Global wind patterns| Earth systems and resources| AP environmental science| Khan Academy

Today we're going to talk about global wind patterns. Wind determines more than just the best places to fly a kite. Global wind patterns help control where it rains, what kinds of species can survive in an area, and even where tropical rainforests and deserts are located. In other words, global wind patterns are really important to life.

One of the reasons we have global wind patterns at all is actually because of the sun. Sunlight shines on the Earth like this. As you can see, the sunlight hits the equator directly, but the light hits the north and south poles at an angle, kind of skimming the surface like this. So, the equator is getting direct sunlight, and the poles are only getting indirect sunlight.

With all the direct sunlight, the air around the equator gets really hot, and the air around the poles doesn't heat up as much because it's only getting indirect sunlight. You may be thinking, what does this all have to do with airflow? Let's take a closer look at the equator to see what's happening.

So imagine you're standing on a piece of land near the equator. When the direct sunlight hits the equator, the hot air near the ground begins to rise up because hot air rises. All the direct sunlight also causes evaporation to increase, which means that this air that's rising up right here is both hot and moist. But once all this hot, moist air reaches a high enough altitude, it begins to expand and cool down.

The water vapor in the air begins to condense into clouds, and it eventually falls as rain. Around the equator, the air, which is now cool and holds less moisture, sinks down to the ground because cold air sinks. The cycle repeats, with the hot, moist air rising and the cool, dry air falling.

The fact that the hot air rises up means that this area right here is an area of low pressure. Because the air is rising up, it creates a space for cooler air in surrounding areas to move in and take its place. Over here, where the cool air is coming down to the ground, that's an area of high pressure because all of that cool, dry air is coming down and pushing the air below it away.

This cyclical movement of air creates something called a convection cell. If the Earth wasn't spinning, we would just have one convection cell in each hemisphere, where the air would heat up at the equator, move up towards the poles, and sink down. In the 18th century, this was how some scientists believed global wind patterns worked.

But because the Earth is spinning, the Earth's rotation pushes air masses from east to west. This movement of air creates a clockwise pattern in the northern hemisphere and a counterclockwise pattern in the southern hemisphere. This is called the Coriolis effect, and this movement of air, because of the Earth's spin, causes us to actually get three convection cells in each hemisphere.

These two, the two closest to the equator, are called the Hadley cells. They're between the equator and the 30-degree latitude marks in both hemispheres. These next two are called the Ferrel cells, or the temperate cells, and these are located between the 30 and 60-degree marks in both the northern and southern hemispheres. Lastly, we have the polar cells, which, as you can probably guess, are right by the poles.

So, we have polar cells up here at the North Pole, and we also have polar cells down here at the South Pole. These convection cells create prevailing winds that move heat and moisture around the Earth. Let's take a look at what happens on the bottom half of each convection cell, the parts closer to the ground.

Because these parts are closer to us, we experience the air movement as wind. So, at the bottom of this convection cell, the Hadley cells, the cold air is moving towards the equator. So that means that the prevailing winds would also be moving towards the equator. Winds are named after where they come from, so these two winds are called the northeast and southeast trade winds because they come from the northeast and southeast and they move west.

The bottom halves of the Ferrel convection cells take cool air from the 30-degree line and pull it towards the 60-degree latitude line. This creates the westerlies, and they're called the westerlies because they pull air from the west to the east. The bottom halves of the polar convection cells take the cool air from the poles and sweep it to the 60-degree latitude lines, and this creates the easterly winds.

It's important to remember that everything in this diagram is just an overall model. Global wind patterns are even more complicated because water-covered areas and land-covered areas absorb solar energy differently. These prevailing wind patterns distribute heat and precipitation unevenly between the tropics, temperate, and polar regions of the Earth.

This uneven distribution creates different biomes, and this helps determine which species can survive where. The tropical rainforest will be in the low-pressure areas near the equator, and right here, between the polar and Ferrel cells, is another area of low pressure, just like near the equator.

Hot moist air rises here, causing more precipitation in the surrounding areas. So along this latitudinal line, you'll find many coniferous forests that thrive because of all that precipitation. There's a high-pressure area right here where the cool, dry air sinks down, so there is not as much precipitation. The air is drier here; you'll find many deserts along this line.

Even though convection cells and prevailing winds are invisible, the ways they shape the environment are not.