WEBVTT 00:00:00.089 --> 00:00:04.860 There's an invisible force shaping our lives, affecting the weather, climate, land, economy, 00:00:04.860 --> 00:00:10.710 and whether a flag looks majestic or just kind of... sits there. I'm talking, of course, about the wind. 00:00:10.710 --> 00:00:14.251 Large parts of the globe are brought warmth and water thanks to wind. In Europe, wind 00:00:14.251 --> 00:00:18.862 energy is one of the most popular renewable energies, thanks to wind turbines that harness its power. 00:00:18.862 --> 00:00:22.170 Ships with sails have followed the path of the wind for centuries, bringing trade and 00:00:22.170 --> 00:00:24.010 entire empires along with them. 00:00:24.010 --> 00:00:27.750 Fierce winds can also bring destruction, stripping soil away from the ground or even ripping 00:00:27.750 --> 00:00:28.860 apart buildings. 00:00:28.860 --> 00:00:32.397 Trying to protect ourselves from the wind might feel like we're battling an imaginary foe. 00:00:32.397 --> 00:00:36.970 But wind is definitely not imaginary -- geographers have defined it and have tools to measure 00:00:36.970 --> 00:00:42.059 it! Whether it's a gentle sea breeze or gale-force gusts, wind is any horizontal movement of 00:00:42.059 --> 00:00:46.829 air. And air is a mixture of nitrogen, oxygen, and other gases that blend together so well, 00:00:46.829 --> 00:00:48.329 they tend to act as one. 00:00:48.329 --> 00:00:51.829 Winds are named based on what direction they come from, and some people are even named 00:00:51.829 --> 00:00:57.280 after winds! My name, Alizé, means the northeasterly trade winds in French -- or les vents Alizés, 00:00:57.280 --> 00:01:01.260 the Alizé winds. With a French sailor for a father who used to love sailing the warm 00:01:01.260 --> 00:01:04.309 northeasterly trade winds, it’s no surprise where this came from! 00:01:04.309 --> 00:01:08.670 So let's get deeper into the science of where wind comes from -- it’ll be a whirlwind 00:01:08.670 --> 00:01:09.670 of an adventure. 00:01:09.670 --> 00:01:12.274 I’m Alizé Carrère and this is Crash Course Geography. 00:01:12.274 --> 00:01:19.979 INTRO 00:01:19.979 --> 00:01:23.509 If we zoom out to look at the globe as a whole, we can see that there are global wind patterns 00:01:23.509 --> 00:01:26.979 just like there are global air temperature patterns. And these are intimately linked. 00:01:26.979 --> 00:01:30.849 We know that insolation from the Sun doesn’t get distributed evenly and ends up heating 00:01:30.849 --> 00:01:35.060 places differently. The temperature of a place is tied to several key factors like latitude, 00:01:35.060 --> 00:01:39.469 elevation, how far it is from the ocean or sea, and even what type of surface it is and 00:01:39.469 --> 00:01:41.299 how much of the Sun’s energy it absorbs. 00:01:41.299 --> 00:01:45.329 No matter where we are though, air that’s warm is lighter, less dense, and tends to 00:01:45.329 --> 00:01:49.700 rise. Cool air, on the other hand, is heavier, more dense, and tends to sink. 00:01:49.700 --> 00:01:53.509 And you did hear me correctly -- there's lighter air and heavier air because air molecules 00:01:53.509 --> 00:01:58.200 all have weight. Not a lot, but still weight. The weight of air then leads to atmospheric 00:01:58.200 --> 00:02:03.094 pressure, which comes from all the air above that's pressing down on whatever air there is below. 00:02:03.094 --> 00:02:06.239 So the pressure is much higher where I’m standing in Miami than if we were filming 00:02:06.239 --> 00:02:11.400 this close to outer space. Down here, there’s all 480 kilometers of atmosphere squishing 00:02:11.400 --> 00:02:15.599 down on us. In fact, it’s likely close to standard sea level pressure -- which is exactly 00:02:15.599 --> 00:02:18.590 what it sounds like: the average atmospheric pressure at sea level. 00:02:18.590 --> 00:02:22.269 We don’t crumple like aluminum cans under this enormous pressure because the air and 00:02:22.269 --> 00:02:26.659 water inside us exert an equal amount of pressure outwards. And the exact atmospheric pressure 00:02:26.659 --> 00:02:31.159 in other places will be different depending on where we are, the season, or even the time of day. 00:02:31.159 --> 00:02:34.629 Wind is actually the atmosphere’s way of smoothing out pressure differences, which 00:02:34.629 --> 00:02:38.709 can be created by the daily and seasonal air temperature patterns across Earth’s surface. 00:02:38.709 --> 00:02:42.459 Meteorologists, who study the atmosphere, use air pressure measurements to forecast 00:02:42.459 --> 00:02:46.629 the weather. Like, a weather report on TV might show a map full of H’s and L’s, 00:02:46.629 --> 00:02:48.430 which is actually a map tracking air pressure. 00:02:48.430 --> 00:02:53.659 A giant L stands for low pressure, or a low. On a global scale, a low is an area where 00:02:53.659 --> 00:02:57.380 the pressure near the surface is less than standard sea level pressure. But on a local 00:02:57.380 --> 00:03:01.060 scale like on your local weather report, a low can also be an area where the pressure 00:03:01.060 --> 00:03:04.700 is less than in the surrounding area because there’s actually slightly less air pressing 00:03:04.700 --> 00:03:06.510 down on that part of the Earth. 00:03:06.510 --> 00:03:10.269 Lows go by lots of names. Like you might hear it called a depression or even a cyclone. 00:03:10.269 --> 00:03:13.959 Though it’s not the giant spinning vortex of air we might think of -- that’s a specific 00:03:13.959 --> 00:03:17.749 weather event that only forms in tropical oceans. But we’ll come back to that in upcoming 00:03:17.749 --> 00:03:18.749 episodes. 00:03:18.749 --> 00:03:22.260 To keep it simple, we’ll just call it a low. Lows exist either because air is being 00:03:22.260 --> 00:03:26.780 heated and expands up and out, or air higher up in the atmosphere is spreading out, so 00:03:26.780 --> 00:03:29.030 there’s less air pressing down on Earth’s surface. 00:03:29.030 --> 00:03:32.859 Down on the ground, we might even be able to tell we’re in a low. As air expands and 00:03:32.859 --> 00:03:36.470 rises, winds are drawn towards the center. The rising air cools, and moisture in the 00:03:36.470 --> 00:03:40.210 air condenses into droplets. So if we happen to be in the center of a low, the weather 00:03:40.210 --> 00:03:41.930 would often be pretty cloudy and rainy. 00:03:41.930 --> 00:03:45.890 The giant H’s on the map mark high pressure areas, which we call a high or anticyclone. 00:03:45.890 --> 00:03:50.560 In a high pressure cell, either the air is cooling and becoming denser, so it sinks, 00:03:50.560 --> 00:03:54.099 or the atmosphere high above is piling up, pushing the air below it downward. 00:03:54.099 --> 00:03:57.969 Sinking compresses air molecules together and makes them warm. So any water vapor in 00:03:57.969 --> 00:04:02.079 the air won’t cool to condense into liquid water. That means high pressure systems bring 00:04:02.079 --> 00:04:03.599 weather that’s clear and sunny, which 00:04:03.599 --> 00:04:05.520 I remember as H stands for “happy”. 00:04:05.520 --> 00:04:10.260 High and low pressure cells are usually large -- like they can be 1000 kilometers across. 00:04:10.260 --> 00:04:13.900 And air moving between these vast areas to balance out energy in the atmosphere helps 00:04:13.900 --> 00:04:17.930 us understand and identify the winds. The key is the difference or change in pressure 00:04:17.930 --> 00:04:22.590 between highs and lows, which is called a  pressure gradient. Like any fluid, air wants 00:04:22.590 --> 00:04:24.110 to flow from high to low pressure. 00:04:24.110 --> 00:04:27.500 Let’s start on a small scale, and look at an island. When the beaches and land warm 00:04:27.500 --> 00:04:32.169 up faster during the day than the surrounding sea, the air over the island expands, rises, 00:04:32.169 --> 00:04:35.540 and lowers the pressure at the surface.  That leaves room for air from the sea to rush 00:04:35.540 --> 00:04:39.520 onto the land, and voilà -- any windsurfer or sun tanner will get a cool sea breeze in 00:04:39.520 --> 00:04:40.520 the afternoon. 00:04:40.520 --> 00:04:44.670 And similar things happen at a bigger scale across the globe! Air at the equator is consistently 00:04:44.670 --> 00:04:48.639 warmed by the Sun and tends to expand and rise, so we get a belt of low pressure around 00:04:48.639 --> 00:04:52.740 the Earth called the equatorial trough. And we’d expect the poles to experience high 00:04:52.740 --> 00:04:54.819 pressure, because the air there is cold and sinking. 00:04:54.819 --> 00:04:59.150 But winds don’t just blow north and south. This is because the Earth rotates. To see 00:04:59.150 --> 00:05:02.509 what really happens to these winds, let’s imagine we’re flying an airplane from the 00:05:02.509 --> 00:05:07.400 North Pole to the South Pole, with a layover in Ecuador on the equator. Let’s go to the Thought Bubble. 00:05:07.400 --> 00:05:08.979 Hello this is Captain Carrère speaking. 00:05:08.979 --> 00:05:13.479 If you look out the windows, you’ll see the surface of the Earth slowly rotating eastwards. 00:05:13.479 --> 00:05:17.580 So in order to stay on a “straight” path, we have to constantly make little turns. 00:05:17.580 --> 00:05:21.870 This phenomenon that causes moving objects -- like our plane or air or water -- to 00:05:21.870 --> 00:05:26.110 seem like they curve as they travel over the rotating Earth is known as the Coriolis effect. 00:05:26.110 --> 00:05:30.550 The Earth is rotating beneath our plane, but also as we travel towards the equator, the 00:05:30.550 --> 00:05:34.400 Earth actually rotates faster because the Earth is bigger at the equator and it has 00:05:34.400 --> 00:05:35.569 to move faster to keep up. 00:05:35.569 --> 00:05:39.569 It’s like a marching band turning a corner -- if they want to stay together in a straight 00:05:39.569 --> 00:05:43.540 line, the marchers on the inside of the circle take much smaller steps and move slower than 00:05:43.540 --> 00:05:44.690 the marchers on the outside. 00:05:44.690 --> 00:05:47.770 So if we’re at the poles, we’d just kind of spin in place. 00:05:47.770 --> 00:05:51.840 But as latitude decreases, our rotational speed increases until we get to the equator 00:05:51.840 --> 00:05:57.000 and the Earth’s surface practically zooms by at 1600 kilometers per hour -- which is 00:05:57.000 --> 00:05:59.541 about twice as fast as our plane. 00:05:59.541 --> 00:06:03.479 Then as our plane gets closer and closer to Ecuador and the equator, our rotational momentum 00:06:03.479 --> 00:06:07.039 comes from the slow speeds at the North Pole, not the rapidly rotating equator. 00:06:07.039 --> 00:06:11.039 Which means we end up getting deflected to the right into the Pacific Ocean and have 00:06:11.039 --> 00:06:12.760 to make little left turns to get to Ecuador. 00:06:12.760 --> 00:06:16.330 Something similar happens on our second flight toward the South Pole. 00:06:16.330 --> 00:06:19.639 But this time we started out rotating faster than our final destination. 00:06:19.639 --> 00:06:24.409 So as we make our final approach to the South Sandwich Islands we’d get deflected left 00:06:24.409 --> 00:06:27.140 and end up east of where we want to be if we didn’t correct. 00:06:27.140 --> 00:06:31.831 Please make sure your seatbelts are fastened and your tray tables are stowed as we prepare for landing! 00:06:31.831 --> 00:06:36.000 Thanks, Thought Bubble. In general, the Coriolis effect deflects objects to the right in the 00:06:36.000 --> 00:06:38.560 Northern Hemisphere and to the left in the Southern Hemisphere. 00:06:38.560 --> 00:06:42.099 Which is how we get those wind spirals around the low and high pressures areas on our weather 00:06:42.099 --> 00:06:46.759 map, and why they’re also called cyclones and anticyclones. The air wants to rush directly 00:06:46.759 --> 00:06:49.949 from the center of the high to the center of the low but gets deflected. 00:06:49.949 --> 00:06:54.229 So in our model, the heated air at the equator first rises upward towards the tropopause, 00:06:54.229 --> 00:06:57.240 which is the boundary between the troposphere and the stratosphere, as it tries to move 00:06:57.240 --> 00:06:59.160 poleward high up in the atmosphere. 00:06:59.160 --> 00:07:02.930 Then as it moves away from the equator, the Coriolis effect causes air traveling northwards 00:07:02.930 --> 00:07:07.759 to turn right, speeding faster east the further north it gets. The air is also cooling, and 00:07:07.759 --> 00:07:11.430 by the time it sinks back to the surface, it’s only reached around 30 degrees latitude. 00:07:11.430 --> 00:07:15.509 So instead of one big circulation cycle, as proposed by George Hadley, an English lawyer 00:07:15.509 --> 00:07:20.470 and amateur meteorologist, who first described it in 1735, we get a more complicated circulation 00:07:20.470 --> 00:07:22.460 system containing the Hadley cell. 00:07:22.460 --> 00:07:25.780 Hadley wanted to understand why surface winds that should have blown straight south towards 00:07:25.780 --> 00:07:29.189 the equator -- along the pressure gradient from high pressure to low pressure -- took 00:07:29.189 --> 00:07:33.449 a turn west. Solving that mystery would help ensure European trading ships would safely 00:07:33.449 --> 00:07:36.210 reach the shores -- and goods -- of the Americas. 00:07:36.210 --> 00:07:39.830 This isn’t the first time our understanding of the winds has gone hand in hand with exploration, 00:07:39.830 --> 00:07:43.669 and trade, wealth, and power were driven by the winds. For instance, new technologies 00:07:43.669 --> 00:07:47.949 created in the 1400s like the quadrant and the astrolabe enabled accurate navigation 00:07:47.949 --> 00:07:50.449 and mapping of ocean currents, winds, and trade routes. 00:07:50.449 --> 00:07:54.289 Over the years many more scientific minds have explored the implications of Hadley’s 00:07:54.289 --> 00:07:57.250 theory, and we’re still learning more as we explore the movement of energy between 00:07:57.250 --> 00:07:58.860 the atmosphere and biosphere. 00:07:58.860 --> 00:08:03.439 We know now that in reality, air in both hemispheres converges in the narrow band around the equator 00:08:03.439 --> 00:08:06.227 called the intertropical convergence zone and rises. 00:08:06.227 --> 00:08:10.090 The surface winds, or doldrums, that form here as the air converges and rises upwards 00:08:10.090 --> 00:08:14.830 are light and not super reliable. Sailing ships could get stuck in the doldrums for days. 00:08:14.830 --> 00:08:18.439 Similarly weak winds are found on the poleward edges of the Hadley cells, where air is being 00:08:18.439 --> 00:08:22.220 forced down, creating high pressure zones centered at about 30 degrees latitude called 00:08:22.220 --> 00:08:24.440 the subtropical high pressure belts. 00:08:24.440 --> 00:08:28.669 Sailors of yore were often forced to eat their horses or throw them overboard in these “horse 00:08:28.669 --> 00:08:32.410 latitudes” to conserve drinking water and lighten the weight while the sailing ships 00:08:32.410 --> 00:08:37.160 waited for the weak winds at the center of these highs to pick up. [Wow, that’s pretty dark.] 00:08:37.160 --> 00:08:39.730 In between these high and low pressure belts, 00:08:39.730 --> 00:08:43.590 there are strong and reliable winds spiraling outwards from the subtropical high pressure 00:08:43.590 --> 00:08:49.871 belt towards the equator. These are the easterly Trade Winds -- and they’re my favorite winds, obviously! 00:08:49.871 --> 00:08:53.320 Many ships have depended on the trade winds, like early Spanish sailing ships as they sought 00:08:53.320 --> 00:08:56.690 God, glory, and gold in what we now call Central and South America. 00:08:56.690 --> 00:09:00.380 Of course, making the return trip was another matter. The ancient mariners of the Spanish 00:09:00.380 --> 00:09:04.420 galleons going home from the Americas plotted a course using the winds blowing poleward 00:09:04.420 --> 00:09:08.900 from the subtropical high pressure belt. These Westerlies are strongly deflected to the right 00:09:08.900 --> 00:09:10.300 and blow from the southwest. 00:09:10.300 --> 00:09:14.250 These strong winds blow towards another low pressure belt called the subpolar lows where 00:09:14.250 --> 00:09:18.740 they clash with the polar Easterlies blowing from the frigid, very high pressure poles. 00:09:18.740 --> 00:09:21.820 In the Southern Hemisphere, they blow with greater strength as there’s very little 00:09:21.820 --> 00:09:24.240 land in these latitudes to interrupt their flow. 00:09:24.240 --> 00:09:28.300 So altogether, on our idealized Earth we’ve seen that there are actually seven pressure 00:09:28.300 --> 00:09:33.990 belts: two polar highs, two subpolar lows, two subtropical highs and one equatorial low. 00:09:33.990 --> 00:09:36.680 And winds flow between these belts of high and low pressure. 00:09:36.680 --> 00:09:41.370 On the real Earth, the belts are not so organized. They form cells of pressure and we see more 00:09:41.370 --> 00:09:45.200 complex patterns of pressure and wind, as the cells shift with the seasons and vary 00:09:45.200 --> 00:09:50.030 between land and water. So our idealized Earth is kind of like a wind and pressure map. It’s 00:09:50.030 --> 00:09:53.600 a simplified model that helps us understand what’s happening on the real Earth. 00:09:53.600 --> 00:09:57.540 Just like the atmosphere works like a cell membrane, the winds are like Earth’s circulatory 00:09:57.540 --> 00:10:01.460 system. So many things vital to our planet flow through the winds. 00:10:01.460 --> 00:10:05.130 During the voyages of discovery in the 15th to 18th centuries -- which we now recognize 00:10:05.130 --> 00:10:08.990 weren’t really discoveries at all -- the knowledge of winds, ocean currents, natural 00:10:08.990 --> 00:10:12.980 harbors and more was an essential foundation for circumnavigating the globe. 00:10:12.980 --> 00:10:17.340 And we continue to rely on the winds to power our economies. As a renewable energy source, 00:10:17.340 --> 00:10:21.140 this silent force will continue to shape our lives in the future. I hope wherever you are 00:10:21.140 --> 00:10:25.220 is in the center of a sunny high pressure area which will be perfect weather to go with 00:10:25.220 --> 00:10:28.113 the flow in the ocean, which we’ll talk about next week. 00:10:28.113 --> 00:10:32.400 Many maps and borders represent modern geopolitical divisions that have often been decided without 00:10:32.400 --> 00:10:37.250 the consultation, permission, or recognition of the land's original inhabitants. Many geographical 00:10:37.250 --> 00:10:40.970 place names also don't reflect the Indigenous or Arboriginal peoples languages. 00:10:40.970 --> 00:10:44.720 So we at Crash Course want to acknowledge these peoples’ traditional and ongoing relationship 00:10:44.720 --> 00:10:47.860 with that land and all the physical and human geographical elements of it. 00:10:47.860 --> 00:10:51.200 We encourage you to learn about the history of the place you call home through resources 00:10:51.200 --> 00:10:55.790 like native-land.ca and by engaging with your local Indigenous and Aboriginal nations through 00:10:55.790 --> 00:10:58.390 the websites and resources they provide. 00:10:58.390 --> 00:11:01.550 Thanks for watching this episode of Crash Course Geography. If you want to help keep 00:11:01.550 --> 00:11:05.535 all Crash Course free for everyone, forever, you can join our community on Patreon.