Increased mid level moisture leads to somewhat decreased instability in the atmosphere. Besides being less dense than dry air, moist air cannot evaporate rain and cloud droplets as efficiently. Evaporation of rain at or below cloud level cools the air caught in the precipitation downdraft making it more dense and thus increasing downdraft instability. Downdraft descent on the rear flanks of supercell thunderstorms have been found to often coincide with tornadic circulation descending to the ground. Moist mid level air results in reduced evaporation within the thunderstorm, thereby allowing more rain to reach the ground.
Meteorologists often use the dewpoint as a measure of atmospheric moisture.
OK Simply stated, the higher the dewpoint, the higher moisture content in the atmosphere. A typical range of summertime dewpoints in West Central Texas is shown to the left. The cap usually forms under a large upper level pressure ridge. Under a ridge, air gradually sinks and warms from compression. A source lift is need to push developing updrafts through the stable capping inversion.