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AP®︎/College Environmental science
Course: AP®︎/College Environmental science > Unit 3
Lesson 3: Earth's air and waterGlobal wind patterns
Explain how environmental factors can result in atmospheric circulation.
Created by Khan Academy.
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- Shouldn't the arrows in the westerlies (Northern Hemisphere) point in a clockwise direction? Two of the arrows are pointing in a counter-clockwise direction.(6 votes)
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Video transcript
- [Instructor] Today, we're going to talk about Global Wind Patterns. Wind determines more than just
the best places to fly 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. And 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 on the equator gets really hot. And the air around the poles
doesn't heat up as much because it's only getting
indirect sunlight. And 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 and the cycle repeats with the hot moist air rising
and the cool dry air falling. And the fact that the hot air rises up, it 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. And 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 create 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. And in the 18th century, this was how some scientists believe 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 counter-clockwise 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
a 30 and 60 degree marks in both the Northern and
Southern hemispheres. And 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. And these convection cells
create prevailing winds that move heat and
moisture around the earth. Let's take a look at what
happens in 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 on 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. And the bottom halves of
the ferrel convection cells, take cool air from a 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 and 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 Easterlies 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. And this uneven distribution
creates different biomes. And this helps determine what
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 longitudinal line, you'll find many coniferous forests that thrive because of
all that precipitation. And there's a high
pressure area right here where the cool dry air sinks down so there's not as much precipitation. The air is drier here. You'll find many deserts
along this line and this line. So even though convection cells and prevailing winds are invisible, the ways they shape the
environment are not.