December 7, 2022

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How does rain work? 6 curiosity to know

Rain is a complex (and vital) mechanism that turns invisible water vapor in the air into droplets that fall to the ground. How exactly does it work? What happens in the transition from heaven to earth? Are extreme events really increasing (ie, does the amount of rain that falls in months fall in one day?) Here is a guide to rain in 6 points, between physical principles and curiosities.

1. Water vapor. The main ingredient in the rain recipe is Water vapor in the atmosphere. “It must condense, that is, pass into a liquid state, and for this to happen the maximum amount of moisture that can be present in the air must be exceeded,” explains Vincenzo Livezzani, author of Clouds book. A practical and theoretical guide to reading the sky (Sayer). But to start the process, a second component is needed: fine particles suspended in the air (aerosols), generated by natural phenomena or human activities. These particles make up me Condensation nucleito which water vapor adheres, leading to the formation of droplets that form Zipper, with a diameter of 10 parts of a millimeter. They grow a bit as more vapor is deposited, but the phenomenon that makes them really big is the collision between them, which Autism It forms droplets of a few millimeters. When gravity overcomes the upstream impulse, it begins to fall: no longer cloud drops, but raindrops ».

2. It rained at our house. If we were in the tropics, this would happen over our heads. But things are complicated in our clouds. “In our country, all the rains somehow went through an icy phase. Even in summer, ”explains Levizani. “Here in the clouds (in whole or in part at altitudes where the temperature is below zero) there may be drops of water, but above all there are Ice crystals: they are always formed around particles (ice nuclei) on which water vapor settles and becomes ice.Then the crystals collide and form clusters – snowflakes – that eventually end in falling: as the temperature decreases, it rises and when it reaches above 0 degrees Celsius, the flakes melt and turn To rain. The same thing happens with other frozen masses that are born in the clouds like hail ».

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3. Air cleaning. “Rain is a natural sweeper of the atmosphere,” Levizani says, “because in autumn it removes particles in the air and partly gases.”

In one experiment, chemists from the Massachusetts Institute of Technology calculated the efficiency of droplets at attracting particles such as sulfate, organic molecules, and carbonaceous particles: smaller droplets work better. For example, rain cleanses cities from the dust that treads on them. The rainwater that reaches the ground is not pure, but in fact it is completely dirty, ” Levizani confirms. It must be said – between what the clouds are surrounded by condensation nuclei or ice and what is collected in the fall – everything comes down with the rain ». Janice Braney of Utah State University, for example, found many raindrops collected in parks and wilderness areas in the USA. Microplastics (especially microfibers from clothing and synthetics), which end up in the air from populated areas. Another example? “With the rain, dust falls from the desert, which is carried by the wind, and we realize that from how we reduce cars,” says Livezani.

4. Cores of all kinds. Let’s go back to the cores on which drops or ice are formed. Fundamental, so much so that the release of substances into clouds that serve this purpose (such as silver iodide) is the basis of techniques aimed at making it rain. “Substances can be very diverse. In order to act as a base for the droplets, they must be soluble in water: for example, sea salt that comes from the surface of the oceans is very important in the sedimentation mechanism, ”says Livezzani. They range from particles produced by Combustion refers to those emitted by volcanoes.”The cores on which ice forms can also be clay from wind-blown soil, cellulose from deteriorating plants in forests, and even bacteria.”Some microbes may use rain as a dispersal strategy. The effects are complex. Small particles form Smaller droplets, which take longer to grow into rain. And they may not succeed at all.” A recent study, led by Cynthia Twohy (Northwest Research Associates, USA), for example, warns that fires can reduce precipitation, exacerbating the drought they cause.

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5. The shape of the drops. Not only is there no longer the middle of the seasons, but the raindrops aren’t what we’d expect: not in the “classic” teardrop shape. “The very small ones are spherical. The larger ones, on the other hand, take on a flattened shape, distorted by the air. Dimensions? The larger drops are associated with large aggregations of ice, for example with the hail that forms in storms: if it melts as it descends, it can generate violent rain » As the expert says.

6. Droughts and floods. Global warming affects precipitation. basic physical principle? «An increase in temperature leads to an increase in the evaporation of water from the seas and thus in the availability of water vapor in the atmosphere, which can store more as the temperature rises (7% more per degree increase, ed.). The theory tells us this. The result, however, is not a generalized increase in precipitation, but a change in the precipitation regime: In wet areas heavy rains and floods, in dry areas even more droughts, ”concludes Levizani.

To cite only the latest research, a study by the University of Newcastle (UK) predicts that severe thunderstorms that move very slowly will be 14 times more frequent in Europe by the end of the century. This means that an area can remain under heavy rain for a very long time, with the risk of flooding. And a German study found an increase in “record” events in the past 10 years in terms of precipitation in a single day.

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