Instantaneous Contraction Rate

This is a measure of the instantaneous rate of change of the distance between two adjacent air parcels. It is related to the process of frontogenesis, which involves an increase of the gradient in some property of the air, such as temperature or mixing ratio, as contraction proceeds. The instantaneous contraction rate (ICON) is related to the kinematic variables according to the formula ICON = 1/2 (DEFres - DIV). Regions of large ICON are likely to experience frontogenesis, if there is some pre-existing weak gradient in some property of the air, like temperature of moisture, in the area experiencing contraction.

Reference:
Cohen, R. A., and D. M. Schultz, 2005: Contraction rate and its relationship to frontogenesis, the Lyapunov exponent, fluid trapping, and airstream boundaries, Mon. Wea. Rev., 133, 1353-1369.