 To see one mechanism by which subsidence in the eye of a hurricane develops, let's examine two air parcels. The blue parcel is located in the eye wall, and the green parcel is located in the periphery of the eye. Initially, both parcels are in gradient balance. Now let's look at the forces acting on each parcel. First we have the pressure gradient force, which is directed in toward the center of the storm. Even though the pressure is lower in the eye, the largest pressure gradient exists in the eye wall, so the pressure gradient force acting on the blue parcel in the eye wall is larger. Next we have the Coriolis force, which is the smaller of the two forces that are balancing the pressure gradient force. The Coriolis force is proportional to wind speed. Finally we have the centrifugal force, which is the larger of the two forces that balance the pressure gradient force. The centrifugal force is proportional to the wind speed squared divided by the radius of curvature. Turbulent mixing along the periphery of the eye causes parcels in the eye to gain momentum. The increase in velocity increases the magnitude of the Coriolis and centrifugal forces, resulting in parcels that are super gradient. When a parcel is super gradient, the Coriolis and centrifugal forces are much larger than they should be for the existing pressure gradient. The imbalance of forces results in a velocity component that is directed opposite the pressure gradient force. This velocity component causes the parcel to spiral outward to a larger radius, which reduces the magnitude of the centrifugal force. As the parcel enters the eye wall, the pressure gradient force also increases, and at some point the parcel again reaches gradient balance thanks to the increasing pressure gradient force and decreasing centrifugal force that occurs as the radius of curvature gets larger. The end result of momentum transfer across the eye wall boundary is that air leaves the eye at low to mid levels. In turn, this increases subsidence as air is drawn down to replace parcels that spiral into the eye wall.