The atmospheric hydrologic cycle plays a key role in regulating the Earth's climate system. In 2007 the Intergovernmental Panel on Climate Change (IPCC) reiterated that clouds remain the largest source of uncertainty in climate projections. It is also well-known that water vapor feedbacks play a significant role in the manner in which the climate system is and will respond to an increase of greenhouse gases. The Atmospheric Infrared Sounder (AIRS) is extremely well suited to characterize and help understand and model these components and processes of the atmospheric hydrologic cycle. In this presentation we will discuss some key AIRS results in the context of the characterization of the essential physical processes in the tropics and sub-tropics that regulate the Earth's climate response to an increase in greenhouse gases.