A thermodynamic index you will run across often is the LI, which stands for Lifted Index. This is the difference between the 500-millibar parcel temperature and the actual temperature at 500 millibars. The 500-millibar parcel temperature is the temperature a parcel of air would have if it were lifted from the lower PBL to the 500-mb level. The formula for LI is the environmental temperature minus the parcel temperature. Therefore, if the parcel temperature is warmer than the environmental temperature then the LI will be negative. Large negative LI's are an indication the troposphere is very unstable with respect to boundary layer based convection. The more negative the LI, the stronger the convective updraft will be. A guide for LI value interpretation follows: 2 or greater (stable) 1 to 0 (almost unstable) -1 to -4 (troposphere marginally unstable) -4 to -7 (large instability) -8 or less (extreme instability) The LI is a good index to use in the warm season when the PBL is warm and humid. It is not a good index to use in the winter (especially behind a cold front or on the cool side of a warm front) since convection is more dynamic rather than thermodynamic is origin. |