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Geostrophic and cyclostrophic winds

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6.1 Geostrophic and cyclostrophic winds


Winds exist because of horizontal and vertical


pressure gradients


, so atmospheric motion can be deduced from isobaric surface charts. In the absence of surface friction, if the


horizontal pressure gradient force


is exactly balanced in magnitude by


Coriolis effect


then accelerations of the air will be relatively small and a


geostrophic wind


(from the Greek 'geo' = earth, strophe = turning )


will flow horizontally at a constant speed that is proportional to the isobaric spacing gradient. The flow will be perpendicular to the two opposing forces and parallel to straight isobars. Air will be accelerated to the extent that these forces are unbalanced. Transitory disturbances and vertical movement will create imbalance. When vertical motion is present the horizontal wind cannot be exactly geostrophic.









Geostrophic flow is predominant above the


friction layer


in very large-scale weather systems, where the pressure gradient force and the Coriolis force are nearly equal and opposite; e.g. the Southern Ocean west wind belt.


Between 15°S and 15°N latitudes there is little geostrophic flow due to weak Coriolis effect (it being zero at the equator), and winds tend to flow across the isobars. (In which case it is more useful to show wind flow on upper air charts as streamlines.






arrow shows the direction of flow, whereas an




is a line along which the speed of flow is constant.)


At the other end of the scale in short-lived


mesoscale systems


, Coriolis has insufficient time to take effect or is relatively weak compared to other forces, so geostrophic balance is not present and air accelerations can be quite large.


If atmospheric circulation was always in perfect balance between geostrophic forces and pressure gradient forces, geostrophic winds would flow and there would be no change in pressure systems. In reality the pressure distribution takes the form of curved isobars resulting in a third force — the


centripetal acceleration


— which pushes the flow inward of the curve.




gradient wind


is the equilibrium wind for the three forces — centripetal acceleration, pressure gradient force and Coriolis (or geostrophic). It is roughly aligned with the isobars on the meteorological surface chart. The vector difference between the geostrophic and the gradient winds is the


ageostrophic wind


. Thus, ageostrophic movement is large for small-scale systems and small for large-scale systems.


When the centripetal acceleration becomes the major control of the gradient wind, there is an extremely strong curvature of the airflow and the winds are called




(Greek = circle – turning)


; for example,


tropical cyclones






. When a body is moving in a curved path, centripetal force is the radial inward force that constrains the body to move in that curved path and, even at constant speed, there is an inward acceleration resulting from the body's continually changing velocity. (The same applies to an aircraft in a constant-speed


level turn


.) The equal and opposite centrifugal force that appears to act outward on a body moving in a curved path is a fictitious force, but convenient to show the equilibrium forces for air moving in a cyclonically curved path; e.g. around a surface low pressure system, thus:









For the gradient wind to follow cyclonically curved isobars, the pressure gradient force must be slightly stronger than Coriolis to provide the centripetal force. As the magnitude of the Coriolis is directly dependent on wind speed, it follows that the wind speed around a low is less than would be expected from the pressure gradient force and the gradient wind is






For air moving in an anticyclonically curved path (e.g. around a high), the opposite occurs, and the Coriolis provides the centripetal force.









For the three forces to be in equilibrium, the Coriolis must exceed the pressure gradient force. Consequently, the gradient wind speed must be greater than would be expected from the pressure gradient force — and thus is






Air moving within a pressure pattern possesses momentum. If the air moves into a different pressure pattern and gradient it will tend to maintain its speed and Coriolis for some time, even though the pressure gradient force has changed. The resultant imbalance will temporarily deflect the airflow across the isobars in the direction of the stronger force — Coriolis or the pressure gradient force.







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Guest cficare

lots of terms the average punter doesnt need to know...


talk to your instructor about 'highs' and 'lows' and wind circulation..



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