What makes raindrops bigger

Drizzle is the lightest form of rain and may seem to float, but unlike fog, it does hit the ground. A typical raindrop is roughly 2 millimeters, but a large raindrop can grow close to 5 millimeters in diameter.

A drop that big usually breaks up into smaller droplets. Rainfall intensity is more-so based on how much rain falls within a certain time. So, the next time it rains, remember that those drops had some growing to do before they could fall in your area.

When this happens, the surface tension of the water that keeps the drop more or less spherical is partially overcome by the airflow pressure on its lower surface as it falls. This causes the bottom of the drop to flatten out, while the top retains its rounded shape.

Thus, contrary to the popular perception of a teardrop shaped raindrop, thanks to the various pressures the water drop is subjected to as it falls to the ground, the raindrop actually tends to look sort of like a kidney bean or the top of a hamburger bun. For very large droplets, it can even start to look something like a jellyfish bell, with this usually happening when a raindrop reaches around 4 mm across.

When the droplets get larger than this at around 5 mm , the air pressure will ultimately overcome the surface tension of the water completely and the raindrop will split apart. As the drop continues to fall, it can continue to grow or shrink as it bumps into and absorbs or is broken apart by other droplets.

Thanks to this process, the average raindrop that actually makes it to your head is only around 1 to 2 mm across. The size of a rain drop depends on the process of formation. In principle, condensation of moist air at high altitudes causes tiny drops to form. As soon as those drops get big enough, they start to fall. Now if these drops encounter other little drops on their way down big drops fall faster, so they catch up with little drops , they will "eat" the little drops and get even bigger.

They will keep doing this until they reach the ground. At the same time, when a drop gets sufficiently large, it will deform and can "explode" back into a bunch of smaller drops. An interesting article including a slow-motion video of this process can be seen here.

Turbulence in the air plays an important role in this formation process. When the air "changes direction" a lot, smaller droplets tend to be dragged along with it - while larger drops have greater inertia compared to their drag.

This means that under turbulent conditions, smaller drops will slam into bigger drops where under calm conditions the big drops go faster and "eat" the little drops. And when there is more wind, it also increases the drop size as explained by the turbulence argument.

A lecture exploring this in more detail can be found here - see especially slide 7 and further. According to a French researcher , raindrops differ in size because of topological changes they undergo as they fall.

They start out spherical when they first form within clouds. Surface tension maintains that shape. However, as they fall and encounter air resistance they flatten like a pancake, and then air pressure pushes up the center and they become umbrella-like. As air pressure builds inside the inverted cup shape of the falling raindrop, eventually it explodes into a chaotic shower of smaller drops of various sizes.

Mist rising from near ground level is very fine and uniform compared to raindrops, as mist droplets do not fall from a great enough height to undergo topological change.

Similarly, the super-large raindrops that fall in the still air that precedes some thunderstorms, form from warm moist air drawn from ground level into low-lying clouds prior to the violent downpour of chaotic droplet sizes that follows the advancing weather front. Sign up to join this community. The best answers are voted up and rise to the top. However, these droplets are too light to fall out of the sky. How will they get big enough to fall? Picture a huge room full of tiny droplets milling around.

If one droplet bumps into another droplet, the bigger droplet will "eat" the smaller droplet. This new bigger droplet will bump into other smaller droplets and become even bigger — this is called coalescence.

Soon the droplet is so heavy that the cloud or the room can no longer hold it up and it starts falling. As it falls it eats up even more droplets. We can call the growing droplet a raindrop as soon as it reaches the size of 0. If it gets any larger than 4 millimeters, however, it will usually split into two separate drops. The raindrop will continue falling until it reaches the ground. As it falls, sometimes a gust of wind updraft will force the drop back up into the cloud where it continues eating other droplets and getting bigger.