absorption lines for air, under indirect illumination, with the direct light source not visible, so that the gas is not directly between source and detector. here, fraunhofer lines
in sunlight and rayleigh scattering
of this sunlight is the "source." this is the spectrum of a blue sky somewhat close to the horizon, pointing east at around 3 or 4 pm (i.e., sun toward the west) on a clear day.