How Can a Dandelion Seed Fly So Far?
― The Secret of the Invisible Ring of Air
Blow on a dandelion seed head and the seeds float up into the air and drift off on the wind, seemingly forever. Look closely and each one is shaped like an umbrella. But it has almost no fabric, just fine hairs spread out with gaps between them. With such a flimsy structure, how does it fall so slowly and travel so far?
Pick up a dandelion seed and look closely. It has a single thin stalk topped by about 100 hairs fanning out in all directions, like a parachute. But unlike the cloth of a parachute, it has almost no solid surface to block the air.
You would expect a structure this full of gaps to catch very little air and drop straight to the ground. In fact, a dandelion seed drifts down incredibly slowly and steadily.
Recent research has shown that the secret of this odd flight lies in an invisible ring of swirling air that forms just above the seed.
The dandelion's fluff is said to be about 90% gaps by area, an extremely sparse structure.
When air passes through the gaps, it has been confirmed that a stable ring of swirling air forms a short distance from the seed.
Let's look step by step at how this ring of swirling air acts as a brake.
Why Can It Float So Well When It's Full of Gaps?
The dandelion's fluff is said to be about 90% gaps by area, a strikingly sparse structure. If it were a solid disc with no gaps, the airflow around it would become unstable as it fell, and the vortex would keep attaching to the disc and detaching again, making it wobble.
But with just the right amount of gaps, as in the dandelion's fluff, some of the air passes through the gaps as the seed falls, and that changes how the vortex forms. Research has confirmed that the vortex stays put, stable, a short distance from the seed.
By letting air pass through, it floats a second, invisible umbrella above itself.
How the Ring of Swirling Air Works as a Brake
This stable ring floating just above the seed is known technically as a "separated vortex ring." It does not touch the seed itself, but it is thought to change how air flows around the seed and greatly increase the effective air resistance. One study reported that a seed's fluff of the same size produces nearly four times as much drag, for its size, as a solid disc. Thanks to this efficiency, the seed uses very little material and yet can fall extremely slowly.
The slower something falls, the longer it takes to reach the ground. During that time the wind keeps blowing it along, so it tends to be carried farther. For a dandelion, falling slowly is itself a strategy for carrying its seed a long way.
Try It Yourself
- Get one dandelion seed (a real one if it's the season, or something fluffy with a similar structure if not)
- Make a small paper ball that weighs about the same as the seed
- Gently drop both at the same time from the same height
- Compare which one falls more slowly, and by how much
You can feel for yourself that, even at the same weight, the speed of the fall changes a lot with shape and with how open the gaps are.
Summary
A dandelion seed has a sparse structure, about 90% gaps by area. Air passing through these gaps creates a stable ring of swirling air a little way from the seed, which is thought to brake the fall like a second, invisible umbrella. Thanks to this efficient brake, the seed falls very slowly, stays in the wind for longer, and so can carry its seed far away.
The secret of the dandelion's flight lies not in the seed itself, but in the invisible ring it creates in the air.
For Those Who Want to Know More ― Terms, Numbers and Links to TextbooksFrom middle-school science to topics still under research, with each part's level clearly marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school basic physics
- High school+High-school full physics, or advanced or sidebar material in textbooks
- UniversityUniversity-level specialist content (fluid dynamics) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught even at university as settled fact
Middle schoolTerms: Words for Dandelion Seeds
- Drag: the force that acts against the direction of motion when an object moves through air or water.
- Terminal velocity: the falling speed at which drag and gravity balance, so the object stops getting faster.
- Vortex: air or water flowing while spinning in a circle.
High schoolChecking with Equations: Fall Time and Distance Carried by the Wind
First we find the main quantity, the seed's falling speed (terminal velocity). Then we calculate the fall time and the distance the wind carries it.
| In symbols | v ＝ √( 2 × m × g ÷ ( ρ × C × A ) ) |
| In words | Falling speed ＝ the square root of (twice the weight, divided by the product of air density, drag coefficient and area) |
| Where it comes from | It is the balance point between downward gravity and upward air resistance (m × g ＝ 0.5 × ρ × C × A × v²), solved for speed. Because the forces balance, there is no further acceleration. |
| Mass of one seed with its fluff, m | 0.0000006 kg (about 0.6 mg), it is said |
| Gravitational acceleration, g | 9.8 (metres per second per second) |
| Air density, ρ | 1.2 (kilograms per cubic metre) |
| Drag coefficient, C | About 1. Despite all the gaps, the vortex ring is said to bring it close to the value for a solid disc |
| Area of the fluff, A | 0.000113 square metres (a circle 12 mm across) |
| Force of gravity (newtons) | 0.0000006 × 9.8 ≒ 0.0000059 |
| Twice that (newtons) | 2 × 0.0000059 ＝ 0.0000118 |
| Denominator ρ × A (C is 1) | 1.2 × 0.000113 ≒ 0.000136 |
| Speed squared | 0.0000118 ÷ 0.000136 ≒ 0.087 |
| Its square root is the falling speed | The square root of 0.087 is about 0.29 (0.29 metres per second) |
Roughly 0.3 metres per second. That is about one-twentieth of walking speed, and it is of the same order as the 0.5 metres per second used as a rough figure in ② below.
Fall time (s) ＝ drop height (m) ÷ falling speed (m/s)
Distance carried (m) ＝ wind speed (m/s) × fall time (s)
| Fall time (s) | 1.5 ÷ 0.5 ＝ 3 |
| Distance carried (m) | 2 × 3 ＝ 6 |
| Result | Carried about 6 m |
| Fall time (s) | 1.5 ÷ 2 ＝ 0.75 |
| Distance carried (m) | 2 × 0.75 ＝ 1.5 |
| Result | Carried only about 1.5 m |
In the same wind, cutting the falling speed to a quarter makes the distance carried four times as long. This shows how much the slow fall produced by the vortex ring helps to carry the seed far.
* The falling speeds and wind speeds are rough values chosen to keep the calculation easy to follow. Real values are said to vary with the condition of the seed and the weather.
High school+The Relationship Between Drag and Terminal Velocity
When an object falls through air, the faster it goes, the larger the drag acting against its motion. Once the drag equals the force of gravity, the object stops accelerating and keeps falling at a constant speed (the terminal velocity). For a dandelion seed, where the vortex ring greatly increases drag, the terminal velocity itself is thought to be very small.
UniversityThe Fluid Phenomenon Called a "Separated Vortex Ring"
In fluid dynamics, researchers study how a stable vortex ring (a separated vortex ring) forms at a distance from a porous, disc-shaped object. Only when the disc's proportion of gaps (its porosity) lies within a certain range does this vortex stay in place, and experiments and numerical simulations have confirmed that it raises drag efficiently. Whether this vortex appears is decided by the Reynolds number, a dimensionless quantity set by the ratio of flow speed and object size to viscosity. The Reynolds number of a dandelion seed is said to lie roughly between 10 and 100, and only in this range, where viscous effects and inertial effects are evenly matched, does the vortex ring stay stable at a distance.
📖 Derivations and further reading: Reynolds number (Japanese Wikipedia)
ResearchWhat Is Still Unclear
- The detailed conditions of porosity and hair arrangement that keep the vortex ring stable are still being studied in fluid dynamics.
- Researchers are also comparing whether the fluff or seeds of plants other than dandelions use a similar mechanism, or have other flight tricks.
- Some research is trying to apply this discovery to develop very light new parachutes and falling sensor devices that get efficient air resistance from very little material.
Even the common dandelion seed hides a rich, still-active research topic where fluid dynamics meets ecology.
Links to Textbooks (by Level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science: how forces act | Basic terms: drag, terminal velocity, vortex |
| High school | Basic physics: force and motion | Calculating fall time and distance carried by the wind |
| High school+ | Physics: drag and terminal velocity (advanced) | The balance of drag and gravity |
| University | Fluid dynamics | Separated vortex rings and porosity |
| Research | Fluid dynamics and plant ecology (under research) | Stability conditions, comparison with other plants, applied technology |
- A research review on separated vortex rings around porous discs, from the field of fluid dynamics.
- An explanation of dandelion seed dispersal strategy, from plant ecology sources.
- An explanation of drag and terminal velocity, from physics textbooks.
- A comparison of the structures of wind-dispersed seeds, from biology sources.
- An explanation of applied research on biomimetic (nature-inspired) technology, from the engineering field.
* Figures such as falling speed and drag efficiency are rough values reported in research. They are said to vary with individual seeds and weather conditions.
* This article is a general-audience science explainer. When observing or collecting plants, check the rules of the place you are in and take care of the surrounding natural environment.