Air in the atmosphere circulates in three convection cells from north to south. This lesson looks at the formation of these different air cells and what they mean for global weather and atmospheric circulation.
General Atmospheric Circulation
If you’ve ever taken a trip to the tropics, you likely went to enjoy the warm climate. The climate near the equator is so warm because this area receives the highest amount of solar radiation, or energy from the sun, throughout the year. Likewise, the polar regions are colder because they receive a much smaller and less consistent amount of annual solar radiation.
Since the tropic region does not get progressively hotter with more solar radiation, and the polar regions do not get progressively colder, there must be some force to balance out all of this energy to keep these areas at a semi-consistent temperature. This force is atmospheric circulation.Solar radiation at the equator causes the air to rise high into the atmosphere. The air will rise until the tropopause, which is the boundary between the troposphere, the lowest layer of the atmosphere, and the stratosphere, the next layer up of the earth’s atmosphere. Continuously rising air will push the air towards the north.
Additionally, the Coriolis force diverts this wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis force is the force that diverts wind due to the rotation of the Earth.As the air rises and travels away from the equator, it cools down and becomes more dense. The denser air falls back closer to the Earth and is pushed back towards the equator.
Eventually, this air will become heated due to solar radiation again, completing the circulation cell.
Three Circulation Cells
Air does not circulate in a single cell from the equator to the poles. Instead, each hemisphere is broken into three circulation cells. The Hadley cell is the circulation cell nearest the equator and has air rising at the equator and sinking near 30 degrees.
The Ferrel cell is the circulation cell in the mid-latitudes where the air rises near 60 degrees and sinks near 30 degrees. The Polar cell is the circulation cell in the polar regions. Let’s look a little deeper into each of these air circulation cells.The Hadley cell creates some of the most important weather patterns on Earth. As air rises and travels northward to the east, it eventually falls around 30 degrees. Where this air descends, it creates a ring of high atmospheric pressure. High-pressure zones are associated with calm, dry weather.
Therefore, a majority of the world’s deserts are located at 30 degrees from the equator. After the air in the Hadley cell falls back to Earth, it travels close to the surface of the Earth in a northeasterly direction, or from northeast to southwest. These northeasterly winds generated by the Hadley cell are called the trade winds.It is important to note at this point that I have been discussing air circulation in the Northern Hemisphere.
In the Southern Hemisphere, the circulation patterns are opposite because the Coriolis force pushes air in the opposite direction. The Hadley cell in the Southern Hemisphere rises at the equator and sinks at 30 degrees south. The wind at the top of the cell is pushed by the Coriolis force to the left. The air along the surface of the earth travels back to the equator in a southeasterly direction, or towards the northwest.The northeasterly and southeasterly trade winds converge near the equator in an area called the intertropical convergence zone, or ITCZ. The ITCZ is always near the equator but migrates throughout the year. It can sometimes be seen on satellite images as a string of clouds near the equator.
The convergence of the trade winds from the Northern and Southern Hemispheres creates some of the most consistent and widespread thunderstorms on Earth.At the ends of the earth, the Polar cells move in the same direction as the Hadley cells. Air rises near 60 degrees, creating a low-pressure zone, and falls near the poles, creating a high pressure zone. Just as with the Hadley cells, the air is deflected by the Coriolis force. The surface winds blowing from the north and south poles to the west are called the polar easterlies.We are discussing the Ferrel cell last because it is driven by circulation in the Hadley and Polar cells. Also, the Ferrel cell operates in the opposite direction than the other two cells.
Here, the air rises near 60 degrees where it is cold and sinks at 30 degrees where it is relatively warm. Why is this behavior opposite in the mid-latitudes? Instead of dissipating energy from solar radiation, the Ferrel cell is actually balancing thermal energy between the Hadley and Polar cells. Cold air travels down to cool down the subtropics and warmer air then travels near the surface to warm the polar region.Interestingly, these circulation cells are also responsible for the largest global wind patterns, the jet streams. The jet streams are high altitude strong winds associated with the downwelling of the Hadley cell near 30 degrees and the upwelling of the Ferrel cell at 60 degrees. These winds are heavily influenced by the Coriolis effect and the rotation of the Earth, so they flow from west to east.
You’ve likely heard the jet streams discussed in the context of plane flights. If you fly from the West Coast of the U.S. to Asia, you’ll notice that it takes a lot longer to fly to Asia than it takes to fly back to the U.S.
That is because when you are flying from Asia back east to the U.S., you can catch the jet stream, which can cut down your flight time by a couple of hours!
Let’s review. Air circulates in the Earth due to energy variations from incoming solar radiation.
At the equator, air heats up and rises. This air is pushed away from the equator and falls back to the Earth once it cools and becomes more dense. There are three circulation cells in each hemisphere. The circulation cell closest to the equator is known as the Hadley cell. The surface winds in the Hadley cell where the air is traveling back towards the equator are known as the trade winds.The stormy region where the trade winds converge is the intertropical convergence zone, or ITCZ. The circulation cells near the poles are the Polar cells and occur where air rises near 60 degrees and sinks near the poles.
The mid-latitude circulation cell is the Ferrel cell. The Ferrel cell has opposite motion to the other two cells because it is equalizing the uneven temperatures between the equator and the poles. Cooler air rises near 60 degrees and falls down near the subtropics. The warmer air then travels along the surface towards the poles.