We have identified two environmental controls that explain a large amount of these variations. One control is the specific humidity, which measures (by mass) the concentration of water in the air. The other is the relative humidity, which measures how close the air is to saturation. In past research, we used idealized model simulations to show that, taken together, these two environmental variables help to determine the amount of water that flows into the storms, the strength of the updrafts in the lower part of the storm, and the amount of cloud water that actually reaches the surface as rain. Here, we focus on real-world observations and on complex computer model predictions of climate change. We find that the variations in these datasets are consistent with our past findings in simpler models.