Phrases Height is the vertical distance above the surface. "Sea surface height" is a time-varying quantity. Amplitude is the magnitude of a wave modelled by a sinusoidal function. A coordinate variable of harmonic_period should be used to specify the period of the sinusoidal wave. The specification of a physical process by the phrase "due_to_" process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase.