Much of the Colorado Plateau is considered arid or semi-arid with annual precipitation amounts in most areas of less than 10 inches. The high plateaus and small mountain ranges, however, receive considerably more precipitation than the more widespread middle and lower elevations due to orographic lifting and cooler temperatures. Most areas above 8000 feet receive 20-25 inches annually, while mountains above 11,000 feet often receive about 35 inches per year.
Climatic patterns vary from south to north across the Colorado Plateau. Much of the northern Plateau shares a climatic regime with the Great Basin. The region generally lies outside the typical major pathways of winter and summer moisture-bearing masses. Winter moisture comes infrequently from Pacific air masses, and summer are generally hot, with infrequent convective rainfall.
The southern Colorado Plateau, however, has a bi-seasonal precipitation regime, with distinct precipitation maxima in both the winter mer seasons. Winter precipitation comes from incursions of Pacific air, while summer precipitation is monsoonal, with a moisture source to the south, southwest, or southeast. Precipitation is low to moderate in the early winter, increasing in February and March, and then dropping off quickly into April. May through June are very dry throughout the region. Precipitation increases in July with the advent of the Arizona summer monsoon, particularly on the southern Plateau. During this time a large dome of high pressure sitting over eastern New Mexico and Texas drives warm, moist air from the Gulf of Mexico into the Southwest. Rapid daytime heating leads to upwelling of the moist air and the formation of widespread and occasionally intense thunderstorms.
The monsoon can continue into mid-September but more commonly ends in late August or early September. With the onset of autumn comes dry conditions and sunny skies. Temperatures begin to cool and often an extended period of comfortable, fair weather ensues. Conditions commonly remain dry until the first winter cyclonic storms develop and pass through the region beginning roughly in late September or November in the north and as late as December on the southern Plateau.
These bi-seasonal and north-south climatic gradients have led to development of a vegetation gradient as well, with species more dependent upon summer precipitation occurring dominantly in the southern and southeastern Colorado Plateau and declining toward the northwest. Locally, these generalizations are modified by high topographic diversity.
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climate studies to understanding the land cover history of the Colorado
Fire-Southern Oscillation Relations in the Southwestern United States. A close linkage between fire and climate could diminish the importance of local processes in the long-term dynamics of fire-prone ecosystems. The structure and diversity of communities regulated by fire may have nonequilibrial properties associated with variations in global climate. Successful prediction of vegetation change hinges on a better understanding of climatically driven disturbance regimes and the relative contributions of regional versus local processes to community dynamics. Adapted from a journal article by Thomas W. Swetnam and Julio L. Betancourt.
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