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Subject: D9) What causes each hurricane to have a
different maximum wind speed for a given minimum sea-level
pressure ?
Contributed by Chris Landsea
The basic horizontal balance in a tropical cyclone above the
boundary layer is between the sum of the Coriolis 'acceleration'
and the centripetal 'acceleration', balanced by the horizontal
pressure gradient force. This balance is referred to as gradient
balance, where the Coriolis 'acceleration' is defined as the
horizontal velocity of an air parcel, v, times the Coriolis
parameter , f. Centripetal 'force' is defined as the
acceleration on a parcel of air moving in a curved path, directed
toward the center of curvature of the path, with magnitude
v2/r, where v is the horizontal velocity of
the parcel and r the radius of curvature of the path. The
centripetal force alters the original two-force geostrophic
balance and creates a non-geostrophic gradient wind. The reason
that different peak winds can result in different central
pressures is caused by the fact that the radius, r, of the
peak wind varies. A storm with 40 m/s peak winds with a 100 km RMW
will have a much lower pressure drop than one with a 25 km RMW.
![](gradient_bal.jpg)
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