Horizontal inflow or outflow near the surface has to be compensated by vertical motion, as illustrated in Figure 6.7, if the low- or high-pressure systems are to persist and there is to be no continuous density increase or decrease. Air rises above a low-pressure cell and subsides over high pressure, with compensating divergence and convergence, respectively, in the upper troposphere. In the middle troposphere, there must clearly be some level at which horizontal divergence or convergence is effectively zero; the mean 'level of non-divergence' is generally at about 600 mb. Large-scale vertical motion is extremely slow compared with convective up- and downdrafts in cumulus clouds, for example. Typical rates in large depressions and anticyclones are of the order of ±5 to 10 cm s-1, whereas updrafts in cumulus may exceed 10 m s-1.
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