Coriolis Force

The Coriolis force like gravity is a force the affects many aspects of our daily lives on earth in a variety of different ways. The wind and the ocean waters are two of the most common elements on our planet that are directly affected by the Coriolis force.

Introduction

Wind is the term used to define the horizontal flow of air as it moves from one place to another under the influence of a pressure differential. The direction of this wind is given by meteorologists as the direction from which it is blowing and its velocity is given in nautical miles per hour, or knots. Wind velocity information provided by a control tower at an airfield gives the direction in degrees magnetic to coincide with the magnetic direction of the runway.

Development of wind

The primary cause of wind is the variation in air density caused by the difference in air temperature. The major currents in the sea are also caused by the difference in density owing to variation in water temperature and/or salinity as discussed in Ocean currents and upwelling. The effect of gravity causes warm air to rise and cold air to sink resulting in a vertical flow of air. At the same time low pressure areas (rising air) and high pressure areas (sinking air) introduce a horizontal motion – air flows from high to how, creating a force called the pressure gradient. Loosely defined, this is the difference in pressure between consecutive isobars, the implication being that the air flows at 90 deg to the isobars which is in fact not the case, as we shall now see if we examine the effect of the earth’s rotation.

Coriolis Force

Newton first law states: A body in a state of rest or uniform motion will continue in that state of rest or uniform motion unless acted upon by an external force. If we view a body, or air mass, from a rotating platform like the earth, the path of the moving mass relative to the platform appears to be deflected or curved. An example of this deflection can be illustrated on a record player, also a rotating platform like earth, for those of you old enough to know or remember what a record player is. Start rotating the turntable, then, using a piece of chalk and a ruler, draw a straight line from the centre to the outer edge of the turntable. Obviously the ruler must be held above and not in contact with the turntable. Relative to the observer, and the ruler, the chalk will have appeared to travel in a straight line. However, if the turntable is stopped the line will, in fact be a spiral. If the turntable rotated in an anti-clockwise direction the spiral would be to the right, and if the turntable rotated clockwise, the spiral would be to the left. An easy way to remember this is – looking at the turntable whichever direction the bottom part of the turntable moves will be the direction of wind deflection. If one were to look down upon the earth from over the North Pole, the earth would be rotating in an anti-clockwise direction; therefore the direction of the wind would be to the right. Looking down upon the earth from the South Pole, the earth’s rotation would be clockwise and therefore the deflection would be to the left. This apparent deflecting force is called is called the Coriolis force. The Coriolis force always acts at 90 degrees to the direction of movement, being of minimum value at the Equator and maximum at the Poles. It is directly proportional to the velocity of the wind. Therefore if the wind velocity increases, then the Coriolis force affecting the wind also increases.

Geostrophic Winds

The Geostrophic wind is the result of the pressure gradient force and the Coriolis force. It is important to bear in mind that these two forces must be balanced for the wind to follow the isobars.

With reference to the figure above.

In the southern hemisphere, consider a particle at rest at position 1. Velocity is zero, Coriolis force is zero. As the particle, (position 2) moved towards the low pressure under the influence of the pressure gradient force (PGF), the Coriolis force (CF) increases proportionally’ acting at 90 degrees to the direction of movement. The resultant of the two forces is the Wind (W) Geostrophic winds As this particle of air accelerates through positions 3 and 4, the Coriolis force increases , until such time that it is exactly equal in magnitude and opposite in direction to the pressure gradient force (position 5) The partice will now continue moving parallel to the isobars at the speed it attained at position 5, provided no external force comes into play. This confirms the statement in 1857 of the Dutch meteorologist Buys-Ballot which became known as Buys-Ballot’s Law, which states “an observer who stands with his back to the wind in the southern hemisphere, will have the low pressure on his right. This is reversed in the northern hemisphere.”   References: Avex Air Training – Aviation Meteorology

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