2.3: Atmospheric Circulation

Convection Cells

Because more solar energy hits the equator, the air warms and forms a low pressure zone. As the air rises, half moves toward the North Pole and half toward the South Pole. As it moves along the top of the troposphere it cools. The cool air is dense and when it reaches a high pressure zone it sinks to the ground. The air is sucked back toward the low pressure at the equator. This describes the convection cells north and south of the equator. If the Earth did not rotate, there would be one convection cell in the northern hemisphere and one in the southern with the rising air at the equator and the sinking air at each pole. But because the planet does rotate, the situation is more complicated. The planet’s rotation means that the Coriolis Effect must be taken into account.

Figure \(\PageIndex{1}\): Hypothetical atmospheric convection cells on a non-rotating Earth. Air rises at the equator and sinks at the poles, creating a single convection cell in each hemisphere. The prevailing winds moving over the Earth’s surface blow from the poles towards the equator in both hemispheres (by Paul Webb via Libretext; CC BY-SA 3.0).

Coriolis Effect

The non-rotating situation in Figure \(\PageIndex{1}\) is of course only hypothetical, and in reality the Earth’s rotation makes this atmospheric circulation a bit more complex. The paths of the winds on a rotating Earth are deflected by the Coriolis Effect.

Definition: Coriolis Effect

An effect whereby a mass moving in a rotating system experiences a force (the Coriolis force ) acting perpendicular to the direction of motion and to the axis of rotation. On the earth, the effect tends to deflect moving objects to the right in the northern hemisphere and to the left in the southern and is important in the formation of cyclonic weather systems.

The Coriolis Effect is a result of the fact that different latitudes on Earth rotate at different speeds. This is because every point on Earth must make a complete rotation in 24 hours, but some points must travel farther, and therefore faster, to complete the rotation in the same amount of time. In 24 hours a point on the equator must complete a rotation distance equal to the circumference of the Earth, which is about 40,000 km. A point right on the poles covers no distance in that time; it just turns in a circle. So the speed of rotation at the equator is about 1600 km/hr, while at the poles the speed is 0 km/hr. Latitudes in between rotate at intermediate speeds; approximately 1400 km/hr at 30^o and 800 km/hr at 60^o. As objects move over the surface of the Earth they encounter regions of varying speed, which causes their path to be deflected by the Coriolis Effect.

In the Northern hemisphere objects moving towards the equator from the north pole are moving from a low speed to a high speed. Imagine a cannon located at 60^o and facing the equator. It will be moving east at 800 km/hr and when its shell is fired towards the equator, the shell will be moving east at 800 km/hr. But, as it approaches the equator it will be moving over land that is traveling east faster than the projectile. So the projectile gets “behind” its target, and will land to the west of its destination. But from the point of view of the cannon facing the equator, the path of the shell still appears to have been deflected to the right (green arrow, Figure \(\PageIndex{2}\)). Therefore, in the Northern Hemisphere, the apparent Coriolis deflection will always be to the right.

In the Southern Hemisphere the situation is reversed (Figure \(\PageIndex{2}\)). Objects moving towards the equator from the south pole are moving from low speed to high speed, so are left behind and their path is deflected to the left. Movement from the equator towards the south pole also leads to deflection to the left. In the Southern Hemisphere, the Coriolis deflection is always to the left from the point of origin.

Figure \(\PageIndex{2}\): The Coriolis Effect. Objects moving from the equator towards the poles (red arrows) move into a region of slower rotational speed and their paths are deflected “ahead” of their point of origin. Movement from high latitudes to low latitudes (green arrows) goes from a region of low speed to a region of higher rotation speed, and there is deflection “behind” their point of origin. In the Northern Hemisphere this deflection is always to the right from the point of origin, and in the Southern Hemisphere the deflection is always to the left (by Paul Webb via Libretext; CC BY-SA 3.0).

Video: The Coriolis Effect

Watch the video below for an animation and explanation of the coriolis effect.

Because of the rotation of the Earth and the Coriolis Effect, rather than a single atmospheric convection cell in each hemisphere, there are three major cells per hemisphere. Warm air rising at the equator cools as it moves through the upper atmosphere, and it descends at around 30^o latitude. The convection cells created by rising air at the equator and sinking air at 30^o are referred to as Hadley Cells, of which there is one in each hemisphere. The cold air that descends at the poles moves over the Earth’s surface towards the equator, and by about 60^o latitude it begins to rise, creating a Polar Cell between 60^o and 90^o. Between 30^o and 60^o lie the Ferrel Cells, composed of sinking air at 30^o and rising air at 60^o (Figure \(\PageIndex{3}\)). With three convection cells in each hemisphere that rotate in alternate directions, the surface winds no longer always blow from the poles towards the equator as in the non-rotating Earth in Figure \(\PageIndex{3}\). Instead, surface winds in both hemispheres blow towards the equator between 90^o and 60^o latitude, and between 0^o and 30^o latitude. Between 30^o and 60^o latitude, the surface winds blow towards the poles (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): On a rotating Earth, there are three atmospheric convection cells in each hemisphere, leading to alternating bands of surface winds (red arrows) (by Paul Webb via Libretext; CC BY-SA 3.0)

Surface winds

The surface winds created by the atmospheric convection cells are also influenced by the Coriolis Effect as they change latitudes. The Coriolis Effect deflects the path of the winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Adding this deflection leads to the pattern of prevailing winds illustrated in Figure \(\PageIndex{4}\. Between the equator and 30^o latitude are the trade winds; the northeast trade winds in the Northern Hemisphere and the southeast trade winds in the Southern Hemisphere (note that winds are named based on the direction from which they originate, not where they are going). The westerlies are the dominant winds between 30^o and 60^o in both hemispheres, and the polar easterlies are found between 60^o and the poles.

Figure \(\PageIndex{4}\): The prevailing wind patterns of Earth (by Paul Webb via Libretext; CC BY-SA 3.0)

Attribution

This article is a modified derivative of:

• Winds and the Coriolis Effect by Paul Webb license a CC BY 4.0