Figure 3-9.-Vertical circulation over developing high.

levels, which may remain quite cold. A warming of 10°C per day at the 500-hPa level is not unusual. Such a rate of warming is not entirely due to subsidence but probably has a considerable contribution from warm advection. However, continuity considerations suggest that the convergence in the 400- to 200-hPa stratum produces some sinking and adiabatic warming in the lower troposphere. See figure 3-9. Thus, in the building of anticyclones, there must be a piling up of air at high levels due to horizontal velocity convergence in the 400- to 200-hPa stratum, which results in the stratospheric cooling observed with developing anticyclones. Insufficient outflow at very high levels results in an accumulation of mass. This is roughly the mechanism thought to be responsible for the development of high-pressure systems. The high-level increase of mass overcompensates the low tropospheric decrease of density, and the high-level effect thus determines the sign of increase of pressure at the surface when highs are intensifying. See figure 3-9. The development of anticyclones appears to be just the reverse of the deepening of cyclones. Outside of cold source regions, and frequently in cold source regions, high-level anticyclogenesis appears to be associated with an accumulation of mass in the lower stratosphere accompanied by ceding. In many cases this stratospheric cooling maybe advective, but more frequently the cooling appears to be clearly dynamic; that is, due to the ascent of air resulting from horizontal convergence in the upper troposphere. Studies of successive soundings accompanying anticyclogenesis outside cold source regions show progressive warming throughout the troposphere. This constitutes a negative contribution to anticyclogenesis. In other words, outside of cold source regions, during anticyclone development, the decrease in DENSITY in the troposphere is overcompensated by an increase in Figure 3-9.-Vertical circulation over developing high. 3-13