14 TOR BERGERON 
a surplus of 4 to 6°C at the top of the hurricane 
cloud, thereby increasing greatly the lability 
energy. If, on the other hand, the temperature 
is depressed a little at O-level, there will be a 
great loss of energy. 
Now, we come to the fact that there is gen- 
erally a very low pressure at the center of a 
mature tropical hurricane. This is an important 
point, to which enough attention has not been 
paid hitherto. Thus, if the central pressure is 
lowered, the curve of lifting will move to the 
right in the diagram, and the lability energy will 
be increased, provided the condensation level 
does not rise or the temperature fall. In fact, 
the air flowing into the Low within the frictional 
layer over the warm sea will conserve its high 
temperature in spite of the expansion. Its tem- 
perature will keep near that of the sea-surface, 
thanks to the very rapid transfer of heat from 
this surface up into the air, and the sea-surface 
cannot by any means be cooled appreciably in 
a short time. Thereby the wet-bulb potential 
temperature of the air is raised. By making, so 
to say, the pressure very low at the center, the 
hurricane will dispose over a greater store of 
ability energy. 
Figure 7 shows a vertical cross-section of the 
major hurricane that passed Miami and New 
Orleans in September 1947, which Patmién and I 
have both been treating. A decisive thing with 
the hurricane mechanism, as I see it, is that 
the updraft of the inflowing air takes place inside 
the region of the most intense precipitation, as 
indicated in the small map-sketch at the bottom 
of Figure 7 and in the third row of Figure 1. 
The central hurricane cloud-mass is often funnel 
shaped; thereby it will not attain enough height 
to produce an efficient precipitation release 
(through ice-nuclei or otherwise) until at some 
distance from the ‘eye,’ presumably outside the 
ring of maximum updraft, a good example of 
cooperation between micro-, meso- and macro- 
physical cloud factors. In the opposite case the 
rain cooling, driving C,, would counteract the 
mechanism C,, and the hurricane would trans- 
form into one or more convective systems in- 
stead; this was probably what happened to the 
hurricane of Figure 8. 
Unfortunately, I was not able to treat just 
the major hurricane of September 1947 from this 
viewpoint. Figure 8 shows another hurricane from 
the same year. When lying Just outside the coast 
[cHicaco@ 85 
90° +734" '+2/" 
40" 
ro 
eS) 
CHICAGO @ 1015 nb 85>" wry 
ae oH 
Ea SA 
5 os 
“~-—~ 16 OCT. 1947 
14 OCT. 1947 DS ec, ie aon 
18" 30™ c.sT. ee ea 00h 30™ cst. 
42/° --------- |sotherm of air Q——— Isallobar 
° % 
+233 ——— Isotherm of sea surface YW Rain area 
Fic. 8—Deeay of a tropical hurricane over Georgia and Alabama, Oct. 14-16, 1947; S indicates areas 
of rising pressure 
