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Global Wind Circulation - Three Cells Polar cells , Hadley Cells and Ferrel Cells . UPSC Geography

This is our second blog on our three blog series for explaining the three wind circulation cells that exist in our atmosphere namely Polar Cells, Hadley Cells, and Ferrel Cells.

In our last blog, we saw how different parts of the world heat up differently and this was because of the revolution of the earth around the sun and because its axis is tilted. This causes the equator to heat up more as it receives more heat from the sun than the poles.

The purpose of Global circulation is to redistribute this heat.

Before moving further it's important to understand the basic property of air :

1. The hot less dense air has a tendency to rise upwards and hence leaves a low-pressure region behind. So where ever the hot air is rising, low pressure is created.

2. When the air sinks it applies the pressure at the surface and hence the pressure increases. The place at which the cold air sinks makes up the high-pressure region.

3. Air always flows from High pressure to a Low-pressure region.

4. So the region where the temperature is cold, would be high -pressure region since the air would sink, and the region where the temperature is hot would be a low-pressure region since air would rise.

As the Hot air rises it begins to cool down, as with increasing height the temperature decreases and hence it cools downs the air. Since the equator is hot the air at the equator becomes hot and rises upwards.

If the Earth did not rotate we would have a single circulatory cell in each hemisphere where the hot air would rise and move towards the poles. The air would sink as it cools and then return towards the equator. But the unequal distribution of land and ocean and the speed of the earth's rotation make it more complex giving us a 3 cell pattern that exists in both hemispheres.

The largest of all the cells is the Hadley cells.

At the equator, the warmer less dense air rises and it rises to the height of about 18km in the troposphere. As it rises it begins to cool down and spreads out towards the poles gradually moving and sinking as it moves before descending towards the surface and flowing back to the equator completing a circulation called Hadley cell. So in short, the hot air rises at the equator creates a low-pressure area, and then moves towards poles as it rises and in the way cools down and then sinks creating the high-pressure region and again moves from high-pressure region to low-pressure region completing the circulation.

Polar cell

The smallest of all the cells is the Polar Cell. Cold dense air descending in the polar regions flows at low levels of about 60 to 70 degrees North or South. As the air leaves the polar regions it starts to warm and rise returning to the poles at high levels. In basic terms since at the pole, the temperature is very cold so the air would sink there cresting a high-pressure region and when we move away from poles the temperature is comparatively warmer and hence there would be a low-pressure region, and air moves from high to low pressure completing the circulation.

Ferrel Cell

This cell is in between Hadley and Polar cell. Unlike the other cells, Ferrel cells are not driven by temperature. These cells flow in opposite directions to Hadley and polar cells acting like a gear. So the basic concept goes the same, air moves from high to low-pressure regions.

The above picture shows the cells on an X-Y axis for better understanding. The air from the Hadley cells descends at around 30 degrees North creating high pressure. The North pole is cold so there its the high pressure and at around 60 degrees north the air gets warmed up and rises and creates a low-pressure. So air from High -Pressure region of 30 degrees North moves to the low-pressure region of 60 Degrees North creating a Ferrel Cell.

We see due to this circulation alternate low and high pressures are created and this gives our different climatic zones like deserts etc.

In the Low-pressure regions as the air rises it cools down and condenses to form clouds and finally precipitates as rainfall. Hence these areas of Low -pressure see more rainfall compared to other Parts.

This is why the largest areas of rainforest are found near the equator.

Where the air is descending an area of high-pressure forms giving largely clear skies and little rainfall which leads to desert regions.

Our next Blog would complete this series Stay tuned!


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