|"K" Index||Chance of Thunderstorm||Lift Rate|
|< -10||None||< 300 ft. /min|
|-10 -> + 5||Blue thermals||300 -> 600|
|+5 -> +10||Increasing convection||500 -> 700|
|+10 -> +15||Isolated strong thermals||600 -> 800|
|+15 -> +20||20% coverage by possible thunderstorms||700 -> 900|
|+20 -> +25||20->40% TRW||800 -> 1000|
|+25 -> +30||40->60% TRW||900 -> 1100|
|+30 -> +35||60->80% TRW||1000-> 1200|
|+35 -> higher||> 80% TRW||1100-> 1300|
|+40 -> higher||near 100% probability of TRW|
|Lifted Index||"K" Index||TRW Probability|
|0 -> -2 Weak indication of TRW||<15||Near 0%|
|-3 -> -6 Moderate||21 -> 25||20-> 40%|
|25 -> 30||40 -> 60%|
|<= -6 Strong TRW||30-> 35||60-> 80%|
"K" Index examines the temperature and moisture profile of the environment. Since a parcel of air is not lifted and compared to the environment, the "K" Index is not truly a stability index. However, the meteorologist, looking at the environmental temperature and moisture profile, can make a good judgment as to the stability of the air. The "K" Index is computed using three terms as follows:
"K" Index = (850) mb temp.- 500 mb temp) + (850 mb dew point) - (700 mb temp to dewpoint spread)
The first term (850 mb temp - 500 mb temp is proportional to the average lapse rate. A large temperature difference shows steep or unstable lapse rate. The greater the difference, the more unstable the air and the higher the "K" value. The second term (850 mb dew point) is a measure of low level moisture. Since the dew point is added, high moisture content at 850 mb increase the "K" value.
The third term (700 mb dew point temperature spread) is a measure of saturation at 700 mb. The greater the spread, the dryer the air. Since the term is subtracted, it lowers the "K" value. However, moist air (small spread) lowers the value less than does dry air (large spread).
The greater the degree of saturation at 700 mb, the larger is the "K" values. Be aware the "K" values can change significantly in summer time over short periods of time due to temperature and moisture advection. In the winter, when temperature are very cold, the moisture terms are very small. So, even the "K" values are fairly large, it does not mean that conditions are favorable for thunderstorms because of lack of moisture.
The Thermal Index (TI) is an indicator of thermal strength frequently used in the U.S. Maximum forecast temperature is lifted dry-adiabatically. As it intersects sounding temperature line, it indicates stability level-attitude. Thus, TI can be measured for any altitude by subtracting the temperature of the lifted air parcel from the sounding temperature. The more negative the number, the stronger the thermals will be. TI of:
This method is more useful in Falkland than in mountainous regions.
The 850 -> 500mb Lapse Rate shows temperature difference between approximately 5,000 ft.(1.5km) and 18,000 ft. (6km) (In Celsius). Lapse rate of 2.0 -> 2.4'C/1000 ft. is a bit marginal and mostly will be blue thermals. Rate of 2.4 ->2.8> produces good soaring conditions and signs of convection clouds. Rate of 2.8 -> 3.4 shows very good thermal activity and also over development; maturing into thunderstorms. Rates of higher than 3.4 will develop partially overcast skies with imbedded thunderstorms.
The Showalter Stability Index is a measure of the stability of atmosphere, expressed as a numerical index. This index is determined by raising an air parcel from 850 mb. Level dry-adiabatically to the point of saturation, then saturation (wet) adiabatically to 500 mb level. At 500 mb level the temperature (in Celsius) of the parcel is compared to that of the environment. The magnitude of the index is the difference between the two temperatures. If the parcel is cooler that its new environment the index is positive; if the parcel is warmer, the index is negative. Positive -> stability; negative -> instability.
The SW Index of +14 show minimal soaring conditions. As this index decreases, the strength of thermals increases. At index of +4, or lower, over development of thundershowers (TRW's) can be expected.