Raindrops are not all the same size because their formation depends on several factors such as falling speed, atmospheric pressure, and interactions with particles suspended in the air, resulting in variability in their size.
In the cloud, the small water droplets spend their time bumping into each other. Sometimes, they merge to form larger drops: this is coalescence. The more they merge, the bigger they get, gaining weight and beginning to fall under the influence of gravity. During this downward journey, they continue to collide with other drops, becoming large enough to create rain that you can feel falling. This random collision game explains why the size of the drops falling on your umbrella varies greatly from one drop to another.
Updrafts play a key role in drop size: the faster the updraft, the longer the drops stay in the cloud, collecting other small droplets along the way. This phenomenon causes them to grow, potentially reaching a substantial size. Conversely, strong downdrafts can quickly push the drops out of the clouds, bringing them to the ground before they have had time to grow. When these large currents continuously mix the drops at different altitudes, a varied mixture of drop sizes is obtained, explaining the diversity observed during a simple shower.
Aerosols and atmospheric particles (like dust, pollution, pollen) significantly influence the size of raindrops. Each drop needs a nucleus around which it begins to form: this is the role of small particles suspended in the air. If there are many of these particles, a large number of droplets will form, but they generally remain quite small due to a lack of space and water vapor to grow further. Conversely, when these small droplets collide with each other, they merge (coalescence) and gradually form larger drops that will eventually fall from the sky. This is why a heavily polluted atmosphere or one laden with dust can lead to rain with many small droplets rather than a heavy downpour with large, heavy drops.
Humidity in the air is a bit like fuel for raindrops: the higher it is, the more water is available for the drops to grow. High humidity therefore promotes the formation of larger droplets. Temperature also plays a role by directly influencing condensation: warm air can hold more water than cold air. When warm, moisture-laden air rises and cools at altitude, this vapor condenses, forming droplets. The difference in temperature during these phases affects the size of the final droplets: rapid cooling produces quick condensation and sometimes many small droplets, while slow cooling gives droplets more chances to grow by attracting other droplets.
Atmospheric pressure mainly affects the falling speed of droplets. The lower the pressure, as at high altitudes, the thinner the air. As a result, droplets fall faster, but they break more easily into small pieces, limiting their average size. Conversely, when the pressure is higher at ground level, the denser air slightly slows the droplets, giving them more time to merge together (coalescence), thus forming larger droplets. The difference in pressure also slightly influences the shape of the droplets: they become flattened when they fall quickly, like small pancakes.
The very fine drops, often referred to as drizzle, fall much more slowly than larger droplets. Some drops can even remain suspended in the air for a long time before reaching the ground.
The shape of raindrops changes as they fall: contrary to popular belief, they do not resemble pointed 'tears.' Small droplets are round, but larger ones take on a flattened shape due to air resistance.
According to certain studies, raindrops can reach a terminal velocity of up to 32 km/h for the larger ones, while the smaller ones generally do not exceed 7 km/h.
The phenomenon of pleasant scent often associated with rain, called 'petrichor', primarily originates from chemical compounds released when rain falls on dry soils or certain natural surfaces.
Indirectly, yes. The wind generates upward currents in the atmosphere, allowing droplets to remain suspended longer and merge with other droplets (a phenomenon known as coalescence), thereby forming larger droplets.
The fineness or heaviness of rain depends on multiple factors: the intensity of available humidity, temperature, the presence or absence of atmospheric particles, and upward currents. High humidity and significant upward movements favor larger droplets.
Atmospheric particles, such as dust or aerosols, serve as condensation nuclei on which water vapor condenses to form droplets. The more numerous these particles are, the smaller and more abundant the formed droplets will be.
Yes, raindrops tend to break apart when they reach a size greater than about 6 to 7 millimeters due to air resistance and aerodynamic forces, which limits their maximum size.
The fall speed of a droplet mainly depends on its size. Larger droplets have greater mass and encounter less resistance per unit of mass, allowing them to fall faster than smaller droplets.
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