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  • 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 forhappy”. 

  • 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 calledpressure 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 surfaceThat 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 “straightpath, 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 equatorour 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 meteorologistwho 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 thesehorse

  • latitudesto 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