The atmosphere and ocean would be in thermodynamic equilibrium if their temperature were equal and if the atmosphere next to the ocean had a relative humidity of 100%. In reality, the atmosphere is a little cooler than the ocean (about 2 degrees F) and the humidity is only about 80%. It is this second factor that is responsible for most of the disequilibrium. When the two fluids are not in equilibrium, heat can flow from the ocean to the atmosphere, and this heat flow is what powers hurricanes.