1. Internal Waves in Lakes: Generation, Transformation, Meromixis – An Attempt at a Historical Perspective

We review experimental and theoretical studies of linear and nonlinear internal fluid waves and argue that their discovery is based on a systematic development of thermometry from the early reversing thermometers to the moored thermistor chains. The latter (paired with electric conductivity measurements) allowed development of isotherm (isopycnal) time series and made the observation of large amplitude internal waves possible. Such measurements (particularly in the laboratory) made identification of solitary waves possible and gave rise to the emergence of very active studies of the mathematical description of the motion of internal waves in terms of propagating time-dependent interface motions of density interfaces or isopycnal surfaces. As long as the waves remain stable, i.e., do not break, they can mathematically be described for two-layer fluids by the Korteweg-de Vries equation and its generalization. When the waves break, the turbulent analogs of the Navier–Stokes equations must be used with appropriate closure conditions to adequately capture their transformation and flux of matter to depth, which is commonly known as meromixis.