Rainfall
From Alpha Centauri Wiki
The amount of Rainfall a land square receives effects the amount of Nutrients that it can produce. Squares can be arid, moderate, or rainy, producing 0, 1, or 2 nutrients respectively.
In addition to coastal and tropical regions, the western (upwind) slopes of mountain ranges tend to be quite rainy, since warm moisture-laden air cools as it rises, and sheds its moisture as rainfall. A player can take advantage of this phenomenon by raising hills to the east of their important bases. Conversely, areas east, or downwind of mountain ranges tend to be quite arid.
