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Raindrops Keep Falling on My Head: A Mosquito’s Lament

This, in case you were wondering, is a mosquito.

Picture of a drawing of a mosquito
Drawing by Robert Krulwich
Drawing by Robert Krulwich

This is a raindrop.

Picture of a drawing of a blue raindrop
Drawing by Robert Krulwich
Drawing by Robert Krulwich

And here’s a puzzle. Raindrops aren’t mosquito friendly. If you’re a mosquito darting about on a rainy day, those drops zinging down at you can be, first of all, as big as you are, and, more dangerously, they’re denser. Water is heavy, so a single raindrop might have 50 times your mass, which means that if one hits you smack where it hurts (between your wings) …

Picture of a mosquito being hit by a drop of water
Photograph by Tim Nowack
Photograph by Tim Nowack

… you should flatten like a pancake. A study says a mosquito being hit by a raindrop is roughly the equivalent of a human being whacked by a school bus, the typical bus being about 50 times the mass of a person. And worse, when it’s raining hard, each mosquito should expect to get smacked, grazed, or shoved by a raindrop every 25 seconds. So rain should be dangerous to a mosquito. And yet (you probably haven’t looked, but trust me), when it’s raining those little pains in the neck are happily darting about in the air, getting banged—and they don’t seem to care. Raindrops, for some reason, don’t bother them.

Picture of a drawing of mosquitos flying through the air, dodging large blue raindrops
Drawing by Robert Krulwich
Drawing by Robert Krulwich

Why not? Why aren’t the mosquitoes getting smooshed?

How Mosquitoes Survive Raindrops

Well, in 2012 David Hu, a professor of mechanical engineering at Georgia Tech, became interested in this problem and decided to pelt some airborne laboratory mosquitoes with water droplets while filming them with a high-speed camera—4,000 to 6,000 frames a second instead of the usual 24. That way he could watch them in super slow motion and figure out what they’re doing when they’re out in the rain. He published his findings in a 2012 paper that I’m going to describe here in “executive summary” form. (His video, by the way, is waiting for you below, so you can see what he saw for yourself.)

What he found is that most of the time anopheles mosquitoes don’t play dodgeball with the raindrops. They do get hit but usually off center, on their long gangly legs, which splay out in six directions. The raindrop can set them rolling and pitching, but they recover quickly—within a hundredth of a second. But even in the worst case, where the mosquito gets slammed right between the wings—a dead-on collision, because the mosquito is so light compared to the heavy raindrop …

Picture of a drawing of a mosquito clinging onto a falling raindrop as it descends through the air
Drawing by Robert Krulwich
Drawing by Robert Krulwich

… it doesn’t offer much resistance, and the raindrop just barrels along with the mosquito suddenly on board as a passenger. Had the raindrop slammed into a bigger, slightly heavier animal, like a dragonfly, the raindrop would “feel” the collision and lose momentum. The raindrop might even break apart because of the impact, and force would transfer from the raindrop to the insect’s exoskeleton, rattling the animal to death.

But because our mosquito is oh-so-light, the raindrop moves on, unimpeded, and hardly any force is transferred. All that happens is that our mosquito is suddenly scooped up by the raindrop and finds itself hurtling toward the ground at a velocity of roughly nine meters per second, an acceleration which can’t be very comfortable, because it puts enormous pressure on the insect’s body, up to 300 gravities worth, says professor Hu.

Picture of a drawing of a mosquito inside a raindrop, falling through the air
Drawing by Robert Krulwich
Drawing by Robert Krulwich

300 Gs is a crazy amount of pressure. Eric Olsen, at his blog at Scientific American, says a jet pilot accelerating out of a loop-de-loop experiences “only about nine gravities (88/m/squared).” One imagines his cheeks all splayed, his face squishy, but hey, that’s a soft-skinned human. We’ve got mosquitoes here. Their heads are harder. They have exoskeletons. Sudden accelerations don’t hurt as much, but what mosquitoes should fear, what they do fear, are crash landings. The ground is a lot harder than a mosquito.

Picture of a drawing of a mosquito being squished by a large blue raindrop
Drawing by Robert Krulwich
Drawing by Robert Krulwich

So what a mosquito has to do is get off that raindrop as quickly as possible. And here comes the best part: In most direct hits, Hu and colleagues write, the insect is carried five to 20 body lengths downward, and then, rather gracefully—maybe helped by a dense layer of wax-coated, water-repellent hairs—gets up and “walks” to the side, then steps off into the air, almost like a schoolchild getting off of a bus (albeit a fast-moving bus hurtling toward its doom). It does this almost matter-of-factly, like it’s no big deal. A mosquito, Hu writes, “is always able to laterally separate itself from the drop and recover its flight.” Always. (Unless the raindrop hits them too close to the ground.) If you want to see this for yourself, take a look at Hu’s video.

Video by David Hu and Andrew Dickerson

The moral here, should we need one, is that if you’re a mosquito on a rainy day, the place to be is high off the ground, and if you’re a human who worries about mosquito safety (not a big group, I know), you can move on. They solved this one roughly 90 million years ago.

Picture of a drawing of a mosquito with its arm around a raindrop, as though they were friends
Drawing by Robert Krulwich
Drawing by Robert Krulwich

23 thoughts on “Raindrops Keep Falling on My Head: A Mosquito’s Lament

  1. Hmmpf! They still poke and bite! I dislike mosquitos! If you wanted to write about them, why not discuss the Texas mosquitos used to drill oil wells? Now, those I’m told, are impressive! Even saw one once on a postcard. Why, one species even secretes an enzyme to dissolve the organic matter in blood leaving only the iron in haemoglobin. Then another enzyme causes the iron atoms to join to form biological drill pipe! These structures are known to be as much as 6 inches in diameter and to extend a mile deep. If you doubt this, just look at diagrams of oil fields. See all the drill pipe and how deep they go? Came from Texas mosquitos. Its a fact!

  2. Now I know why trying to swat a mozzie while I’m in the shower by directing the water towards it doesn’t work. Thanks for that rather impressive information, I feel that such dedicated research should at least get an “ig-nobel” prize.

  3. awesome. found this article by way of a hacker news link.
    I have missed the Krulwich illustrations and stories (from npr.org) and am glad to find them.
    sorry for the tangent. I enjoyed the story. never actually thought about the question.

  4. While it is cute the illustration of the raindrop pepetuates a common myth. Raindrops are not teardrop shaped while they are falling through the air. Proof of This can be seen clearly in the referenced video. I point this out mearly in the hope that one day what the shape of a raindrop is in the zeitgeist will more accurately reflect reality.

  5. As I’m sure you’ve heard, there is much work being done on genetically altering mosquitoes via a virus, allowing us to kill them off in one fell swoop. I realize, the real reason this research is being done probably has more to do with some really rich people trying to find a way to depopulate the earth, but in the meantime, if it gets rid of mosquitoes, I’m happy.

    So, raindrop resilience won’t save them for long.

  6. I was enjoying the article until you started mixing up velocity and acceleration. Sorry. 88/m/squared?? Wtf is that?

  7. Ok fine, the mosquito is too small and makes e. Which just postpones the problem: What about the slight bigger insects? I wanna know about the area where the insects and droplets have a bigger problem with each other.

  8. Aw man. Why doesn’t the video have footage of one of the direct hits? Those sounded like the most interesting interactions.

  9. I was hoping that the video would show “the best part” of a direct hit, where the mosquito is carried 5 and 20 body lengths by the raindrop before simply stepping off but it doesn’t.

    The referenced paper (http://www.pnas.org/content/109/25/9822.abstract) offers some supplemental information (http://www.pnas.org/content/suppl/2012/05/28/1205446109.DCSupplemental) including a supplemental video that sort of shows this “best part” starting around the 0:55 mark.

    That video is also online on Andrew Dickerson’s Youtube channel (https://www.youtube.com/user/andrew52987/videos) here:

    1. Loisaida Sam,
      Thank you so much for finding the very thing I trolled and trolled for. These little movies you found on Andrew Dickerson’s Youtube channel are perfect illustrations of what I’m talking about here. I don’t know why I didn’t find them, since I was searching for this very thing, but I’m glad you did. Now we’re out of my fantasy space and looking at real animals and real raindrops, and suddenly the story is stronger. Truth is, while it’s a tad embarassing to have my readers out-report me, in the end it’s a win/win for all of us. So again, big thanks! (from Robert K.)

  10. Robert,

    A bus is much more than “roughly 50 times” the weight of a human. That’s closer to a car. 50*200 = only 10,000

  11. Wow! Robert – you and I were thinking the same thing. Exactly 2 days ago (when this article was published) I was sitting in my Upper Michigan home pondering this very question while watching a cloud of mosquitoes during a pretty heavy rainstorm. It has been a mosquito infested June this summer. The fact that the rainstorm was not seeming to affect (let alone killing) the swarm I was watching had me lost for an explanation…and a little bit disappointed, I suppose. Thank you for the explanation.

    1. Hey matzukaze2001,
      When I write these things, I just imagine that maybe, just maybe, there’s someone out there who might be wondering the same thing, so when you pop up and say “Me too!”, I do a little happy dance. Thank you.

  12. What is the mosquito’s purpose in nature? Every species seems to have a purpose — predators check the growth of herbivore populations that check the growth of vegetation that supply oxygen, etc. (My understanding is very rudimentary, sorry, everyone.)

  13. I am not a expert but lately I am observing these creatures (mosquito) in my room say or bathroom, There is only one way to enter to my room for mosquito i.e. bathroom.Sometimes I observed during evening time they are more volatile and to kill them at that time,I just start exhaust fan. But it works only first time.Also your raindropping experiment i have already tried before i read this article but not in 4000 frames per second but through visible eye only,what I observed I kill by splashing waters by greater force and by killing with cup moving in downward direction.But they fly ultimate,sometimes soothe but sometimes untraceable.

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