- [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.