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. . partially, to increased air temperatures during winter months. High elevation and structurally unconstrained drainage are subject to rapid cooling and have little capacity to store radiant heat energy. The complex effects of local and microscale climate on riparian habitats requires additional research. Precipitation Precipitation in the arid West is influenced by regional and global air circulation patterns and elevation. Convective lifting occurs because air warmed by contact with the ground is less dense than cool air and rises. Orographic lifting of air masses occurs when moving air masses are pushed up over topographic obstacles, such as ridges or mountains. As air rises it cools, causing water vapor to condense and allowing precipitation to fall. Thus, precipitation is likely whenever large air masses are lifted. For the Colorado Plateau region with its prevailing westerly winds, precipitation is often greater on west-facing flanks of mountain ridges, such as the Wasatch Front, where orographic lifting increases precipitation. Consequently, the Colorado Plateau lies in a partial rainshadow created by the Wasatch Range and ranges further west. As a result of elevation, orographic lifting and other factors, precipitation on the Colorado Plateau ranges from less than 10 cm/yr at lowest elevations to nearly 100 cm/yr in mountainous regions (Windell et al. 1986; Table 4). For example, the Paria River headwaters receive 42.0 cm of precipitation annually (as much as 3 m of snow annually), whereas annual average precipitation in the central Paria basin is about 30.0 cm/yr, and scarcely more than 5 cm/yr falls at the mouth of the Parianear Lees Ferry (Seller and Hill 1974; Hereford 1984). Precipitation in the Paria River basin is bimodal, with a peak in February-March and another in August-September (Hereford 1984). Rain and Snow Precipitation on the Colorado Plateau is variable, seasonal and bimodal in distribution, with substantial winter snow and summer monsoonal rains (Fig. 8). Precipitation depends on topographic (convective and/or orographic) interactions with various kinds of storm systems, especially including tropical low pressure storms. Hansen et al. (1981) identified six kinds of storm systems in addition to the predominant tropical cyclonic systems, including: cut-off lows, high-latitude lows, low-latitude lows, low-latitude/breakthrough lows, mid-latitude lows, and Gulf of Mexico lows. Winter storms are generally derived from northwesterly frontal systems; spring and fall storms to be . . . . . . . . 16 .