Coriolis+Force

Coriolis force
The unequal distribution of air across the globe establishes the horizontal pressure gradients that cause the movement of air as wind. If no other forces were involved the wind would always flow in the direction of the pressure gradient force. However, the situation is also influenced by the coriolis force which arises from planetary rotation. All points on the earth except the equator undergo some rotation, all moving objects experience this displacement which is the result of the coriolis force. As well as objects, the force acts on the movement of the atmosphere as well. In between the equator and the poles there is a gradual increase in the strength of the coriolis force with increasing latitude. No displacement occurs at the equator as there is no twisting motion. Instead, any point on the surface of the equator travels the full circumference of the earth (40,000 km) in 24 hours. The coriolus force can be described using the north pole as an example. As the northern hemisphere rotates in a counterclockwise direction, any object or weather system in this case moving across the pole would appear to be turning to the right. The same relationship between latitude and magnitude of the coriolis force can also be seen in the southern hemisphere. The only difference is that the deflection is to the left as opposed to the right, this is because the southern hemisphere rotates in a clockwise direction.

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The coriolis force changes the direction of a moving object, never its speed. To produce detectable effects the coriolis force must act over relatively long periods of time. It is seen mainly for the for the motion of objects travelling long distances including the air circulating around a hurricane. In the northern hemisphere the coriolis force turns winds to the right, however, hurricanes move in an anticlockwise direction. This happens because coriolis is only one of several forces acting on the hurricane. While the coriolis force is pushing the winds towards the right, the pressure gradient force caused by air pressure differences pushes the air towards the center of the low pressure system. This pressure gradient is stronger than the coriolis force and so results in an anticlockwise movement of air round the area of low pressure system. The winds in the poleward portion of a hurricane are pulled slightly poleward by the coriolis force. However, the equator ward portion of the hurricane contains winds which are pulled slightly towards the equator. However, because the coriolis force weakens towards the equator, the net drag of the hurricane is pole ward. Therefore, hurricanes in the northern hemisphere usually turn north and tropical cyclones in the southern hemisphere usually turn south. Air masses are large enough for the coriolis force to act on, however, small objects such as boats are far too small to experience significant deflections in direction.

References: -Edward Aguado, James E. Burt (2007) Understanding weather and climate, fourth edition. -John.H.Seinfield, Spyros.N.Pandis (1998) Atmospheric chemistry and physics, from air pollution to climate change.