  We show a parallelism between the expansion and compression of the atmosphere
in the secondary cycle of a tropical cyclone with the fast expansion and
compression of wet air in a bottle. We present a simple model in order to
understand how the system (cyclone) draws energy from the air humidity. In
particular we suggest that the upward (downward) expansion (compression) of the
warm (cold) moist (dry) air follows a polytropic process, $PV^\beta$= constant.
We show both experimentally and analytically that $\beta$ depends on the
initial vapor pressure in the air. We propose that the adiabatic stages in the
Carnot-cycle model for the tropical cyclone be replaced by two polytropic
stages. These polytropic processes can explain how the wind wins energy and how
the rain and the dry bands are produced inside the storm.
