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Which accuracy does hydrostatic balance hold to for synoptic scale motion?

  • A.About one part in ten, which is comparable with the accuracy of the geostrophic balance in the horizontal
  • B.About one part in a hundred
  • C.It holds exactly, since the vertical acceleration of a parcel of air is always identically equal to zero
  • D.About one part in a hundred million

Answer: D

Vertical accelerations in large scale flow are around \( 10^{-7} \) metres per second squared against a gravitational acceleration near 10, so the two vertical terms balance to about one part in \( 10^{8} \). That is why pressure can be used as a vertical coordinate with confidence. The horizontal geostrophic balance, by contrast, is good only to about ten per cent. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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