not all become, because not all of the cloud particles will grow up well, many of which would have been a part of staying in the air, even evaporate again. This determines the efficiency of the process into the rain, which reflects a certain area in a certain period of actual precipitation and precipitation ratio of the maximum possible.
The second problem is energy, which is why we have to consider in trying to influence the natural process. You can see from the above analysis, several processes of atmospheric water cycle involves a lot of energy needs. We must consider whether there is the ability to provide energy to change the natural course of development? Natural energy is enormous, and we need to make some quantitative estimates. The annual number of global human energy use as a ruler, put it as a unit (the specific value of about 1020 joules in the order), to estimate the energy involved in atmospheric processes.
All movement of Earth's atmosphere by the sun's energy is provided by the driver, according to the Journal of Atmospheric Sciences estimated that each year the sun to provide energy to the Earth is about 40 000 units, of which the energy for evaporation of ground about 10 000 units, while in the adult energy cloud process for raising the gas block movement of about 100 units. Because humans only hold grip a unit of energy, tremendous energy to face the evaporation process and the processes involved in cloud, humans apparently powerless.
But very lucky, atmospheric science also found to rain process is actually not related to energy demand, and its efficiency depends mainly cloud microphysical structure, and therefore we can appropriately changing cloud microphysical structure, adjust it efficiency of rain into the process, so as to achieve the purpose of rainfall or minus rain. This is what we do based artificial rainfall.
What will rain clouds
Artificial precipitation hope through human intervention, changing cloud microphysical structure, thereby improving the efficiency of the rain into the process, so that the ground increases in precipitation. To do this we need to understand the natural clouds, is how raindrops form? Ie under natural conditions as rain What is the process underway?
Atmospheric Sciences pointed out that cloud droplets or ice crystals in clouds formed in the process are very small, with a diameter of about 10 microns, such particles in the atmosphere has increased airflow is impossible to reach the ground. Make a cloud in cloud droplets grow to raindrops (diameter of at least 200 microns, and even 1 mm) requires two processes, one condenses the growth process, but because of the efficiency of this process with more and more low cloud droplet diameter increases, diameter greater than 10 microns cloud droplets, it is difficult to achieve by coagulation volume the other is the touch and the growth process, if the size of the particles in the cloud are different, the rate of decline will be greater than large particles smaller particles, so it will catch up with it and small particles and collisions, merging (referred to as "touch and"), so that the large particles fall along the way are small particles and large particles to go into this, which quickly becomes large, and the larger the particle it the touch and the higher growth efficiency. Precipitation particles of nature, whether it is liquid or solid raindrops snow, sleet, hail are mainly formed by this process.
Since there are nature during the formation of precipitation particles, why some cloud will rain, clouds and some rain is not it? This is the key difference lies inside cloud droplets microphysical structure, that is, the difference in particle size distribution of the cloud . On the front and the growth of cloud droplets touch model can see and touch the clouds to occur growth of cloud droplets must be of different sizes, which can happen to catch big cloud droplets and droplet collision and Xiaoyun phenomenon. If the cloud of particles in the same size, they are in the same speed, who can catch who can not talk about it so that large particles quickly increases situation that we are in the cloud, said glue steady state no particle increases as the raindrops, it is impossible to produce precipitation.
Usually we often see a cloud floating in the sky, sometimes quite thick clouds (looks dark), but that it does not rain, the cloud should be in a stable state of gum. So how to change this situation? We can add manually some big drops in the cloud, destroying the cloud glue steady state, so you can make the cloud and touch process begins, and soon the formation of precipitation particles. Of course, things have to think in two ways. If there is no large particles in the cloud, in a glue stable, precipitation is not easy to be but if a lot of large particles, is not the formation of precipitation will certainly benefit? This is not necessarily. If the particles are too large, they are limited to those who fought cloud drops, and small particles is limited, who would grow up, may finally unable to form an effective precipitation. So, to say in order to make the cloud gum of a steady state as soon as possible to produce precipitation, where you want to spread the appropriate number of large cloud droplets, but if a large cloud drops too much, it will prevent the precipitation occurred, which became rain reduction a.
How to implement artificial rainfall
So how to broadcast the withdrawal of a large cloud droplet cloud it? Intuitive idea is to directly broadcast the withdrawal of 30 to 50 micron droplets, but its efficiency is very low. Currently artificial precipitation for cloud body conditions, using an appropriate amount of warm cloud seeding cold cloud catalyst or catalyst.
For warm cloud, that is, the temperature is above freezing cloud, this cloud is composed of liquid water droplets composed of all, we have a choice of scale (a few microns in diameter) hygroscopic particles as a warm cloud catalyst to make them quickly to the cloud absorption of water vapor to form a large cloud droplets. Is still more inorganic salts such as ammonium chloride or certain organic compounds, as long as their ability to absorb moisture strong, and capacity to disperse into small particles can be considered a trial.
The other is the hybrid cloud, this cloud is thick, its upper temperature is below zero, while the lower part is higher than zero. This cloud above the zero line often exists supercooled water droplets. This is an unstable phenomenon may occur in cleaning the air, if there is a disturbance, these supercooled water droplets will immediately freezes. If the aircraft entered the clouds, supercooled water that will be icing on the ground, if this cloud of supercooled water droplets formed by the encounter some obstacles, such as wires, that quickly freeze onto the wire causing the wire icing phenomenon. These are harmful natural phenomena, but it is also to make use of artificial rainfall. Atmospheric Research found that, in this cloud of supercooled water droplets having a body, if several small ice crystals appear so small ice crystals will grow rapidly, while those of supercooled water droplets will quickly evaporate the moisture concentrate these ice crystals to go, this is Becky Long process (BergeronProcess). So for those who have the cloud of supercooled water, can quickly form a group of small ice crystals as ice cores by spreading it, these small ice crystals grow quickly by Becky Long process, to tens of microns can begin after the collision and through the process of forming quickly grew to become raindrops, precipitation occurs. Currently the most used cold cloud catalyst is silver iodide, also includes a composite catalyst having a similar lattice structure of silver iodide.
These are the basic ideas of the current artificial precipitation. In short, the basic idea is that for those artificial rain cloud in colloid stability, it is because of the lack of initial large cloud droplets and therefore can not form an effective ground by touching and precipitation process, then we in the cloud sow some catalysts, prompting the formation of a large number of cloud droplets, so that the clouds into rain efficiency, increase the ground precipitation. Similarly, other cloud body, due to lack of its large number of cloud droplets, may also be considered to add some large cloud droplets, thereby increasing the efficiency of a rain cloud body, increase the ground precipitation.
Of course, things may go in the opposite, that is, the original big body cloud cloud droplet is appropriate, as is already high efficiency rain, too much added to the large cloud droplets will be fought over those limited to cloud drops, may actually make precipitation efficiency is reduced, or even eliminate rain consequences. Because of this change cloud large cloud droplet concentration of work does not consume a lot of energy, and therefore humans can consider methods used.
Anterior still twists and turns
That approach is really effective? The answer this question is quite difficult. Although these physical processes mentioned above, such as touch and procedures, processes, etc. Long Baiji, in physics are correct, we can simulate the theoretical model, even in the laboratory experiments, test, but to understand Natural cloud in the end is what the situation, whether foreign or domestic, so far has not been a good solution.
We have been using various means of direct or remote sensing, the estimated maximum possible precipitation in the cloud, and cloud physics aircraft observation equipment installed to detect the cloud, hoping to understand the situation of the cloud droplet size distribution of the cloud, and further estimate Natural clouds into rain efficiency. But these effects are detected so far less than ideal, especially strongly undulating cloud parameters, very laws, so that the results of a single observation is difficult to have a representative. In addition, arrangements and implementation plans in terms of jobs, and other aircraft flew cloud cloud observation in order to understand the situation it is already too late, and we very much hope that the ground will be able to understand a wide range of cloud microphysics by the method of remote sensing characteristics of the structure, in order to determine the efficiency of the current natural precipitation cloud, this can decide how we should like to change the cloud microphysical structure, increasing the surface precipitation. These techniques require great efforts to develop in the future.
It is because of our lack of understanding of the situation of the cloud, so that the current artificial rainfall operations have a certain blindness. In this case, we can only assess the effect of the job from the final result in this regard, it is hoped that there is data to prove by precipitation artificial rainfall operations area is indeed increased, which is believed to be on target. But this thing is simply impossible, because when we say that the job increases the amount of precipitation, mean estimate precipitation job after job is more than not how much? But because the job has been done, we only the job can be observed after precipitation, without precipitation natural cloud job is unknown. Current general approach is to use statistical methods, such as statistical target area and comparative area analysis or statistical analysis of randomized trials. From the statistical requirements, the only randomized trial designed to meet the significance tests only statistically, but in real-world conditions such randomized trials is very difficult, we can only carry out in some areas there are strict randomized experimental design.
(Author: MAO Thai, Chinese famous atmospheric science expert, former Chinese president atmospheric physics)
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