Transverse horizontal cross‐coherence lengths as a function of signal frequency were measured by a hydrophone array 80 m long under homogeneous winter conditions with heavy sea state. The decrease of the normalized coherence length with the acoustic wavenumber is shown as an example of sound propagation parallel to the wave crests and is compared with published reverberation measurements. The cross‐coherence length is interpreted in a simple model computation as the reciprocal of a medium‐dependent angular uncertainty.