Less Than 15
0% Thunderstorm Potential
15 to 20
Less Than 20% Thunderstorm Potential
21 to 25
20% to 40% Thunderstorm Potential
26 to 30
40% to 60% Thunderstorm Potential
31 to 35
60% to 80% Thunderstorm Potential
36 to 40
80% to 90% Thunderstorm Potential
Greater Than 40
Near 100% Thunderstorm Potential

The K index combines moisture and stability but DOES NOT depend on lifting. It is computed using the terms as follows: K=(850mb temp minus 500mb temp) + (850mb dew point) minus (700mb temp-dew point spread). A high K index supports cloudiness and precipitation.