Storms developing in one environment can be different in character than those developing in another. Modelers can alter the stability of the atmosphere in which they simulate storms by changing the vertical distribution of temperature and moisture.
Thunderstorms develop when surface and low level air is allowed to rise without restriction into the upper troposphere. If air rises in an environment without restriction, the environment is said to be unstable. This means that stability is simply the resistance the atmosphere imposes on rising (or sinking) air.
If, however, the rising air cools at a slower rate than the surrounding atmosphere, it will be warmer (and less dense) than its surroundings. Here, the rising air would continue rise.
Sometimes, the environment is stable near the ground and must be forced to rise into unstable air by a front or other mechanism.