In our last blog, we saw how atmospheric circulation is split into three cells in each hemisphere, Hadley cell, Ferrel cell, and polar cell. In this blog, we will look at the winds within these cells and how the rotation of the Earth influences these winds to give us jet streams and prevailing wind patterns, as well as being split into three cells. As we can see in the above gif that winds deflect as they move on the surface of the earth.
What causes this deflection?
The answer lies in the concept of Coriolis Force. So as you can see the winds deflect to their right as they move from poles towards the equator and from the equator towards poles in the northern hemisphere. (Imagine the directions while facing towards the direction of Wind Movement) and similar deflection happens in the southern hemisphere where the wind deflects towards the left as it moves from poles to the equator and from the equator towards the poles.
So what causes Coriolis force?
The key to the Coriolis effect lies in the fact that the earth's surface rotates faster at the equator than at the poles. This is because the earth is wider at the equator. So a particle has to cover more distance at the Equator to complete one rotation as compared to the particle at the poles in the same time. Due to the difference in the speeds as a particle moves from the equator towards the poles it gets deflected. Note that Coriolis force only causes deflection and it's not a physical force. Deflection is just due to the air flowing from a region that is moving faster to a region that is moving more slowly
To understand Better let us visualize the force :
Imagine an air parcel as a ball. The ball is thrown from the equator towards a point near the North Pole. Even though it moves in a straight line, the ball will appear to observe on the ground to curve away and land to the right of its targets as the points near the North Pole is moving more slowly and is not caught up.
If the ball is now thrown from the North Pole towards a point near the equator, it will again appear to a surface observer to land to the right of its target. But this time it is because the earth's surface at the equator is moving faster and has moved ahead of the ball. This effect only happens on objects that are in motion, this deflection is a major factor in explaining why winds blow anticlockwise around low pressure and clockwise around high pressure in the northern hemisphere and vice versa in the southern hemisphere.
How does this lead to eastwards flowing jet streams and prevailing winds?
As air moves away from the equator at the top of the Hadley cells towards higher latitudes, it starts to be deflected by the Coriolis force. Just as a skater spins faster by bringing their arms and legs closer to their bodies, Air moves faster near the surface and hence deflection is less, when it moves upwards towards the sky the speed decreases, and hence Coriolis force is able to deflect the wind more.
This process is known as the conservation of angular momentum. The magnitude of the Coriolis force increases towards the poles and is ZERO at the Equator. So by the time the air reaches 30 to 40 degrees north or south, it is moving in the east direction.
This jet stream sits between the rising branches of the polar and Ferrel cells, it marks the boundary between the cold polar air and warm tropical air known as the polar front. The polar front jet occurs at a height of 11 to 13 kilometers. It is primarily the result of the temperature contrast across the polar front. The stronger the temperature contrast across the front, the stronger the jet. So it follows that the polar front jet is stronger in the winter than in the summer.
The surface flow of the Hadley cells forms the persistent trade winds as air flowing towards the equator is deflected towards the west in both hemispheres, forming the northeast trade winds in the northern hemisphere and the southeast trade winds in the southern hemisphere. The persistence of these winds allowed sailing ships to cross the Atlantic and opened up trade routes between Europe and America, giving them their name
The surface wind of the Ferrel cells would flow from a southerly direction in the Northern Hemisphere. But the Coriolis effect causes this wind to be deflected to the right leading to the prevailing westerly and southwesterly winds often experienced over the U.K.
For Video Explanation Watch: https://bit.ly/3uSjN7T
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Lecture 3: https://bit.ly/3ojlYPg
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