Level of nondivergence
From Glossary of Meteorology
level of nondivergence
A level in the atmosphere throughout which the horizontal velocity divergence is zero.
Although in some meteorological situations there may be several such surfaces (not necessarily level), the level of nondivergence usually considered is that midtropospheric surface that separates the major regions of horizontal convergence and divergence associated with the typical vertical structure of the migratory cyclonic-scale weather systems. Interpreted in this manner, the level of nondivergence is usually assumed to be in the vicinity of 500 mb. The assumption of such a level in theoretical work facilitated the construction of early models in numerical forecasting.
