A storm is a column of air before it is a cloud. This is a vertical slice of the atmosphere: the red line is temperature falling with height, the blue line is the dewpoint. Where the two lines stay far apart there is dry air; where they converge the air is saturated and ready.
The shaded area between the parcel's path and the environment is the fuel — meteorologists call it CAPE. More area, more violent the storm.
Drag any red or blue point to reshape the profile and watch the verdict change.