def cloud_sd_func(x, a):
    """Function for fitting the standard deviation of wet and dry cloud to the
    mean of wet or dry cloud

    This function returns y with

    .. math::
        y = a^2 x (1 - x)

    Parameters
    ----------
    x: np.ndarray
        The x input data
    a: float
        The parameter as mentioned in the equation above"""
    return a * a * x * (1 - x)
