Stability Assessment » K Index (KI) Definition, Strengths and Limitations
Definition: K Index
The K index (KI) is particularly useful for identifying convective and heavy-rain-producing environments. Its computation takes into account the vertical distribution of both moisture and temperature. It does not require a skew-T diagram; it is simply computed from temperatures at 850, 700, and 500 hPa, and dewpoints at 850 and 700 hPa. The higher the moisture and the greater the 850-500 temperature difference, the higher the KI and potential for convection.
Thunderstorm probability ranges from very low when KI < 20 (KI < 15 west of the Rocky Mountains) to a likelihood of widespread activity when KI > 35 (KI > 30 west of the Rocky Mountains).
Strengths and Limitations
The K index is a useful tool for diagnosing the potential for convection. However, it can't be used to infer the severity of convection. Because it uses 850 hPa data, it is not applicable in the Rocky Mountain region, where the surface pressure is typically less than 850 hPa.