The Coriolis force causes apparent deflections in winds and other moving objects. It is caused by the Earth's rotation and causes deflections to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The strength of the Coriolis force depends on latitude and the speed of the moving object. It does not affect speed but only direction. Geostrophic winds blow parallel to isobars resulting from a balance between the pressure gradient force and Coriolis force. Gradient winds occur where friction is present, causing winds to flow perpendicular to the pressure gradient.

1. The Coriolis ForceThe Coriolis Force

2. When air has been set in motion by theWhen air has been set in motion by the
pressure gradient forcepressure gradient force, it undergoes an, it undergoes an
apparent deflection from its path. Thisapparent deflection from its path. This
apparent deflection is called theapparent deflection is called the CoriolisCoriolis
forceforce. It is a result of the earth's rotation.. It is a result of the earth's rotation.

3. Coriolis Force (Cont.)Coriolis Force (Cont.)

4. The Coriolis force, named after French mathematician Gaspard
Gustave de Coriolis (1792–1843). This force has traditionally
been derived as a matter of coordinate transformation by an
essentially kinematics technique.
Application of Coriolis’s principles elucidates cause
and effect aspects of the dynamics and energetics of
the atmosphere, the geostrophic adjustment process,
the circulation around jet streams, the meridional
extent of the Hadley cell, the strength and location of
the subtropical jet stream, etc.

5. Coriolis Force (Cont.)Coriolis Force (Cont.)
 Deflection forceDeflection force
 Depends on: latitude, wind speed, earth’s rotation rateDepends on: latitude, wind speed, earth’s rotation rate
 Acts at right angles to the windActs at right angles to the wind
 Deflection is proportion to wind speed. That’s why, slowly blowing winds willDeflection is proportion to wind speed. That’s why, slowly blowing winds will
be deflected only a small amount, while stronger winds will be deflectedbe deflected only a small amount, while stronger winds will be deflected
more. On the other hand, winds blowing closer to the poles will be deflectedmore. On the other hand, winds blowing closer to the poles will be deflected
more than winds at the same speed closer to the equator.more than winds at the same speed closer to the equator.
 This force is important mainly for the motion of objects traveling longThis force is important mainly for the motion of objects traveling long
distances, such as air circulating around a hurricane, the sea-breezedistances, such as air circulating around a hurricane, the sea-breeze
circulation, etc.circulation, etc.

6. Latitude and the Strength of Coriolis ForceLatitude and the Strength of Coriolis Force
In the above diagram, Blue arrows denoteIn the above diagram, Blue arrows denote
a possible straight line path. Red arrowsa possible straight line path. Red arrows
indicate the Coriolis-induced deflections.indicate the Coriolis-induced deflections.
These are to the right in the northernThese are to the right in the northern
hemisphere and to the left in thehemisphere and to the left in the
southern hemisphere.southern hemisphere.

7. Earth RotationEarth Rotation
The reason for the reversal - right or left - is just the consequence of direction ofThe reason for the reversal - right or left - is just the consequence of direction of
motion in the two hemispheres.motion in the two hemispheres.

8. Fundamental Characteristics of Coriolis ForceFundamental Characteristics of Coriolis Force
 The Coriolis force produces an apparent deflection in all movingThe Coriolis force produces an apparent deflection in all moving
objects, regardless of their direction of motion. The deflection is toobjects, regardless of their direction of motion. The deflection is to
the right in the Northern Hemisphere and to the left in the Southernthe right in the Northern Hemisphere and to the left in the Southern
Hemisphere.Hemisphere.
 The Coriolis force is zero at the equator (because the planet’s 24-The Coriolis force is zero at the equator (because the planet’s 24-
hour rotation imparts no twisting motion at the equator) andhour rotation imparts no twisting motion at the equator) and
increases with latitude, reaching a maximum level at the poles.increases with latitude, reaching a maximum level at the poles.
 The Coriolis force acting on any moving object increases with theThe Coriolis force acting on any moving object increases with the
object’s speed.object’s speed.
 The Coriolis force changes only the direction of a moving object,The Coriolis force changes only the direction of a moving object,
never its speed.never its speed.

9. Winds of the AtmosphereWinds of the Atmosphere

10. Geostrophic WindGeostrophic Wind
 Balance between PGF and Coriolis Force.Balance between PGF and Coriolis Force.
i.e. when the pressure gradient forcei.e. when the pressure gradient force
equals the Coriolis force.equals the Coriolis force.
 Geostrophic flow occurs only in the upperGeostrophic flow occurs only in the upper
atmosphere where friction is absent andatmosphere where friction is absent and
only the Coriolis and pressure gradientonly the Coriolis and pressure gradient
force apply.force apply.
 Wind blows parallel to isobars.Wind blows parallel to isobars.

11. Geostrophic Wind (Cont.)Geostrophic Wind (Cont.)

12. Gradient WindGradient Wind
 Due to the absence of friction, the air flows parallelDue to the absence of friction, the air flows parallel
to the contours. To follow the contours, there mustto the contours. To follow the contours, there must
be a continual mismatch between the pressurebe a continual mismatch between the pressure
gradient and Coriolis forces. The movement due togradient and Coriolis forces. The movement due to
the mismatch are refer to asthe mismatch are refer to as Gradient flowGradient flow oror
Gradient windGradient wind..
 Gradient flow develops only in theGradient flow develops only in the absence ofabsence of
frictionfriction and the wind flows perpendicular to theand the wind flows perpendicular to the
pressure gradient.pressure gradient.

13. Gradient Wind (Cont.)Gradient Wind (Cont.)

14. Gradient Wind (Cont.)Gradient Wind (Cont.)

15. ReferencesReferences
 Aguado, E. and Burt, J. E. 2010. Understanding Weather and Climate.Aguado, E. and Burt, J. E. 2010. Understanding Weather and Climate.
Fifth Edition, Prentice Hall, New Jersey, USA. pp. 114-119.Fifth Edition, Prentice Hall, New Jersey, USA. pp. 114-119.
 For DiagramsFor Diagrams
 Class Lecture of Michael Palmer, Atmospheric Physics, University ofClass Lecture of Michael Palmer, Atmospheric Physics, University of
Oxford, UK.Oxford, UK.
 http://ww2010.atmos.uiuc.eduhttp://ww2010.atmos.uiuc.edu
 http://rst.gsfc.nasa.gov/Sect14/Sect14_1c.htmlhttp://rst.gsfc.nasa.gov/Sect14/Sect14_1c.html
 http://www.sci.uidaho.edu/scripter/geog100/lect/04-atmos-oceanic-http://www.sci.uidaho.edu/scripter/geog100/lect/04-atmos-oceanic-
circ/ch4-part-4-corolis-force.htmcirc/ch4-part-4-corolis-force.htm