Lifted Index & K-Index Discussion
Lifted IndexThe "lifted index" is computed as if a parcel of air near the surface were lifted to 500 mb (18,000ft). As the air is "lifted" it cools, at 3 degrees Celsius per 1,000ft, due to expansion. The temperature the parcel would have at 500 mb is then subtracted from the actual (environmental) 500mb temperature. This difference is the "lifted index" which is positive, negative, or zero and indicates the stability of the parcel of air.
A positive index means that a parcel of air, if lifted, would be colder than the surrounding air at 500mb. The air is, therefore, stable and would resist vertical motion. Large positive values (+8) would indicate very stable air.
A negative index means that the low-level air, if lifted, to 500 mb, would be warmer than the surrounding air. The air is unstable and suggests the possibility of convection. Large negative values (-4 or less) would indicate very unstable air.
A zero index means that the parcel of air, if lifted to 500mb, would have the same temperature as the actual air at 500 mb. This air is said to be neutrally stable (neither stable or unstable).
When using a lifted index chart, remember that the lifted index assumes the air near the surface will be lifted to 500mb. Whether or not the air near the surface will be lifted to 500mb depends in what is happening below. It is possible to have a negative LI with no thunderstorm development because either the air below 500mb is not being lifted high enough or there is not enough moisture in the air. For use, the lifted index is more indicative of the severity of thunderstorms, if they occur, rather than the probability of general thunderstorm occurrence. Also note that the LI can change dramatically just by daytime heating and nighttime cooling. Daytime heating tends to make the LI value less positive (more unstable) and nighttime cooling tends to make the LI more positive (more stable).
During thunderstorm season, a large K index indicates conditions favorable for air mass thunderstorms. However, K index values and meanings can decrease significantly for thunderstorm development associated with a synoptic scale low pressure system (non air-mass thunderstorms). In winter, because of the lack of moisture, even fairly large values do not mean conditions are favorable for thunderstorms. Be aware that the K values can change significantly over a short time period due to temperature and moisture advection.
|Lifted Index (LI)||Severe Potential||K-Index *||Airmass Thunderstorm Probability|
|0 to -2||Weak||<15||near 0%|
|-3 to -5||Moderate||21-25||21-40%|
| It is essential to note that an unstable Lifted Index
does NOT automatically mean thunderstorms. Look at the synoptic situation
and if thunderstorms are expected to develop in the unstable air, this
table may be used.
* Use caution when applying these values in the western mountainous terrain due to elevation