The Sierra Nevada region of California is its alpine region, covering a large portion of the state's inland territory. Rugged mountains and awe-inspiring canyons in the area's national parks and forests are truly part of the United States' national wilderness treasures.
It's a region of grandiose sizes; Mount Whitney, the highest peak in the contiguous U.S., is here, as well as the giant sequoias, the largest living things on earth. The human population, however, is relatively small. Most of the towns and cities in the Sierra Nevada region are ski resort towns, or gateways to the area's parks.
What is winter?
On maps of the Sierra, some roads will be marked "Closed in winter." Travelers are often advised to "carry chains in winter." You might think that such advice refers to that time of year the rest of the world calls "winter" which lasts from Winter Solstice through Spring Equinox. You would be wrong.
What Californians are referring to is the time of year when it snows in the Sierra. Snow can begin as early as October, and last as late as May. Roads may be closed as late as July. When snow falls in mid October, it usually kills a few unprepared backpackers in the Sierra who hadn't carefully checked the weather reports. If you don't have chains during an early October or late May snowstorm, continuing to drive may kill you too.
The Sierras are a high mountain environment containing some of the snowiest places in the US. During winter you should always carry tire chains in your car and know how to use them.
When hiking or backpacking where you will be far from help, check the weather first. The first snows of Winter may come earlier than you expect.
In summer during the North American Monsoon, thunderstorms often build up near the crest of the Sierra Nevada, particularly during late afternoon. It is unwise to be in a high, treeless place during a thunderstorm. If hiking far in summer, it's a good idea to carry a light rain jacket just in case a thunderstorm pops up out of nowhere.