How are raindrops formed?

Source: French to English Tester   Published on: 2026-04-28

Source: The Conversation – in French– By Auguste Gires, Chief Engineer of Water, Bridges and Forests, researcher at the Hydrology, Meteorology and Complexity (HM&Co) laboratory, École Nationale des Ponts et Chaussées (ENPC)

In their workWalk in the rain, published in 2025 by Presses des Ponts, Auguste Gires, chief engineer of the corps of Bridges, Waters and Forests, and researcher at the Hydrology, Meteorology and Complexity laboratory of the École Nationale des Ponts et Chaussées, and Eleonora Dallan, environmental engineer and researcher at the Department of Earth, Environment, Agriculture and Forests at the University of Padua in Italy, take us on a walk to discover a very common phenomenon, but ultimately little known: rain.

For the fourth “stopover” of this walk-through book, they explain to us how raindrops physically form in clouds. Excerpt.

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Isn’t it surprising that all these cumulus clouds (low-altitude fluffy clouds) seem to form at the same altitude? In fact, this is related to how clouds, and more broadly precipitation, are generated. On Earth, temperature and pressure conditions allow water to naturally exist in three states: liquid (such as in oceans, rivers, or tap water), solid (such as ice, snow, hail, freezing rain), and gas, in the air as invisible vapor. The interactions between these three states are, among other things, the origin of rain.

It all starts with evaporation, that is to say, a part of the liquid water present on the Earth’s surface evaporates under the action of the sun. The water vapor mixes with the ambient air near the surface. This air, heated by the sun, becomes warmer than the air above it. Its density becomes lower than that of the surrounding air (it is said to be lighter in common language), which facilitates its ascent. Indeed, the molecules it contains move faster under the effect of heat and then occupy more space, while keeping the same mass.

Due to the decrease in temperature with altitude, generally about 6.5 °C every 1,000 meters, initially warm air cools progressively as it rises. It turns out that the amount of water vapor that air can hold decreases with its temperature (about 7% per degree). In colder air, the molecules are closer to each other, which reduces the space available for other gas molecules like water vapor. This is the Clausius-Clapeyron relation.

Thus, at a given altitude, which depends on the temperature and the initial amount of water vapor, the air becomes saturated and can no longer hold as much water vapor as at the moment of evaporation, when the air was still close to the surface. Specialists call this the dew point. Part of the water condenses and returns to its original liquid form. The formation of these cloud droplets generally occurs around condensation nuclei, such as dust, ice, or salt. They are much smaller than the raindrops we mentioned earlier, with a size close to 0.02 millimeters. These droplets are so small that the turbulence of the surrounding air is strong enough to keep them suspended in the air, preventing them from falling to the ground. This concentration of droplets in the air forms a cloud. The process can also occur at negative temperatures, and in this case, the cloud consists of small ice crystals.

In a way, you are all witnesses to cloud formation on a daily basis. Let’s take advantage of the winter temperatures. Breathe in, breathe out… You will notice that a kind of small cloud forms as you exhale. Yes, it really is a small cloud! The air coming out of your lungs, warm and moist, meets cooler air in the outside atmosphere. Part of the water vapor it contains condenses and forms droplets.

The same phenomenon occurs when you boil water. Just above the hot water in the pot, the air is very hot and contains a lot of water vapor. As it rises, it meets cooler air and some of the water condenses into droplets. What you see is therefore certainly not water vapor, which is invisible, but a small cloud.

The amount of liquid water contained in clouds, i.e., the liquid water content, varies greatly depending on the type of clouds. It is expressed as the mass of liquid water per unit volume of air, in grams per cubic meter (g/m³).³). It shows a very great variability from one cloud to another, or even within the same cloud. Theorders of magnitudecan range from 0.06 g/m³For non-precipitating clouds like cirrus, high-altitude clouds, at 0.4 g/m³Â for clouds generating rain, such as stratocumulus, which are at a lower altitude. The values are higher for cumulonimbus, which will be discussed at the next stop.

What about the rain? For it to form, the droplets contained in the clouds must grow large enough to be heavy enough to fall. This mainly happens thanks to two processes.

The first, the Wegener-Bergeron-Findeisen process, involves the growth of ice crystals in mixed-phase clouds. These clouds, characterized by negative temperatures, simultaneously contain water in its three states: ice, supercooled water (water below 0 °C but not yet solidified), and water vapor. In this environment undersaturated with liquid water, the liquid rapidly evaporates while the water vapor deposits just as quickly onto the ice crystals, causing a sudden growth of the latter. The second is coalescence. In clouds containing enough droplets, some of them collide during their movement, merge, and progressively grow.

The authors do not work for, advise, own shares in, or receive funds from any organization that could benefit from this article, and have declared no other affiliation than their research institution.

–ref. How are raindrops formed?https://theconversation.com/how-raindrops-form-280285