Box 3: What is artificial rainfall?

As the saying goes, "the weather is unpredictable." However, with the development of science and technology, this view has become a thing of the past. For thousands of years, mankind's desire to "sow clouds and rain" has now become a reality. The scientist who realized artificial rainfall for the first time was irving langmuir, an outstanding American physical chemist.

Irving langmuir,1881131was born in Brooklyn, new york, USA. Langmuir was very interested in natural science and applied technology since he was a child. When he was young, he had a great ideal: to realize artificial rainfall and get rid of the fate of relying on the weather for food.

Langmuir fully understood the farmers' thirst for rain in the dry season. Facing the farmers' eyes for rain and boundless blue sky, as a scientist, he made a rational and scientific exploration. After in-depth research, he finally figured out the mystery.

It turns out that the water vapor on the ground is a "cloud" when it rises and condenses when it meets cold. The tiny freezing point in the cloud is only about 0.0 1 mm in diameter and can be suspended in the air for a long time. Small ice crystals can be formed when they meet some impurity particles (called ice cores). Once the ice crystals appear, water vapor will quickly condense on the surface of the ice crystals, making small ice crystals grow into snowflakes, and many snowflakes stick together to become snowflakes. When the snow is big enough, it will fall from the sky, which is snowfall. If a snowflake collides with a cloud drop during falling, and the cloud drop condenses on the snowflake, this opaque ice hockey is called hail. If the snowflake falls to a warm area with a temperature higher than 0℃, it will melt into water droplets and start to rain.

However, clouds do not necessarily rain. This is because the ice cores in the cloud are not rich and the number of ice crystals is too small.

At that time, there was a popular view among the people that raindrops were composed of dust particles as "ice crystals". If it rains, there must be dust particles in the air besides water vapor. This popular view seriously restricts people's experiments and research on artificial rainfall, because it is difficult to raise the dust all over the sky on cloudy days.

Langmuir is a rigorous and pragmatic scientist. He was then the deputy director of the research laboratory of General Electric Company in Schenectady, new york. There are artificial clouds in his laboratory, which are water vapor filled in the refrigerator. Langmuir tried his best to make the water vapor in the refrigerator the same as that in the atmosphere before it rained. He also kept adjusting the temperature and adding all kinds of dust to carry out experiments.

1946 One day in July, the sun was shining and the heat was unbearable. Langmuir is conducting an experiment nervously. Suddenly, the refrigerator stopped cooling because of some equipment failures, and the temperature in the refrigerator could not drop. He decided to use dry ice to cool down. The gasification heat of solid carbon dioxide is very high, which is 87.2 calories/g at -60℃. It can be quickly converted into gas under normal pressure, and the ambient temperature can be reduced to about -78℃ by absorbing environmental heat for refrigeration. When he just put some dry ice into the freezer, a wonderful picture appeared: small ice particles danced and circled in the freezer, rain and snow fell from the floor, the whole freezer was chilly, and artificial clouds turned into ice and snow.

Langmuir analyzed this phenomenon and realized that dust is not an absolutely necessary condition for rainfall. Dry ice has a unique function of condensing water vapor, that is, ice crystals or ice cores in the cloud are used as "seeds". The temperature drop is also one of the important factors that water vapor turns into rain. He constantly adjusted the amount of dry ice and changed the temperature, and found that artificial rainfall was possible as long as the temperature dropped below -40℃.

The dry ice cloud method developed by Langmuir is a breakthrough in the study of artificial rainfall, and it has got rid of the shackles of old ideas. Interestingly, this breakthrough invention was made in the refrigerator in hot summer. Langmuir is determined to apply the dry ice cloud method to the practice of artificial rainfall. Although 1946 is a 66-year-old man, he is still burning with enthusiasm for exploring the mysteries of nature like a young man. /kloc-one day in 0/946, under the command of Langmuir, a plane took off and flew over the sea of clouds. The experimenter scattered 207 kilograms of dry ice into the sea of clouds, just like a farmer sowing seeds in a wheat field. After 30 minutes, a strong wind suddenly blew and rain poured down on the earth. The first artificial rainfall experiment was successful.

Langmuir initiated a new era of artificial rainfall. According to the theory of ice crystal nucleation in supercooled clouds, scientists also found that silver iodide (AgI) can be used as a "seed" for artificial rainfall. And in terms of effect, silver iodide is better than dry ice. Silver iodide can be scattered on the ground and floated into the clouds in the air under the action of updraft, which is simpler and easier than dry ice rainfall.

"Artificial rainfall" as a new type of "meteorological weapon" is not uncommon in the war. During the US-Vietnam War, the "Ho Chi Minh Trail" from Cambodia to Vietnam was busy, and foreign war materials supporting the Vietnamese people in fighting against American invaders were sent to the front line through this unique passage. However, there are often heavy rains and floods, which will damage bridges and dams. A large number of transport vehicles are struggling on muddy mountain roads, and the traffic is greatly affected, which is as destructive as bombing. At first, Vietnam knew nothing about this sudden rainstorm. Later, after many investigations, I learned that this was a secret weather operation approved and implemented by US President Johnson for six years, that is, the United States carried out "artificial rainfall" in the air on that road.

The significance of "unpredictable weather" lies not only in "artificial rainfall", but also in inspiring people to control the weather reasonably. Langmuir also studied this. He hopes to nip it in the bud by manual control before storm warning. This idea is not only reasonable, but also feasible, and has been widely used now.