. Compendium of meteorology. Meteorology. TROPICAL CYCLONES 893 likely at some altitude high above the storm center, has simimarized how considerable outflow may take place otherwise. Following Durst and Sutcliffe [6] he has considered the effect of ang-ular momentum of rising surface air. Because of the decrease of pressure- gradient force with height, the rising air seeks to draw away from the center as soon as the centrifugal force exceeds the pressure-gradient force. Values indicating the variations of the pressure-gradient force with height are tabulated below. The values in the right-han
. Compendium of meteorology. Meteorology. TROPICAL CYCLONES 893 likely at some altitude high above the storm center, has simimarized how considerable outflow may take place otherwise. Following Durst and Sutcliffe [6] he has considered the effect of ang-ular momentum of rising surface air. Because of the decrease of pressure- gradient force with height, the rising air seeks to draw away from the center as soon as the centrifugal force exceeds the pressure-gradient force. Values indicating the variations of the pressure-gradient force with height are tabulated below. The values in the right-hand column are the ratios (expressed in per cent) of the pressure-gradient force at different elevations to that at the surface. 1 2 3 4 5 6 7 8 9 Per cent . 100 . 98 . 92 . 90 . 80 . 70 . 63 . 54 . 28 . 25 H(kxa) 10.,.. Per cent 3 -17 -31 -35 -60 -84 -59 -131 -142 (After Riehl) If it is assumed that the ascending current retains its angular momentum, withdrawal from the central area would start approximately at the 3-km level, according to Rielil. At this elevation the magnitude of the pressure-gradient force directed toward the hur- ricane center begins to drop significantly with height. In reality, outflow will begin at even lower heights, owing to the decrease of the frictional force above the surface. Durst and Sutcliffe also postulate an increase of the vertical velocity with height and show that such an increase may contribute to the radial outflow of the air aloft. In view of the probability that convergence is restricted to the vicinity of the surface, Riehl con- cludes it is unlikely that the upward vertical motion increases with height except near the surface. Height of Tropical Cyclones. Tropical cyclones are associated with stronger pressure gradients than are observed in any other weather phenomenon where we must deal with hydrostatic equilibrium. The equaliza- tion of this pressure difference between the
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