Enhanced Full Spectral Temperature‐Dependent Refractive Index of Liquid Water From Supercooled to Ambient Conditions

Abstract A new compilation of the complex refractive index of liquid water is presented, spanning temperatures from (near homogeneous freezing) to K and wavelengths from μm to 10 m. The real part of the refractive index is derived using the Kramers–Kronig relation, where the imaginary part is constrained by measurements reported in literature and validated through the f‐sum rule. The result reveals a significant temperature dependence, especially at wavelengths beyond the near‐infrared. Sensitivity analyses in the infrared split‐window and microwave spectral regime demonstrate substantial differences in bulk optical properties between supercooled and ambient conditions. These findings manifest the importance of accounting for temperature‐dependent refractive indices in optical radiative transfer and simulations.