chain of lakes that began with Mono Lake, in the north, and continued through basins down the Owens River Valley, through Searles and China Lakes and the Panamint Valley, to the immediate west. As the area turned to desert, the water evaporated, leaving an abundance of evaporitic salts, such as common sodium salts and borax, which were later exploited during the modern history of the region, primarily 1883 to 1907. Climate Death Valley has a subtropical, hot desert climate (Köppen: BWh), with long, extremely hot summers; short, warm winters; and little rainfall. The valley is extremely dry, because it lies in the rain shadow of four major mountain ranges (including the Sierra Nevada and Panamint Range). Moisture moving inland from the Pacific Ocean must pass eastward over the mountains to reach Death Valley; as air masses are forced upward by each range, they cool and moisture condenses, to fall as rain or snow on the western slopes. When the air masses reach Death Valley, most of the moisture in the air has already been lost and thus there is little left to fall as precipitation. The extreme heat of Death Valley is attributable to a confluence of geographic and topographic factors. Scientists have identified a number of key contributors: Solar heating: The valley's surface (consisting of soil, rocks, sand, etc.) undergoes intense solar heating because the air is clear and dry, and the land is dark and sparsely vegetated. This is especially noticeable in mid-summer, when the sun is nearly directly overhead. Trapping of warm air: Warm air naturally rises and cools; in Death Valley this air is subject to continual reheating as it is trapped by high, steep valley walls and recycled back to the valley floor. Warm air also is trapped by the valley's north–south orientation, which runs perpendicular to prevailing west-to-east winds. Migration of warm air from other areas (advection): Warm desert regions adjacent to Death Valley, especially to the south and east, often heat air before it arrives in Death Valley. Warm mountain winds: As winds are forced up and over mountains (e.g. the numerous ranges west of Death Valley), the winds can be warmed in several ways. The resulting dry, warm winds are known as foehn winds. Their warmth can in part be caused by the release of latent heat, which occurs when water vapor condenses into clouds. Severe heat and dryness contribute to perpetual