234 



MALKUS 



[chap. 4 



at this central pressure. The total heat content of Jiormal tropical air raised un- 

 dilute {without entrainment) to the level of zero buoyancy is insufficient to generate 

 pressures substantially below 1000 mb. 



Formerly it was believed that descent and compressional warming in an 

 outward-sloping eye could account for the rapid pressure drop in the inner 

 rain area. The aircraft data have refuted this ; no large enough eye-wall slopes 

 have yet been observed ; in most cases the hot towers grow almost vertically to 

 great heights. A typical "inner rain area" sounding (curve D, Fig. 68) from 

 hurricane Daisy, 1958, contains the heart of the new physical modelling and 

 points to the existence of the extra heat source. This sounding very closely 



Fig. 68. Thermodynamic features of mean tropical atmosphere (curve E), wet adiabatic 

 ascent of mean tropical surface air (dashed) and mean moderate hurricane (D, 

 actually from observations of hvirricane Daisy, 1958) plotted on tephigram. Abscissa 

 is temperature in °C. Slanting ticks are pressures in mb. Oi'dinate (not shoMTi) is 

 potential temperature 6. Arrow labelled Tq is sea-surface temperature (29°C) pre- 

 vailing for hurricane Daisy situation. Isopleths of total heat-energy content, Q, would 

 be parallel to dashed line, increasing toward upper right. Dotted extension of lower 

 part of curve E shows properties of surface air, if cooled adiabatically, in flowing 

 horizontally toward center. 



represents the moist adiabatic ascent of surface air whose total heat-energy 

 content has been increased by somewhat more than 2 cal/g over mean tropical 

 air. By a hydrostatic computation we find the surface pressure associated with 

 curve D to be 960-970 mb.i Thus we may generate low pressures by increasing 

 the heat content of the rising air, which moves the adiabat ascended towards 

 the upper right, or to warmer temperatures. The surface pressure goes down 

 nearly linearly at about 13 mb per cal/g added. 



But is it required that the entire inner rain area rises wet adiabatically? 

 What of the concentration in hot towers and the intervening space between 

 the undilute cores in Fig. 67? We see from curve D of Fig. 68 that the tempera- 

 ture excess over mean tropical air increases upward, becoming a maximum in the 

 upper troposphere. In fact, the hydrostatic computation shows that 80% of 



1 The range depends upon relative humidity and the undisturbed upper level assumed. 



