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The ratio of the thickness of thermal boundary layer to the thickness of hydrodynamic boundary layer is equal to (Prandtl number) n, where n is equal to

δ = the thickness of the hydrodynamic boundary layer; the region of flow where the velocity is less than 99% of the far-field velocity.

δT = the thickness of the thermal boundary layer; the region of flow where the local temperature nearly reaches the value (99%) of the bulk flow temperature

\(\begin{array}{*{20}{l}} \begin{array}{l} \dfrac{\delta }{{{\delta _T}}} = {\left( {{\rm{Pr}}} \right)^{\dfrac{1}{3}}}\\ \: \end{array}\\ {\dfrac{{{\delta _T}}}{\delta } = {{\left( {{\rm{Pr}}} \right)}^{ - \dfrac{1}{3}}} \Rightarrow n = - \dfrac{1}{3}} \end{array}\)

The ratio of the thickness of the thermal boundary layer to the thickness of the hydrodynamic boundary layer is equal to -1/3.