Nature's Carnot Engine: The Hurricane

The typical curriculum in a thermodynamics course includes the Carnot cycle, a series of processes that constitute the most efficient heat engine between two constant temperature reservoirs. The conventional view of this cycle is that it is an ideal that cannot be achieved in practice. The spate of hurricanes that battered the U.S. in 2005 has given rise to extensive thermodynamic analysis of these storms with the finding that they in fact replicate the Carnot cycle. Heat energy fuels the storm through evaporation of the warm ocean waters at constant temperature. Work is produced by adiabatic expansion upward of the saturated air, resulting in strong winds. Energy is then released by radiation to space at the approximately constant temperature of the upper atmosphere. Adiabatic compression follows as the air descends back to sea level. Unlike the textbook Carnot engine, the work output of the hurricane is returned to the cycle by frictional dissipation due to the wind at the sea surface boundary layer. This paper will describe the hurricane in thermodynamic terms so as to appreciate its great power and efficiency. It may be a worthwhile topic to include in a thermodynamics course.