Why does a mosquito shrug off a raindrop many times heavier than itself?
― It's too light for the drop to bother stopping
A single raindrop can weigh many times more than a mosquito. For a person, it would be like being pelted from the sky by objects heavier than yourself, over and over. Yet the mosquito stays airborne. What protects it isn't a tough body — it's astonishing lightness.
It's a summer evening, and rain suddenly starts falling. You duck under the eaves, and right in front of you a mosquito is drifting about.
Fat raindrops come clattering down. Surely if one of those hits the mosquito, it's finished.
But once the rain stops, the mosquito is back, looking perfectly fine. How did it survive that downpour?
Just two reasons it's fine
The mosquito is so light that the raindrop keeps falling at almost the same speed, carrying the mosquito along with it. Because the drop's momentum barely changes, the force on the mosquito stays small.
A mosquito's body and wings are covered in fine hairs that repel water. Even if it gets swept along by a drop, it slips free a little lower down and takes off flying again.
The instinct that "getting hit by something heavy is dangerous" only holds when the thing being hit can't move. For a light body floating in mid-air, it works the other way round entirely.
Whether the target can "move" changes the force
The size of an impact force depends on how abruptly the speed changes. The same raindrop, hitting the ground, stops in an instant. Because its speed changes so much, the force is strong.
But when it hits a mosquito in mid-air, the mosquito gets carried downward along with it. Since the mosquito is far lighter than the drop, the drop only slows a little and keeps falling. Compare the left and right sides of Figure 1.
A US research team used high-speed cameras to fire water droplets at mosquitoes. The mosquito reportedly gets carried along for a dozen or so body-lengths before breaking free and flying off. During that moment, its body is said to experience acceleration around 100 to 300 times gravity. That would be unsurvivable for a human, but the mosquito's light body isn't damaged by it.
What about other creatures?
This "survive by being light" trick works completely differently depending on body size.
Humans. A raindrop weighs only a few millionths of a person's body weight. It stops completely on our skin, but its momentum is so small to begin with that it neither hurts nor tickles. Unlike the mosquito, we're on the "too heavy to notice" side.
Mosquitoes flying near the ground. The real danger for a mosquito is said to be getting hit by a raindrop right near ground level. If it collides with the ground or a puddle while still being carried by the drop, it ends up in the same "immovable target" situation as the left side of Figure 1 — trapped between the raindrop and the ground.
Bats. For a bat, thousands of times heavier than a mosquito, the impact of a raindrop is no issue at all. The problem is that wet fur chills the body and adds weight. One experimental report found that flying in the rain roughly doubles the energy a bat spends on flight.
Flying through heavy rain, a mosquito can still take a direct hit from a raindrop. In experiments, a struck mosquito was carried a few centimetres to about 10 centimetres before righting itself and flying off. To us, raindrops are tiny — but to a mosquito, it's a world where lumps of water bigger than itself keep falling from the sky.
Summary
The force of an impact isn't decided by the weight of the thing that hits you alone. Whether the target can move along with it changes how the speed shifts, and that changes the size of the force. A mosquito is so much lighter than a raindrop that it barely slows the drop down at all. That keeps the force on it small, and its water-repellent body lets it break free and fly on.
A mosquito doesn't beat the rain by being tough.
It's too light, so the raindrop won't stop for it.
For why the raindrop itself doesn't hurt, see Raindrops fall from a thousand metres up, so why don't they hurt?. For how body size changes the feel of water, see Why does water feel like "honey" to a water flea?. For how mosquitoes find people, see Why are some people bitten by mosquitoes more than others?.
- Hang one sheet of tissue paper from a thread tied to the ceiling or a rod. Lay another sheet flat on a table and hold down one edge with your hand.
- Using a straw, drop water one drip at a time from the same height onto both sheets.
- The hanging sheet sways as the drop pushes it, letting the momentum escape through movement. The held-down sheet can't move. Compare how the drops land and bounce on each.
A hanging tissue isn't nearly as light as a mosquito, but it gives you a feel for how "the more a target can move, the more the impact escapes." Do this somewhere spills are fine.
Want to know more? ― Terms, maths, and links to the textbooksWe label which level each part belongs to, from junior-high science to university specialist courses
- JHSCovered in junior-high-school science
- HSCovered in high-school "Physics Basics / Physics"
- HS+High-school extension content, or textbook sidebar material
- UnivNot covered in high school — university specialist subjects (fluid dynamics, biophysics)
- ResearchNot yet settled even at university level — what researchers are currently investigating
JHSTerms: this phenomenon has names
- Inertia: an object's tendency to keep moving the way it already was. The heavier it is, the harder its motion is to change.
- Momentum: mass multiplied by speed. The combined total for two colliding objects stays the same before and after.
- Water repellency (hydrophobicity): a surface's tendency to repel water. The fine hairs on a mosquito's body and wings produce this.
JHSHSCheck with maths: how much does the raindrop slow down when it hits a mosquito?
Consider the simplest case: the raindrop and mosquito stick together and move as one. The meaning and units of each symbol are in the table below.
| Symbol | Meaning and unit |
| M | Mass of the raindrop (mg) |
| m | Mass of the mosquito (mg) |
| V | Speed of the raindrop before impact (m/s) |
| v | Combined speed after impact (m/s). v = M ÷ (M + m) × V |
| Mass M of a 3mm-diameter raindrop | said to be about 14mg |
| Falling speed V of that raindrop | said to be about 8 m/s |
| Mass m of one mosquito | about 2mg |
| A person's body weight | 60kg (60,000,000mg) |
| Combined mass of drop and mosquito | 14 + 2 = 16 mg |
| Share of that taken by the raindrop | 14 ÷ 16 ≒ 0.875 |
| Speed after impact, v | 0.875 × 8 = 7 m/s |
| Drop in raindrop speed (hitting a mosquito) | 8 − 7 = 1 m/s |
| Ratio to the drop when stopping on the ground | 8 ÷ 1 = 8 times |
| How many times heavier a person is than the drop | 60,000,000 ÷ 14 ≒ 4,300,000 times |
If the collision lasts the same length of time, the force on the raindrop is proportional to how much its speed changes. Hitting a mosquito works out to about one-eighth the force of stopping on the ground. And since that small force is all that pushes on the mosquito's body, its light frame can withstand it. A person is about 4.3 million times heavier than the raindrop, so even a drop that stops dead on our skin generates a force too small to feel at all.
HSHS+Force is "the rate of change of momentum"
HSThe impulse an object receives equals the change in its momentum. Since the combined momentum of the raindrop and mosquito is conserved, the light mosquito only picks up a small share of it. That means a small impulse, and so a small impact.
HS+A collision where two objects stick together and move as one is called a "perfectly inelastic collision." In reality the raindrop deforms and the mosquito breaks free partway through, so it's a bit more complex. Even so, the conclusion holds: the lighter the target, the less the raindrop slows down.
UnivDroplet deformation and the role of surface tension
A raindrop isn't a rigid ball — on impact it flattens and spreads out. A number that compares the drop's momentum against the strength of surface tension (the Weber number) determines whether it splashes apart or stays intact. In the mosquito's case, its water-repellent hairs are thought to let it slide sideways off the drop rather than staying trapped inside it. The fine arrangement of hairs on insect body surfaces is a subject actively studied in water-repellency research.
ResearchWhat isn't fully understood yet
- How often this happens outdoors. Most experiments used indoor water droplets, and it's not well understood how many times a mosquito actually gets hit in real rain, or what fraction survive.
- How they right themselves. Exactly how a mosquito uses its wings and legs to recover posture after such huge acceleration is still being studied in detail.
- Differences among other small flying insects. Whether the same trick works for flies, moths, beetles, and other insects with different weights and body stiffness is a field where comparative studies are still scarce.
In other words, this article too reflects "what's understood so far." The calculations above are simplified estimates meant to convey the underlying mechanism.
Links to the textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science — motion and energy (inertia) | Heavier objects resist changes in motion |
| HS | Physics Basics / Physics (momentum and impulse) | The raindrop's deceleration, checked with maths |
| HS+ | Physics (inelastic collisions) | Sticking together and moving as one |
| Univ | Fluid dynamics / biophysics | Droplet deformation and water repellency |
| Research | Insect flight and environmental adaptation | Real-world conditions, righting behaviour |
| ― | Connection to daily life | Why mosquitoes come back after the rain stops |
- Dickerson, A. K., Shepherd, P. G., & Hu, D. L. (2012). Mosquitoes survive raindrop collisions by virtue of their low mass. PNAS, 109(25)
- Gunn, R., & Kinzer, G. D. (1949). The terminal velocity of fall for water droplets in stagnant air. Journal of Meteorology, 6, 243–248.
- Voigt, C. C., et al. (2011). Rain increases the energy cost of bat flight. Biology Letters, 7, 793–795.
- Japan Meteorological Agency, "Rain intensity and how it falls" (気象庁「雨の強さと降り方」)
※This article is a general-audience science explainer. The figures given are estimates meant to convey the underlying mechanism. Actual values vary with rain intensity, raindrop size, and mosquito species.