How does humidity affect the water potential?

As you decrease humidity, I believe the water potential of the environment also decreases. Conversely, if you increase the humidity, the water potential increases. An example can be found in water movement of leaves. As the humidity increases, the rate of diffusion goes down because water moves from areas of high water potential to low water potential. Thus, as the humidity goes down, the rate of diffusion of water from leaves to the environment increases because water potential of the environment is lower (more negative) than the leaf's, so water flows out of the leaf.