All Stories

Ever Wonder About Raindrops?

These days I am more aware of the weather—the rain in particular, now that I have a dog to walk. This led me to wonder how raindrops form. Here is the story, as far as I have been able to piece it together.


Once upon a time, a speck found itself inside a cloud. It may have been a bit of dust or smoke from the city far below, a crystal of salt left behind when some sea water evaporated or even a bacterium carried up by the winds. The size of this speck will affect the size of the drop that forms.


Clouds are saturated with water vapour molecules. As molecules of water vapour run into this speck, they lose energy and condense into liquid around the particle. This will happen more often under higher pressures and lower temperatures.


Hydrogen bonds between the water molecules on the liquid surface are relatively strong and pull on each other more than air molecules. This bond strength results in surface tension that tends to minimize the liquid surface area and produce a spherical raindrop (not pointy!).


At first, the raindrop was tiny and it was easy for air drafts within the cloud to keep it suspended. Slowly, more water molecules joined it, because growing by condensation takes a while and a cloud can evaporate within an hour.


With the help of wind turbulence, the raindrop began to grow more quickly by colliding with other drops. The drops coalesced until the resulting drop was heavy enough to fall out of the cloud.


As the drop fell, the air pushed against its lower surface, flattening out the bottom of the drop so that it looked more like a dinner roll. The top remained spherical, not pointy. If the drop was any bigger than about 4mm, it got flattened into a pancake and then into a parachute that burst into smaller drops (according to some experiments). This reminded me of the time I went parachuting and the instructor told us how it hurts to fall in the rain because all the little points stick into you. But the problem is not the shape, it's the speed.


When it rains hard, there is a wider range of sizes of raindrops that  fall. Galileo talked about how gravity makes all things fall faster and faster, regardless of weight, but he was ignoring the effects of air flow and drag. On small things like raindrops, these effects become important and limit the speed of a raindrop. The problem with parachuting in the rain is that drops fall around 10 m/s, whereas people can fall at more than 50 m/s. I think parachuting in the rain hurts because you are effectively crashing into the raindrops at more than 40 m/s.

Do you have any raindrop observations to share?