From Glossary of Meteorology
The problem in turbulence analysis that occurs when Reynolds averaging is applied to Navier–Stokes equations; the result is that there are more unknowns than equations.
In order to solve the problem, assumptions have to be made concerning the unknown quantities in the equations. These unknowns appear as correlations between the fluctuating quantities. The simplest closure is the so-called ''K'' theory. A more advanced form is the direct interaction approximation.