Why don't mosquitoes get hurt by raindrops many times heavier than they are?
― They're so light, the raindrop is the one that has to slow down
A single raindrop can weigh several times as much as a mosquito. For a person, that would be like being pelted from the sky by objects heavier than yourself, one after another. Yet mosquitoes don't fall — they keep on flying. What protects them isn't a tough body. It's astonishing lightness.
It's a summer evening, and rain suddenly starts to fall. You duck under the eaves, and right in front of you a mosquito is fluttering about.
Fat raindrops come clattering down. Surely if one hit the mosquito, that would be the end of it.
And yet, once the rain stops, the mosquito drifts back over, looking perfectly fine. How did it survive that downpour?
Just two reasons it's fine
A mosquito is so light that the raindrop keeps falling at nearly the same speed even while carrying it along. Because the drop's momentum hardly changes, the force acting on the mosquito stays small.
A mosquito's body and wings are covered in fine hairs that repel water. Even if it gets stuck to a drop and carried down a little, it slips free again and takes back off.
The idea that "a heavy impact is dangerous" only holds when the thing being hit can't move. For a light body floating in mid-air, the situation is almost the opposite.
Whether the target can move changes the force
The size of a collision force depends on how abruptly the speed changes. The same raindrop, when it hits the ground, stops instantly. Its speed changes hugely, so a strong force acts on it.
But when it hits a mosquito in mid-air, drop and mosquito move downward together. Since the mosquito is much lighter than the drop, the drop only slows down a little and keeps falling. Compare the left and right sides of Figure 1.
A research team in the United States used high-speed cameras to film water drops striking mosquitoes. They reported that a mosquito would ride the drop for a distance of ten-odd times its own body length before breaking free and flying off. During that impact, the mosquito's body was estimated to experience acceleration of roughly 100 to 300 times gravity — a level no human could survive, but one a light mosquito body shrugs off.
What about other creatures?
This "protection through lightness" strategy works very differently depending on body size.
Humans. A raindrop weighs only a few millionths of a person's body weight. It stops completely on human skin, but its momentum is far too small to feel — no pain, no itch. Unlike the mosquito, we're on the "too heavy to notice" side of things.
Mosquitoes flying close to the ground. In fact, the truly dangerous moment for a mosquito is said to be when it's hit right near the ground. If it's struck by a drop while just above the ground or a puddle, it ends up in the same "immovable target" situation as the left side of Figure 1. The mosquito can then get 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 problem at all. The trouble is that wet fur chills the body and adds weight. One experimental report found that flying in 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 10cm before righting itself and flying off. To us, a raindrop is 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 a collision isn't decided by weight alone. Whether the thing being hit can move along with the impact changes how the speed shifts, and that changes 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 quickly and fly on.
Mosquitoes don't survive the rain because they're tough.
They survive because they're too light for the raindrop to stop.
For why a raindrop itself doesn't hurt, see Raindrops fall from a kilometre up — so why don't they hurt?. For how body size changes the way water feels, see Why does water feel like honey to a water flea?. And for how mosquitoes find people to bite, see Why do mosquitoes bite some people more than others?.
- Hang one sheet of tissue paper from a thread off a ceiling or rod. Lay a second sheet flat on a table and hold down one edge with your hand.
- Using a straw, drop water one drop at a time from the same height onto both sheets.
- The hanging sheet swings when hit, shedding momentum through its movement. The held-down sheet can't move. Compare how the drops strike and bounce on each.
The hanging tissue isn't nearly as light as a mosquito, but it gives you a feel for how a movable target absorbs less impact. Do this somewhere spills are fine.
Want to go deeper? ― Terms, maths, and where this fits in the curriculumLabelled by level, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics / Physics"
- HS+High-school extension material, or textbook sidebar content
- Univ.Not covered in high school — university-level fluid dynamics or biophysics
- ResearchNot yet settled even at university level — something researchers are actively investigating
MSTerms: this phenomenon has names
- Inertia: an object's tendency to keep moving the way it already is. The heavier it is, the harder it is to change its motion.
- Momentum: mass multiplied by velocity. The combined total of two colliding objects stays the same before and after.
- Hydrophobicity: a surface's tendency to repel water. The fine hairs on a mosquito's body and wings create this.
MSHSChecking with maths: how much does the raindrop slow down?
Consider the simplest case, where the raindrop and mosquito stick together and move as one. The symbols and units are as follows.
| 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 | about 14mg |
| Falling speed V of that raindrop | about 8 m/s |
| Mass m of one mosquito | about 2mg |
| Human body weight | 60kg (60,000,000mg) |
| Combined mass of raindrop and mosquito | 14 + 2 = 16 mg |
| Share of that mass from the raindrop | 14 ÷ 16 ≒ 0.875 |
| Speed v after impact | 0.875 × 8 = 7 m/s |
| Drop in the raindrop's speed (hitting a mosquito) | 8 − 7 = 1 m/s |
| Ratio versus stopping on the ground | 8 ÷ 1 = 8 times |
| Human body weight vs. raindrop mass | 60,000,000 ÷ 14 ≒ 4,300,000 times |
If the impact lasts the same length of time, the force the raindrop feels is proportional to its change in speed. Hitting a mosquito costs it only about one-eighth of what stopping on the ground would. And since that small force is all that pushes on the mosquito's body, its light frame can withstand it. A human weighs about 4.3 million times as much as a raindrop, so even though the drop stops completely on our skin, the force it generates is too small to feel at all.
HSHS+Force as "the rate of change of momentum"
HSThe impulse an object receives equals the change in its momentum. Since the total momentum of the raindrop and mosquito is conserved, the light mosquito receives only a small share of it. That means a small impulse, and a small impact.
HS+A collision where 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 things are a bit more complex. Even so, the conclusion holds: the lighter the target, the less the raindrop slows down.
Univ.Droplet 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 to the strength of its surface tension (the Weber number) determines whether it breaks apart or stays intact. In a mosquito's case, its water-repellent hairs are thought to let it slide sideways out of the drop rather than staying trapped inside it. The fine arrangement of hairs on insect body surfaces is a heavily studied subject in water-repellency research.
ResearchWhat's still not fully understood
- How often this happens in the wild. Most experiments have used indoor water drops, and it isn't well known how often mosquitoes are actually struck in real rain, or what fraction survive.
- How they right themselves. Exactly how a mosquito uses its wings and legs to recover its posture after such a large acceleration is still being studied in detail.
- Differences among other small flying insects. How far the same strategy applies to 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 "the best explanation we have so far." The calculations above are a simplified estimate meant to convey the underlying mechanism.
Where this fits in the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | 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) | Collisions where objects stick and move together |
| Univ. | Fluid dynamics / biophysics | Droplet deformation and hydrophobicity |
| Research | Insect flight and environmental adaptation | Real-world conditions, righting behaviour |
| ― | Everyday connection | Why mosquitoes come back after rain |
- 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 rough estimates meant to help you understand the underlying mechanism. Actual values vary with rain intensity, raindrop size, and mosquito species.