def emissivity(T_air):
    """Apparent atmospheric emissivity

    Apparent emissivity (Sedlar and Hock, 2009: Cryosphere 3:75-84)
    Atmospheric emissivity (clear-sky) Idso and Jackson (1969)

    Parameters
    ----------
    T_air : ee.Image
        Air temperature (Kelvin)

    Returns
    -------
    e_atm : ee.Image

    """
    e_atm = T_air.expression(
        '1.0 - (0.2811 * (exp(-0.0003523 * ((T_air - 273.16) ** 2.0))))', {
        'T_air': T_air})
    return e_atm
