Vertical slice through the center of a mature hurricane showing the secondary circulation. In the lower troposphere, air spirals inward towards the center of the hurricane (red arrows), then upward in the eyewall (orange arrows), and then outward at the top of the troposphere (blue arrows). Eventually, the air sinks back towards the lower troposphere (smaller, blue arrows), and the circulation continues with new and/or recycled air spiraling inward towards the eyewall. The secondary circulation works like a heat engine. For heat energy to be converted to mechanical energy, air must flow into the hurricane (in the lower troposphere) at a higher temperature than it exits the hurricane (at the top of the troposphere).