def fwhm(fwhm_now, lambda_now, secz_now, lambda_next, secz_next):
    """Predict the fwhm.

    Parameters:
    -----------
    fwhm_now    : The fwhm value of the current exposure
    lambda_now  : The wavelength of the current exposure
    secz_now    : The airmass of the current exposure
    lambda_next : The wavelength of the next exposure
    secz_next   : The airmass of the next exposure
    
    Returns:
    --------
    fwhm_next   : The predicted fwhm of the next exposure
    """
    lambda_now, secz_now = map(float,[lambda_now, secz_now])
    lambda_next, secz_next = map(float,[lambda_next, secz_next])
    fwhm_next = fwhm_now * (lambda_next/lambda_now)**(-0.2) * (secz_next/secz_now)**(0.6)
    return fwhm_next