Abstract – Water covers two-thirds of the earth’s surface and is an unmistakable presence in the atmosphere above it. It forms clouds of various colors, shapes and sizes, it falls from the sky as rain or snow, and it produces breathtaking optical phenomena such as rainbows and halos. Yet it is the water that lies unseen in the form of gaseous water vapor that exerts perhaps an even more most profound influence on our planet’s climate. Given its ubiquitous nature, it is surprising how rare water vapor molecules actually are – on average, they comprise fewer than one out of every hundred molecules in the atmosphere. Yet the small concentrations of water vapor belie its importance. Water vapor is the dominant greenhouse gas, trapping more of the planet’s heat than any other atmospheric constituent. In sufficient quantities, water vapor condenses to form clouds and precipitation, each of which are highly variable in both space and time. However, despite this apparent complexity in its distribution, many aspects of the water cycle are actually governed by relatively simple physical constraints. These constraints allow us to make confident predictions about how certain facets of the water cycle and climate will respond to increasing greenhouse gases. These simple physical arguments help us to understand climate projections made by much more sophisticated earth system models. This presentation will highlight these simple physical constraints and explain how they help us make confident predictions of the water cycle’s response to a warming climate.