Climate determines the types of forests which can occupy a particular region, and climate as modified by topography largely controls the spatial patterns of forest types within a region. Since analyses of trends in annual growth increments are central to the regional air pollution effects studies described in other chapters, an understanding of the general patterns and variability of climate in the West is necessary for interpreting the results of those studies. In the chapter, the general patterns and causes of climate over the western United States are described. More detailed climatic descriptions are then presented for the five regions of the western United States discussed later in the volume: Northwestern Washington; the Colorado Front Range; Arizona and New Mexico; Sierra Nevada; and southern California. Mean seasonal temperature and precipitation are characterized for selected climatic divisions within the regions. The Palmer Drought Severity Index is used to discuss temporal variability in the climate of these regions.