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Which parameter determines whether the balance holds?

  • A.The magnitude of the horizontal pressure gradient force acting at the level being considered at the time
  • B.The aspect ratio of the motion
  • C.The Rossby number, which compares the inertial acceleration against the acceleration due to the rotation
  • D.The Richardson number

Answer: B

Scale analysis shows that the ratio of the vertical acceleration to gravity is of order the square of the aspect ratio, so a flat disturbance is hydrostatic and a tall narrow one is not. With a ratio of one in a hundred for synoptic motion the balance is extremely good. The Rossby number governs the horizontal balance instead and has no bearing on the vertical one. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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