Deserts are defined, in part, by scant precipitation, less than ten inches per year. North American deserts are isolated from generous rainfall by massive mountain ranges, such as the Sierra Nevada in California and the Sierra Madre in Mexico. Precipitation arrives annually, if at all in some locations such as Death Valley, in one or two seasons that can bring torrential downpours and flooding. Deserts can be lands of extremes. The Mojave is the highest and lowest, the hottest and coldest, and the driest desert in North America. Occasionally, large climate changes, notably El Niño, ride over the isolating barriers and nourish deserts with exceptional precipitation.