full text of the classic FAA guide
THEY DON'T JUST HAPPEN
For a thunderstorm to form, the air must have (1) sufficient water vapor, (2) an unstable lapse rate, and (3) an initial upward boost (lifting) to start the storm process in motion. We discussed water vapor in chapter 5 and stability in chapter 6; but, what about lifting? Surface heating, converging winds, sloping terrain, a frontal surface, or any combination of these can provide the lift.
Thunderstorms have been a subject of considerable investigation for many years as they are today. Figuratively speaking, let's look inside a thunderstorm.