Lift Index

Theoretical index, known in aerology by the acronym LI (Lift Index), which makes it possible to estimate the degree of atmospheric instability from the information provided by a radiosonde sounding. It is defined as the algebraic difference between the temperature observed at the 500 hPa level (an intermediate level of the troposphere) and the temperature that a bubble of air rising adiabatically from near the Earth's surface would have upon reaching that level. If the index is positive (LI > 0), the environment is thermodynamically stable and the possibility of convection being triggered and storms forming is practically nil. Negative values of the lift index (LI < 0) indicate that there is instability. The more negative it is, the more intense the convection will be. For LI values between 0 and –3, the air is not thermodynamically very unstable, so any storms that may form will not be very significant. If the lift index takes values between –3 and –6, storms can be strong. For values between –6 and –9, atmospheric instability is notable, favoring the development of severe storms, with a high probability of hail. Finally, if LI < –9, the environment will be one of extreme instability, favorable for the development of supercells and tornadoes.