def jonswap_peak_frequency(U10: float, fetch: float, grav: float) -> float:
    """Returns the JONSWAP equilibrium peak frequency in Hz, 
    given input wind speed at 10-m height (m/s), fetch (m), 
    and gravitational acceleration (m/s**2)."""
    return 3.5 * (grav**2 / (U10 * fetch))**(1. / 3)