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Middle school Earth and space science - NGSS
Course: Middle school Earth and space science - NGSS > Unit 3
Lesson 3: Global winds and currentsGlobal winds and currents
Large-scale currents are found throughout Earth's atmosphere and oceans. In the atmosphere, air currents are caused by the uneven heating of Earth's surface. In the ocean, water currents are caused by winds or differences in density. Created by Khan Academy.
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- A current is the steady flow of a fluid (such as air or water) within a larger body of that fluid. Large-scale currents are found throughout Earth's atmosphere and oceans. In the atmosphere, air currents are caused by the uneven heating of Earth's surface. In the ocean, water currents are caused by winds or differences in density.(6 votes)
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- Hi there!
The water that the oceans contain still remains salty even after the water cycle, because the water leaves the ocean when it evaporates, and the salt gets left behind. Then the water goes through the rest of the water cycle, and returns to the ocean through rain. The rain comes into contact with the salty water, and is rejuvenated with those minerals. And hence, the water is salty again. Hope this helped!(2 votes)
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Video transcript
- [Instructor] One of
my favorite things to do is go camping. For me, there's nothing
better than getting outside, breathing in some fresh air and taking a swim in my favorite river. Have you ever jumped into a river and felt that the deeper, cooler water closer to your feet was moving faster than the shallow, warmer
water at your knees? That's a current, which is the a word we use to describe how water or even air flows within a larger
body of water or air, but what causes a current? Well, let's start with the Sun. The Sun actually heats Earth unevenly. We know that it's hotter near the equator and it gets colder as
you go towards the poles. Near the equator, the Sun's rays hit Earth
surface more directly, while near the poles, the Sun's rays hit Earth
surface less directly. In both regions, the same amount of solar
energy is heading Earth, but near the equator, this
energy is concentrated into a smaller area and near the poles it's
spread out over a larger area. So the regions near the
equator get more solar energy, which makes them warmer and the regions near the
poles get less solar energy, which makes them cooler. This uneven heating at Earth
also affects air pressure, where it's cooler near the poles, cool air will sink making
the air pressure high, but where it's warmer near the equator, warm air will rise
resulting in low pressure. This is where the terms low-pressure cells and high-pressure cells come from. The low-pressure warmer air at the equator rises into the upper
atmosphere where it cools and flows away towards higher latitudes away from the equator. Because the air is now cooler, it starts to sink again and creates a high-pressure
band near these latitudes. This process repeats and creates a pattern of high and low-pressure bands from the equator to the poles. We know that air flows
from areas of high pressure to areas of low pressure. This creates air occurrence or winds. Now you might think that these winds would blow in straight lines from high to low-pressure areas. But the global wind patterns, which we call prevailing winds, look like they curved the right
in the Northern Hemisphere and to the left in the
Southern Hemisphere. This curving has to do
with the rotation of Earth and is called the Coriolis effect. As these prevailing winds blow across the surface
of the land and water, they also pushed against
the surface of the ocean and produce wind-driven surface currents, which helped to move ocean water. Here's what the global pattern of ocean surface currents looks like. Like wind currents, ocean surface currents are also curved due to the Coriolis effect. We can see that in these currents that are traveling north and south, which curved to the right
in the Northern Hemisphere and to the left in the
Southern Hemisphere. Together as these surface
currents of the ocean connect, they formed giant rotating systems of ocean currents called gyres. The currents that drive these gyres extend from the surface
to about one kilometer down into the ocean and helped move water
all around the globe. But these gyres aren't just moving water, they're moving heat energy as well. Water is pretty good at holding onto heat it absorbs from the Sun. So as the water in our
oceans moves around the world through this gyre circulation, the water also carries heat. Here, warm water generally moves from the equator to the poles and cold water moves from
the poles to the equator. But the ocean has other deeper currents that are affected by differences
in temperature and density. Remember how we talked
about areas heated directly and less directly by the Sun? And how that results in low
and high-pressure areas? Same thing with water,
except that water density is affected by both
temperature and salinity, which is a measure of
how salty the ocean is. Cooler and saltier water is more dense, so it tends to sink, just like cool air. Whereas warmer and less
salty water is less dense and tends to rise, just like warm air. So with these deeper ocean currents, water actually moves
vertically or up and down. For example, water near
the poles gets very cold. It also gets very salty,
because when sea ice is formed, the salt can't go into the ice. Instead, the salt stays
behind in the water and so the water gets
saltier or more saline. Together, the coldness and salinity makes the water very dense causing it to sink deep into the ocean. And other parts of the ocean, wind drags deep water up to the surface and a process called upwelling. These vertical currents are connected by horizontal currents at the
surface and in the deep ocean. Collectively, this system of currents is known as the overturning circulation. You might also hear it called the global ocean conveyor belts. So here's a map showing the
overturning circulation. This map might look a little bit strange, but here we're looking at
Earth from the South Pole. So Australia and the
southern tips of Africa and South America are closes
to the center of the map, while Europe, most of
Asia and North America are at the edges. Now, if you follow the currents in the overturning circulation, you can see that they flow
all over the world's oceans. From the southern ocean
around the South Pole, to the Pacific, the Indian and all the way into the North Atlantic. Like the currents in the gyre circulation, the currents in the
overturning circulation also carry and disperse heat
energy all around the world. Here I am, back in my
favorite river, waiting around and enjoying currents of cool
water flowing around my feet. And even though this river is small, the currents that flow through it are similar to the global
wind and ocean currents that flow all around the world. So these currents connect in
our atmosphere and oceans, which means that we are all connected. So currents connect us all.