The earth cooled and so did the worldwide ocean. This reduced the rate of evaporation. The cloud layer thinned. There was still much carbon dioxide in the atmosphere that absorbed a lot of solar radiation. However there was less water vapour and fewer cloud droplets so some light was able to penetrate to ocean.

It would be interesting to know which wavelengths of solar radiation were most likely to penetrate to the oceans under these conditions. Short wave radiation is most penetrative but also the most easily scattered. The existence of short wave radiation was important to produce in the atmosphere the small quantities of oxygen necessary for the origins of life. It is also thought that the chemicals in the oceans upon which life is based were produced under the influence of short wave radiation.