A stability index that is identical to the lifted index (LI) except that it modifies the standard 1200 and 0000 UTC soundings or numerical-model soundings by incorporating hourly surface observational data, and the characteristics of the lifted parcel are from surface values of temperature and dewpoint. The advantage of this index is that it can be computed at hourly intervals and it typically reflects short-term changes in the boundary layer. The surface-based lifted index is typically more unstable than the LI. As with the LI, thunderstorms become increasingly likely the further the surface- based lifted index decreases below a threshold of zero.