What is Coriolis Effect and How it Affects Global Wind Patterns


‘Coriolis effect’ or Coriolis force can be defined simply as deflection of wind.

Winds and ocean currents are in constant motion. This movement does not follow a straight line but bends and curves. This is the Coriolis effect which is noticeable in the curvature of global winds, and all that is free flowing across the surface of the earth. The curvature results from the earth’s rotation on its axis. This was first discovered by Gaspard C. Coriolis, a nineteenth century French engineer.

According to him, if the path of any object is set in motion above a rotating surface, the object will curve in relation to objects on that surface. Coriolis explained his position with the help of mathematical formulas.

The Coriolis Effect is a force that causes objects in motion to deflect – in relation to the earth, to the right in the northern hemisphere and to the left in the southern hemisphere. This is due to the rotation of the earth. It is an important meteorological force that is used to predict the path of storms and the formation of cyclonic weather systems.

Wikipedia defines Coriolis effect as,

In physics, the Coriolis force is an inertial force (also called a fictitious force) that acts on objects that are in motion relative to a rotating reference frame. In a reference frame with clockwise rotation, the force acts to the left of the motion of the object.

Why Does Coriolis Effect Happen?

The regions near the equator receive the direct ray of the sun. The sun’s effect is strongest here than in the poles. The areas close to the equator, experience high temperatures all year around. For this reason, the air around this area is warm. Warm air is light and rises above towards the atmosphere. Cold dense air from the poles rushes in to take its place. Air always flows from a high pressure area to a low pressure area.

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The earth is in constant motion. This movement of the earth affects the direction of the winds that blow from the north and south towards the equator. Their path is deflected by the rotation of the earth. This is the Coriolis effect. This is the reason why wind streams on the northern side (north hemisphere) spin counter-clockwise and that blows south of the equator, the southern hemisphere, spin clockwise.

Causes of the Coriolis Effect

Rotation of the earth

It is the rotation of the Earth that creates the Coriolis effect which is an inertial force. The earth spins in a counter-clockwise direction, from west to east, on its axis. It has been observed that anything flowing over a long distance above its surface is deflected. This occurs because as something moves freely above the earth’s surface, the earth is moving east under the object at a faster speed.

Increase in latitude

The speed of the earth’s rotation decreases with the increase in latitude. With this the Coriolis effect increases. Flying on the equator, a pilot can continue without any apparent deflection. If he moves a little to the north or south of the equator, he would be deflected. Near the poles the pilot will experience the most deflection.

Speed of the earth

The speed of the earth as it rotates on its axis is faster at the Equator than at the poles. This is due to the fact that the Earth is wider in its diameter at the Equator and narrower at the poles.

Impact of the Coriolis Effect


The strongest impact of the Coriolis Effect is felt on the weather which is influenced by the rotation of the earth. The Earth rotates on its own axis and creates weather patterns. The Coriolis effect makes storms swirl clockwise in the Southern hemisphere and counter clockwise in the Northern Hemisphere.

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Movement of ocean currents

Because currents are driven by the movement of wind across the water of the ocean, the Coriolis effect also affects the movement of the ocean’s currents.


Many of the ocean’s largest currents circulate around warm, high pressure areas creating vortex called gyres. These air vortex or gyres have a spiralling formation which is the deflection caused by the Coriolis effect.

Westerly winds

In the Northern Hemisphere, above the subtropical highs, and in the Southern Hemisphere, below the subtropical highs, winds blow from the west towards the east. These winds are called westerly winds, after the direction from where they come. In the Northern Hemisphere, these winds deflect to the right and in the Southern Hemisphere to the left. Thus the westerly winds move from the subtropical areas to the poles.

Moving winds

The air moving directly north or south is greatly affected by the Coriolis effect. It not only alters the air flow but also any warhead or rocket traveling through the air. If a ballistic shell is fired directly towards the south from the North Pole, due to the Coriolis effect, it will touch down to a certain extent on the west of a target placed due south.

In the northern hemisphere, air moving southward turns westward for the Coriolis effect. Near the equator air that is flowing southbound seems to be moving to the west as the Earth turns east. As a result, not only the weather patterns are affected but movements of planes and boats.

The Coriolis effect has been called an observer effect. It has been compared to a passenger on a moving train watching another stationary object outside the train. The person outside the train seems to be moving. Likewise, air currents seem to be moving off to one side under the Coriolis effect, from their original path. However, it is the observer that moves. The rotating Earth can be compared to a moving train. It carries an observer who sees the air to change direction. The directional change of air is always toward the west.

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Movement of rotating objects

The invisible force that appears to deflect the wind is the Coriolis force. The Coriolis force applies to movement on rotating objects. It is determined by the mass of the object and the object’s rate of rotation. The Coriolis force is perpendicular to the object’s axis. The Earth spins on its axis from west to east. The Coriolis force, therefore, acts in a north-south direction. The Coriolis force is zero at the Equator.

Though the Coriolis force is useful in mathematical equations, there is actually no physical force involved. Instead, it is just the ground moving at a different speed than an object in the air.

In the Northern Hemisphere, wind from high-pressure systems pass low-pressure systems on the right. This causes the system to swirl counter-clockwise. Low-pressure systems usually bring storms. This means that hurricanes and other storms swirl counter-clockwise in the Northern Hemisphere. In the Southern Hemisphere, storms swirl clockwise. The Earth rotates fairly slowly, compared with other planets. The slow rotation of the Earth means the Coriolis effect is not strong enough to be seen in small movements, such as the draining of water in a bathtub.

The Coriolis effect increases with an item’s increasing speed, it significantly deflects air flows and as a result the wind.

Finally, the Coriolis effect is important to man-made objects in addition to these natural phenomena. One of the most significant impacts of the Coriolis effect is a result of its deflecting planes and missiles.

Photo by: Unsplash

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