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Climate and meteorology

The climate of the Sierra Nevada is influenced by the Mediterranean climate of California. During the fall, winter and spring, precipitation in the Sierra ranges from 20 to 80 in (510 to 2,030 mm) where it occurs mostly as snow above 6,000 ft (1,800 m). Precipitation is highest on the central and northern portions of the western slope between 5,000 and 8,000 feet (1,500 and 2,400 m) elevation, due to orographic lift. Above 8,000 feet (2,400 m), precipitation diminishes on the western slope up to the crest, since most of the precipitation has been wrung out at lower elevations. Most parts of the range east of the crest are in a rain shadow, and receive less than 25 inches of precipitation per year. While most summer days are dry, afternoon thunderstorms are common, particularly during the North American Monsoon in mid and late summer. Some of these summer thunderstorms drop over an inch of rain in a short period, and the lightning can start fires. Summer high temperatures average 42–90 °F (6–32 °C). Winters are comparatively mild, and the temperature is usually only just low enough to sustain a heavy snowpack. For example, Tuolumne Meadows, at 8,600 feet (2,600 m) elevation, has winter daily highs about 40 °F (4 °C) with daily lows about 10 °F (−12 °C). The growing season lasts 20 to 230 days, strongly dependent on elevation. The highest elevations of the Sierra have an alpine climate.

The Sierra Nevada snowpack is the major source of water and a significant source of electric power generation in California. Many reservoirs were constructed in the canyons of the Sierra throughout the 20th century, Several major aqueducts serving both agriculture and urban areas distribute Sierra water throughout the state. However, the Sierra casts a rain shadow, which greatly affects the climate and ecology of the central Great Basin. This rain shadow is largely responsible for Nevada being the driest state in the United States.

Precipitation varies substantially from year to year. It is not uncommon for some years to receive precipitation totals far above or below normal.

The height of the range and the steepness of the Sierra Escarpment, particularly at the southern end of the range, produces a wind phenomenon known as the "Sierra Rotor". This is a horizontal rotation of the atmosphere just east of the crest of the Sierra, set in motion as an effect of strong westerly winds.

Because of the large number of airplanes that have crashed in the Sierra Nevada, primarily due to the complex weather and atmospheric conditions such as downdrafts and microbursts caused by geography there, a portion of the area, a triangle whose vertices are Reno, Nevada; Fresno, California; and Las Vegas, Nevada, has been dubbed the "Nevada Triangle", in reference to the Bermuda Triangle. Some counts put the number of crashes in the triangle at 2,000, including millionaire and record-breaking flyer Steve Fossett. Hypotheses that the crashes are related in some way to the United States Air Force 's Area 51, or to the activities of extra-terrestrial aliens, have no evidence to support them.