SECT. 2] 



LAR(iK-S('ALE INTERACTIONS 



233 



\Mth the basic assuin])tion of an undisturbed top near the tropopause, so 

 that lateral pressure gradients are produced by density variations within the 

 troposphere, it is readily shown that ascent and condensation, however rapid, 

 of unmodified tropical air cannot produce a hurricane. Curve E on the tephi- 

 gram in Fig. 68 shows the structure of the mean tropical atmosphere in the 

 hurricane season, the environment and air source for the developing storm. If 

 its sub-cloud air (T = 26.0''C; g=18.5 g/kg) is lifted dry adiabatically to 



Fig. 67. Schematic diagram of core region of typical moderate strength tropical hurricane, 

 which is defined to consist of an "inner rain area" and an "eye" (see text). Major 

 spii'al bands of cumulonimbus along trajectories and forming eye wall idealized. 

 Arrow at top denotes direction of storm motion. Stippled region, marked v^, is 

 region of maximum wind speed, frequently oriented as shown. Typical surface 

 pressures and wind speeds indicated. Dimensions of real hurricanes vary widely and 

 figures in this diagram should not be taken too literally. 



condensation and from thence moist adiabatically, the dashed curve results. 

 This is the maximum warming of air columns achievable by converting the 

 original latent heat content to sensible heat content. It is uniquely fixed by 

 the initial total heat-energy content Q[Q = CpT + Agz + Lq^ {CpT + Lq)o] of the 

 lifted air, which is approximately 84 cal/g in mean tropical air. According to 

 the hydrostatic equation, the lowest surface pressure obtained through this 

 ascent will be about 1000 mb (Riehl, 1954). This is a threshold value and it is 

 interesting to note that many tropical storms (cf. Fig. 56d) reach equilibrium 



