The key to understanding energy conversion processes is simplification. It is difficult to understand engines without the concept of isentropic compressors and expanders. Sadi Carnot used polytropic reversible process to understand how heat can be converted to work. While the isentropic process concept is natural for engineers who work with engines, the concept is foreign to atmospheric scientists. An engineer knows that an isentropic expander must have a shaft to take work out of the system. Atmospheric scientists sometimes assume that expansion can be isentropic without providing means of removing work from the system.