The Blue Skies Anomaly
Measurements show that one quarter more solar radiation is absorbed in a clear atmosphere than predicted by the best models. At ground level it represents a difference of between 25 and 30 Wm-2. To put this effect to scale it is five times larger than the worst predictions of the Greenhouse effect (<=5 Wm-2). Possible causes for this anomaly can be absorption of sunlight in the atmosphere by water clusters or small aerosols or scattering back of sunlight by small dust particles in the earth's atmosphere.
Water already accounts for about 70% of the known atmospheric absorption. The missing absorption is probably in the near infrared and visible. Water transitions in this region are numerous but weak. Also the absorption of water molecules in the near infrared and visible part of the spectrum is only poorly characterized experientally. Research on sunspots helps to elucidate the situation. The excited states involved here are also observed in sunspots. So the atmosperic absorption can be modelled using linelists generated for stellar atmospheres. How this linelists evolve you can find out in our research on water molecules.