Raindrops are spherical because the surface tension of water pushes them to adopt the most compact shape possible, which is the sphere, thereby reducing their surface energy.
Water molecules have polarity: that is, each molecule has a positively charged side (hydrogen) and a negatively charged side (oxygen). This polarity makes water molecules very attracted to each other: this is called molecular cohesion. These attractive forces cause water droplets to prefer to group together rather than spread out in every direction. As a result, the molecules naturally stick together to minimize their common energy, spontaneously adopting a grouped, compact shape. It is this phenomenon of polarity that is responsible for the remarkable properties of water, and it partly explains why droplets prefer to form a nice little sphere rather than some other strange shape.
Water molecules are strongly attracted to each other, creating a sort of elastic skin at the surface of the liquid: this is called surface tension. This skin always tries to minimize its surface area because a smaller surface means less energy expended. And the most compact shape possible with the lowest outer surface is, of course, a sphere. You can imagine this like an inflated balloon: it takes on a round shape because it's simply the easiest way to keep the air inside with the least surface area possible. Raindrops do exactly the same thing with water.
When gravity acts on a droplet of water, it tries to stretch it downwards, which deforms its beautiful rounded shape. For very small droplets, the effect is weak and the droplet remains almost spherical. However, the larger ones undergo a real battle between the surface tension that tries to keep them round and gravity that pulls them down — as a result, they end up resembling somewhat flattened disks. The larger the droplet, the more it takes on this flattened disk appearance, because the bottom is pulled towards the ground while the air exerts resistance above. In extreme cases, large droplets even end up bursting as they fall, forming several small round droplets.
The size of the droplets directly influences their shape. Small droplets are more spherical due to the strong surface tension relative to their reduced mass. But when a droplet grows, the situation changes: it flattens slightly. The reason is that gravity pulls the water downward and begins to overcome this surface tension. On a molecular level, water molecules like to cling together because of their hydrogen bonds. The stronger these interactions are, the more they keep the droplet compact, limiting its deformation up to a certain size. Beyond a certain critical size, however, gravity clearly prevails, and the droplet eventually divides into smaller, more stable droplets.
Atmospheric pressure mainly plays an indirect role in the shape of droplets. At high altitudes, where the pressure is lower, water droplets can slightly swell in size and become less compact before falling. As they descend towards the ground, the pressure increases and slightly compresses the droplets, making them a bit smaller and denser. But be careful, this compression remains weak, not enough to radically change the droplet. The real influencer here is primarily the combination of pressure with air resistance—together, they modify the size and stability of the droplets during their fall, without causing them to lose their generally spherical appearance.
Contrary to popular belief, raindrops are not tear-shaped, but rather spherical or flattened at their base when they are large, due to the combined effects of surface tension and air resistance.
The heaviest rainfall ever recorded on Earth occurred in India, in Cherrapunji, where it reached 26,461 mm in a single year (from August 1860 to July 1861)!
On the International Space Station (ISS), the absence of gravity causes water droplets to form perfectly spherical shapes, floating in the air before being consumed or absorbed with a cloth.
Tiny particles called aerosols play an essential role in the formation of raindrops by serving as condensation nuclei where water vapor can condense.
Yes, a large droplet can fragment upon falling due to turbulence and aerodynamic forces encountered. This phenomenon, known as fragmentation, typically occurs when a water droplet becomes too massive, leading to an instability that triggers its separation into several smaller droplets.
In reality, no. As the droplets fall, their shape evolves due to aerodynamic forces and air resistance. Smaller droplets remain relatively spherical, while larger droplets will adopt a flattened shape, even lens-shaped, as they descend.
The size of raindrops depends on several factors, including specific processes within clouds, such as coalescence (the merging of small droplets). The more droplets merge before falling, the larger the final drop will be. Updrafts also significantly influence their final size.
Atmospheric pressure indirectly influences the falling speed and formation of droplets by affecting the density of the air. The lower the pressure, the less dense the air becomes, which can slightly alter the speed or trajectory of the droplets as they fall to the ground.
The spherical shape of a droplet mainly results from surface tension. This tension arises from the attractive forces between water molecules at the surface, which pushes them to minimize their surface area and thus creates a spherical shape, geometrically the configuration with the least potential energy.
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