We’ve been having a strange afternoon for rain. It’s been humid and hot for the past few days, and today the towering thunderheads rolled in. For about 15 minutes late this morning there was a brief shower with enormous raindrops. They were just huge. And widely spaced. And about two hours later the rain came down in earnest. Now we’re looking at a freshet because of all the runoff. Our storm drains can barely handle it. This rain was a mix of pelting rain with big drops and a finer rain that was actually more intense, followed by a brief respite. Which is when I dashed from the car.
According to Wikipedia, “Scientists traditionally thought that the variation in the size of raindrops was due to collisions on the way down to the ground. In 2009 French researchers succeeded in showing that the distribution of sizes is due to the drops’ interaction with air, which deforms larger drops and causes them to fragment into smaller drops, effectively limiting the largest raindrops to about 6 mm diameter.”
Another place in Wikipedia cites the largest raindrops to be about 9 mm diameter, with the largest ever at 10 mm in Brazil. So keep these figures loose in your mind until you can consult an expert.
So why are drops sometimes large and sometimes small? It depends on temperature, mostly. Clouds are water droplets that have sort of clumped together, but clouds don’t just rain all the time, so something is needed to make the water droplets get their act together and fall. When several clouds converge (or one cloud is experiencing turbulence, say from updrafts and downdrafts) the water droplets in the clouds, which want to not zip around too awful much, converge. The newly-created, larger drops are called “daughter droplets” – isn’t that just great? As these daughter droplets converge they drop in the cloud and continue to converge, eventually becoming heavy enough to drop as rain. It’s important to note here that rain falls because it is heavy, not because it is large. This process usually happens in warm weather, rather than freezing, and causes warm rain – a relative term, I suppose.
The warm rain doesn’t necessarily have large raindrops, though. Sometimes, larger drops occur when there is more moisture present in a smaller area (causing more convergence), which is more typical in “warm” clouds than in freezing ones, since in freezing clouds, the water droplets are little icicles and REALLY don’t want to move, decreasing just a little their likelihood of converging multiple times. Another time you’ll have larger raindrops is if there is something in the cloud for the raindrops to coalesce around, like smoke particles or (dare I say) pollution. Finally, the largest drops of all are associated with melting hail. Hail comes about when bits of ice shellac on to larger bits of ice as the air movement within the cloud forces warmer air upward to cooler reaches and then cycles back around again. This most often occurs in cumulus clouds. The hail falls to earth when it reaches sufficient weight – hail is varying sizes depending on how much weight any particular cloud can produce, since hail can form rapidly, and the more cloud resources that go into forming hail, there is less cloud to help suspend hailstones. But we’re talking about rain. The smaller hail that begins to fall into a very warm atmosphere will melt quickly, but will still be more or less congealed by the time it hits the ground. Larger droplets hit the ground faster than smaller droplets. A large drop can hit the ground at 20 miles per hour. A small drop at around 4 miles per hour.
Raindrops are not shaped like the classic teardrop shape, incidentally. They have nothing to hold them back in that manner (the teardrop leaves a little trail, giving it that shape, because of friction). The smallest drops are spherical. As they get gradually larger, the shape flattens on the bottom and finally becomes parachute-shaped, as the oncoming airflow forces a pocket of air against the flattened bottom, which eventually escapes around the sides. Very large drops may start out making the parachute shape, but the force of the air on the long descent causes them to break into smaller droplets.
Finally, Wikipedia leaves us with this: Euphemisms for a heavy or violent rain include gully washer, trash-mover and toad-strangler.