"The cold air entering the dayside at the west terminator is heated and rises, causing a permanent warm front on that side of the planet; rain occurs whenever the rising air cools below the precipitation point. A considerable amount of water vapour does not precipitate out of the atmosphere in this western habitable zone, but carries on over the hot side as transparent vapour until it cools on the eastern terminator; precipitation here is usually even more frequent than in the west. The warm winds cause the ice near the eastern terminator to melt, adding more water to the eastern habitable zone."