Where Does Wind Come From? Crash Course Geography #8

There's an invisible force shaping our lives,
affecting the weather, climate, land, economy,

and whether a flag looks majestic or just
kind of... sits there. I'm talking, of course, about the wind.

Large parts of the globe are brought warmth
and water thanks to wind. In Europe, wind

energy is one of the most popular renewable
energies, thanks to wind turbines that harness its power.

Ships with sails have followed the path of
the wind for centuries, bringing trade and

entire empires along with them.

Fierce winds can also bring destruction, stripping
soil away from the ground or even ripping

apart buildings.

Trying to protect ourselves from the wind
might feel like we're battling an imaginary foe.

But wind is definitely not imaginary -- geographers
have defined it and have tools to measure

it! Whether it's a gentle sea breeze or gale-force
gusts, wind is any horizontal movement of

air. And air is a mixture of nitrogen, oxygen,
and other gases that blend together so well,

they tend to act as one.

Winds are named based on what direction they
come from, and some people are even named

after winds! My name, Alizé, means the northeasterly
trade winds in French -- or les vents Alizés,

the Alizé winds. With a French sailor for
a father who used to love sailing the warm

northeasterly trade winds, it’s no surprise
where this came from!

So let's get deeper into the science of where
wind comes from -- it’ll be a whirlwind

of an adventure.

I’m Alizé Carrère and this is Crash Course
Geography.

INTRO

If we zoom out to look at the globe as a whole,
we can see that there are global wind patterns

just like there are global air temperature
patterns. And these are intimately linked.

We know that insolation from the Sun doesn’t
get distributed evenly and ends up heating

places differently. The temperature of a place
is tied to several key factors like latitude,

elevation, how far it is from the ocean or
sea, and even what type of surface it is and

how much of the Sun’s energy it absorbs.

No matter where we are though, air that’s
warm is lighter, less dense, and tends to

rise. Cool air, on the other hand, is heavier,
more dense, and tends to sink.

And you did hear me correctly -- there's lighter
air and heavier air because air molecules

all have weight. Not a lot, but still weight.
The weight of air then leads to atmospheric

pressure, which comes from all the air above
that's pressing down on whatever air there is below.

So the pressure is much higher where I’m
standing in Miami than if we were filming

this close to outer space. Down here, there’s
all 480 kilometers of atmosphere squishing

down on us. In fact, it’s likely close to
standard sea level pressure -- which is exactly

what it sounds like: the average atmospheric
pressure at sea level.

We don’t crumple like aluminum cans under
this enormous pressure because the air and

water inside us exert an equal amount of pressure
outwards. And the exact atmospheric pressure

in other places will be different depending
on where we are, the season, or even the time of day.

Wind is actually the atmosphere’s way of
smoothing out pressure differences, which

can be created by the daily and seasonal air
temperature patterns across Earth’s surface.

Meteorologists, who study the atmosphere,
use air pressure measurements to forecast

the weather. Like, a weather report on TV
might show a map full of H’s and L’s,

which is actually a map tracking air pressure.

A giant L stands for low pressure, or a low.
On a global scale, a low is an area where

the pressure near the surface is less than
standard sea level pressure. But on a local

scale like on your local weather report, a
low can also be an area where the pressure

is less than in the surrounding area because
there’s actually slightly less air pressing

down on that part of the Earth.

Lows go by lots of names. Like you might hear
it called a depression or even a cyclone.

Though it’s not the giant spinning vortex
of air we might think of -- that’s a specific

weather event that only forms in tropical
oceans. But we’ll come back to that in upcoming

episodes.

To keep it simple, we’ll just call it a
low. Lows exist either because air is being

heated and expands up and out, or air higher
up in the atmosphere is spreading out, so

there’s less air pressing down on Earth’s
surface.

Down on the ground, we might even be able
to tell we’re in a low. As air expands and

rises, winds are drawn towards the center.
The rising air cools, and moisture in the

air condenses into droplets. So if we happen
to be in the center of a low, the weather

would often be pretty cloudy and rainy.

The giant H’s on the map mark high pressure
areas, which we call a high or anticyclone.

In a high pressure cell, either the air is
cooling and becoming denser, so it sinks,

or the atmosphere high above is piling up,
pushing the air below it downward.

Sinking compresses air molecules together
and makes them warm. So any water vapor in

the air won’t cool to condense into liquid
water. That means high pressure systems bring

weather that’s clear and sunny, which

I remember as H stands for “happy”.

High and low pressure cells are usually large
-- like they can be 1000 kilometers across.

And air moving between these vast areas to
balance out energy in the atmosphere helps

us understand and identify the winds. The
key is the difference or change in pressure

between highs and lows, which is called a 
pressure gradient. Like any fluid, air wants

to flow from high to low pressure.

Let’s start on a small scale, and look at
an island. When the beaches and land warm

up faster during the day than the surrounding
sea, the air over the island expands, rises,

and lowers the pressure at the surface. 
That leaves room for air from the sea to rush

onto the land, and voilà -- any windsurfer
or sun tanner will get a cool sea breeze in

the afternoon.

And similar things happen at a bigger scale
across the globe! Air at the equator is consistently

warmed by the Sun and tends to expand and
rise, so we get a belt of low pressure around

the Earth called the equatorial trough. And
we’d expect the poles to experience high

pressure, because the air there is cold and
sinking.

But winds don’t just blow north and south.
This is because the Earth rotates. To see

what really happens to these winds, let’s
imagine we’re flying an airplane from the

North Pole to the South Pole, with a layover
in Ecuador on the equator. Let’s go to the Thought Bubble.

Hello this is Captain Carrère speaking.

If you look out the windows, you’ll see
the surface of the Earth slowly rotating eastwards.

So in order to stay on a “straight” path,
we have to constantly make little turns.

This phenomenon that causes moving objects
-- like our plane or air or water -- to

seem like they curve as they travel over the
rotating Earth is known as the Coriolis effect.

The Earth is rotating beneath our plane, but
also as we travel towards the equator, the

Earth actually rotates faster because the
Earth is bigger at the equator and it has

to move faster to keep up.

It’s like a marching band turning a corner
-- if they want to stay together in a straight

line, the marchers on the inside of the circle
take much smaller steps and move slower than

the marchers on the outside.

So if we’re at the poles, we’d just kind
of spin in place.

But as latitude decreases, our rotational
speed increases until we get to the equator

and the Earth’s surface practically zooms
by at 1600 kilometers per hour -- which is

about twice as fast as our plane.

Then as our plane gets closer and closer to
Ecuador and the equator, our rotational momentum

comes from the slow speeds at the North Pole,
not the rapidly rotating equator.

Which means we end up getting deflected to
the right into the Pacific Ocean and have

to make little left turns to get to Ecuador.

Something similar happens on our second flight
toward the South Pole.

But this time we started out rotating faster
than our final destination.

So as we make our final approach to the South
Sandwich Islands we’d get deflected left

and end up east of where we want to be if
we didn’t correct.

Please make sure your seatbelts are fastened
and your tray tables are stowed as we prepare for landing!

Thanks, Thought Bubble. In general, the Coriolis
effect deflects objects to the right in the

Northern Hemisphere and to the left in the
Southern Hemisphere.

Which is how we get those wind spirals around
the low and high pressures areas on our weather

map, and why they’re also called cyclones
and anticyclones. The air wants to rush directly

from the center of the high to the center
of the low but gets deflected.

So in our model, the heated air at the equator
first rises upward towards the tropopause,

which is the boundary between the troposphere
and the stratosphere, as it tries to move

poleward high up in the atmosphere.

Then as it moves away from the equator, the
Coriolis effect causes air traveling northwards

to turn right, speeding faster east the further
north it gets. The air is also cooling, and

by the time it sinks back to the surface,
it’s only reached around 30 degrees latitude.

So instead of one big circulation cycle, as
proposed by George Hadley, an English lawyer

and amateur meteorologist, who first described
it in 1735, we get a more complicated circulation

system containing the Hadley cell.

Hadley wanted to understand why surface winds
that should have blown straight south towards

the equator -- along the pressure gradient
from high pressure to low pressure -- took

a turn west. Solving that mystery would help
ensure European trading ships would safely

reach the shores -- and goods -- of the Americas.

This isn’t the first time our understanding
of the winds has gone hand in hand with exploration,

and trade, wealth, and power were driven by
the winds. For instance, new technologies

created in the 1400s like the quadrant and
the astrolabe enabled accurate navigation

and mapping of ocean currents, winds, and
trade routes.

Over the years many more scientific minds
have explored the implications of Hadley’s

theory, and we’re still learning more as
we explore the movement of energy between

the atmosphere and biosphere.

We know now that in reality, air in both hemispheres
converges in the narrow band around the equator

called the intertropical convergence zone and rises.

The surface winds, or doldrums, that form
here as the air converges and rises upwards

are light and not super reliable. Sailing
ships could get stuck in the doldrums for days.

Similarly weak winds are found on the poleward
edges of the Hadley cells, where air is being

forced down, creating high pressure zones
centered at about 30 degrees latitude called

the subtropical high pressure belts.

Sailors of yore were often forced to eat their
horses or throw them overboard in these “horse

latitudes” to conserve drinking water and
lighten the weight while the sailing ships

waited for the weak winds at the center of
these highs to pick up. [Wow, that’s pretty dark.]

In between these high and low pressure belts,

there are strong and reliable winds spiraling
outwards from the subtropical high pressure

belt towards the equator. These are the easterly
Trade Winds -- and they’re my favorite winds, obviously!

Many ships have depended on the trade winds,
like early Spanish sailing ships as they sought

God, glory, and gold in what we now call Central
and South America.

Of course, making the return trip was another
matter. The ancient mariners of the Spanish

galleons going home from the Americas plotted
a course using the winds blowing poleward

from the subtropical high pressure belt. These
Westerlies are strongly deflected to the right

and blow from the southwest.

These strong winds blow towards another low
pressure belt called the subpolar lows where

they clash with the polar Easterlies blowing
from the frigid, very high pressure poles.

In the Southern Hemisphere, they blow with
greater strength as there’s very little

land in these latitudes to interrupt their
flow.

So altogether, on our idealized Earth we’ve
seen that there are actually seven pressure

belts: two polar highs, two subpolar lows,
two subtropical highs and one equatorial low.

And winds flow between these belts of high
and low pressure.

On the real Earth, the belts are not so organized.
They form cells of pressure and we see more

complex patterns of pressure and wind, as
the cells shift with the seasons and vary

between land and water. So our idealized Earth
is kind of like a wind and pressure map. It’s

a simplified model that helps us understand
what’s happening on the real Earth.

Just like the atmosphere works like a cell
membrane, the winds are like Earth’s circulatory

system. So many things vital to our planet
flow through the winds.

During the voyages of discovery in the 15th
to 18th centuries -- which we now recognize

weren’t really discoveries at all -- the
knowledge of winds, ocean currents, natural

harbors and more was an essential foundation
for circumnavigating the globe.

And we continue to rely on the winds to power
our economies. As a renewable energy source,

this silent force will continue to shape our
lives in the future. I hope wherever you are

is in the center of a sunny high pressure
area which will be perfect weather to go with

the flow in the ocean, which we’ll talk
about next week.

Many maps and borders represent modern geopolitical
divisions that have often been decided without

the consultation, permission, or recognition
of the land's original inhabitants. Many geographical

place names also don't reflect the Indigenous
or Arboriginal peoples languages.

So we at Crash Course want to acknowledge
these peoples’ traditional and ongoing relationship

with that land and all the physical and human
geographical elements of it.

We encourage you to learn about the history
of the place you call home through resources

like native-land.ca and by engaging with your
local Indigenous and Aboriginal nations through

the websites and resources they provide.

Thanks for watching this episode of Crash
Course Geography. If you want to help keep

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