Raindrops, a common sight during precipitation, typically appear as relatively small spheres. While they can vary in size, it’s observed that raindrops larger than 5 millimeters in diameter are exceptionally rare. This is due to the atmospheric processes and physical forces that govern the formation and stability of these falling water droplets.
From Vapor to Visible: How Raindrops Form
The journey of a raindrop begins with microscopic particles suspended in the atmosphere, known as cloud condensation nuclei (CCN). These tiny particles, which can include dust, salt, pollen, and pollutants, provide surfaces for water vapor to condense upon. As moist air cools, water vapor molecules gather around these nuclei, forming minuscule cloud droplets, often just 10 to 20 micrometers in diameter.
These tiny cloud droplets are too small to fall as rain and remain suspended in the cloud. They grow into larger raindrops primarily through a process called collision-coalescence. In this mechanism, larger, faster-falling droplets collide with and absorb smaller, slower-moving droplets. This merging allows droplets to gain significant mass and size, eventually becoming heavy enough to overcome air currents and fall towards the Earth’s surface as precipitation.
The Forces Shaping Falling Drops
As a raindrop descends through the atmosphere, it is subject to physical forces. Gravity acts as the primary downward force, constantly pulling the water droplet towards the Earth. Opposing this downward pull is air resistance, also known as drag, which increases with the drop’s speed and its surface area.
Surface tension is another important force, acting to hold the water molecules together and striving to maintain the drop’s spherical shape. For small raindrops, surface tension dominates, keeping them nearly spherical. However, as raindrops grow larger and their fall velocity increases, the upward pressure from air resistance at their base begins to deform them. This results in larger drops flattening into a shape often described as a “hamburger bun” or oblate spheroid.
Why 5 Millimeters is the Limit
The rarity of raindrops exceeding 5 millimeters stems from instability caused by these interacting forces. As a raindrop grows in size, its terminal velocity (the constant speed it reaches when the downward force of gravity is balanced by the upward force of air resistance) also increases. This higher speed leads to greater aerodynamic stress on the drop.
When a raindrop’s diameter approaches 5 millimeters, the aerodynamic stress overwhelms surface tension. The drop’s flattened shape becomes unstable, leading to oscillations. The drop deforms, often into a parachute-like shape or hollow bag, before fragmenting into multiple smaller droplets. This process, known as “bag breakup” or “parachute breakup,” is the primary reason why raindrops cannot grow indefinitely and why those larger than 5 millimeters are exceedingly uncommon.