You’re on an airplane 
when you feel a sudden jolt.

Outside your window nothing 
seems to be happening,

yet the plane continues to rattle 
you and your fellow passengers

as it passes through turbulent air
in the atmosphere.

Although it may not comfort 
you to hear it,

this phenomenon is one of the 
prevailing mysteries of physics.

After more than a century 
of studying turbulence,

we’ve only come up with a few 
answers for how it works

and affects the world around us.

And yet, turbulence is ubiquitous,

springing up in virtually any system 
that has moving fluids.

That includes the airflow 
in your respiratory tract.

The blood moving through your arteries.

And the coffee in your cup, 
as you stir it.

Clouds are governed by turbulence,

as are waves crashing along the shore 
and the gusts of plasma in our sun.

Understanding precisely how this 
phenomenon works

would have a bearing on so many 
aspects of our lives.

Here’s what we do know.

Liquids and gases usually have 
two types of motion:

a laminar flow, which is stable 
and smooth;

and a turbulent flow, which is composed 
of seemingly unorganized swirls.

Imagine an incense stick.

The laminar flow of unruffled smoke 
at the base is steady and easy to predict.

Closer to the top, however,

the smoke accelerates, becomes unstable,

and the pattern of movement changes
to something chaotic.

That’s turbulence in action,

and turbulent flows have certain
characteristics in common.

Firstly, turbulence is always chaotic.

That’s different from being random.

Rather, this means that turbulence 
is very sensitive to disruptions.

A little nudge one way or the other

will eventually turn into 
completely different results.

That makes it nearly impossible 
to predict what will happen,

even with a lot of information 
about the current state of a system.

Another important characteristic of 
turbulence

is the different scales of motion 
that these flows display.

Turbulent flows have many 
differently-sized whirls called eddies,

which are like vortices of 
different sizes and shapes.

All those differently-sized eddies 
interact with each other,

breaking up to become smaller and smaller

until all that movement is 
transformed into heat,

in a process called the “energy cascade."

So that’s how we recognize turbulence–

but why does it happen?

In every flowing liquid or gas there 
are two opposing forces:

inertia and viscosity.

Inertia is the tendency of fluids 
to keep moving,

which causes instability.

Viscosity works against disruption,

making the flow laminar instead.

In thick fluids such as honey,

viscosity almost always wins.

Less viscous substances like water or air 
are more prone to inertia,

which creates instabilities that 
develop into turbulence.

We measure where a flow falls 
on that spectrum

with something called the Reynolds number,

which is the ratio between a flow’s 
inertia and its viscosity.

The higher the Reynolds number,

the more likely it is that 
turbulence will occur.

Honey being poured into a cup, 
for example,

has a Reynolds number of about 1.

The same set up with water has a Reynolds 
number that’s closer to 10,000.

The Reynolds number is useful for 
understanding simple scenarios,

but it’s ineffective in many situations.

For example, the motion of the atmosphere
is significantly influenced

by factors including gravity and the 
earth’s rotation.

Or take relatively simple things 
like the drag on buildings and cars.

We can model those thanks to many 
experiments and empirical evidence.

But physicists want to be able to predict 
them through physical laws and equations

as well as we can model the orbits 
of planets or electromagnetic fields.

Most scientists think that getting there 
will rely on statistics

and increased computing power.

Extremely high-speed computer simulations 
of turbulent flows

could help us identify patterns that could
lead to a theory

that organizes and unifies predictions
across different situations.

Other scientists think that the phenomenon
is so complex

that such a full-fledged theory 
isn’t ever going to be possible.

Hopefully we’ll reach a breakthrough,

because a true understanding of turbulence
could have huge positive impacts.

That would include more 
efficient wind farms;

the ability to better prepare for 
catastrophic weather events;

or even the power to manipulate 
hurricanes away.

And, of course, smoother rides 
for millions of airline passengers.