If we think of the atmosphere as a sponge, we can imagine that the sponge is very dry in the western US, and almost full over humid regions in the eastern US. The addition of moisture to the atmosphere from the land surface in these humid regions makes the sponge leak (makes rain fall). In the west, the dry sponge soaks up any moisture the land surface can contribute without initiating rainfall: rainfall in the west is only initiated by other mechanisms. Credit: Kirsten Findell, NOAA/GFDL
