The traditional book diagram involves an air parcel starting at rest within a horizontal pressure gradient. As the parcel accelerates toward lower pressure, the Coriolis force causes a deflection to the right. Eventually the air parcel is moving parallel to the isobars, with the leftward-directed pressure gradient force exactly balancing the rightward-directed Coriolis force.

But how did it reach equilibrium? The problem with the example is with the component of acceleration down the pressure gradient. Initially the parcel experiences the full force of the pressure gradient. Through time the pressure gradient force is gradually compensated by the Coriolis force. By the time it reaches equilibrium (according to the traditional diagram), the Coriolis force exactly balances the pressure gradient force.

So let's recap. The parcel starts off experiencing a strong pressure gradient force, and accelerates down the pressure gradient. As it moves, the net force in that direction weakens but is still present, so the parcel continues to accelerate in that direction. In other words, it continues going even faster down the pressure gradient. When the Coriolis force finally balances the pressure gradient force, the parcel should actually be traveling at peak speed down the pressure gradient, but in the book diagram its motion down the pressure gradient has somehow disappeared competely!

A real air parcel not acted on by friction will actually overshoot and oscillate about the equilibrium position, never coming into equilibrium until friction damps its motion.

What Happens to Drains Near the Equator

No way I could ever do a better job debunking this one than Alistair Fraser. Go to his page.

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Questions or Comments

Technical: E-mail John Fulton < jdfult@nimbus.met.tamu.edu >
Scientific: E-mail Dr. John Nielsen-Gammon. < nielsen@ariel.met.tamu.edu >

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